1. Introduction
Anybody can write code. With a few months of programming experience, you can write 'working applications'. Making it work is easy, but doing it the right way requires more work, than just making it work.
Believe it, majority of the programmers write 'working code', but not ‘good code'. Writing 'good code' is an art and you must learn and practice it.
Everyone may have different definitions for the term ‘good code’. In my definition, the following are the characteristics of good code.
· Reliable
· Maintainable
· Efficient
Most of the developers are inclined towards writing code for higher performance, compromising reliability and maintainability. But considering the long term ROI (Return On Investment), efficiency and performance comes below reliability and maintainability. If your code is not reliable and maintainable, you (and your company) will be spending lot of time to identify issues, trying to understand code etc throughout the life of your application.
2. Purpose of coding standards and best practices
To develop reliable and maintainable applications, you must follow coding standards and best practices.
The naming conventions, coding standards and best practices described in this document are compiled from our own experience and by referring to various Microsoft and non Microsoft guidelines.
There are several standards exists in the programming industry. None of them are wrong or bad and you may follow any of them. What is more important is, selecting one standard approach and ensuring that everyone is following it.
3. How to follow the standards across the team
If you have a team of different skills and tastes, you are going to have a tough time convincing everyone to follow the same standards. The best approach is to have a team meeting and developing your own standards document. You may use this document as a template to prepare your own document.
Distribute a copy of this document (or your own coding standard document) well ahead of the coding standards meeting. All members should come to the meeting prepared to discuss pros and cons of the various points in the document. Make sure you have a manager present in the meeting to resolve conflicts.
Discuss all points in the document. Everyone may have a different opinion about each point, but at the end of the discussion, all members must agree upon the standard you are going to follow. Prepare a new standards document with appropriate changes based on the suggestions from all of the team members. Print copies of it and post it in all workstations.
After you start the development, you must schedule code review meetings to ensure that everyone is following the rules. 3 types of code reviews are recommended:
Peer review – another team member review the code to ensure that the code follows the coding standards and meets requirements. This level of review can include some unit testing also. Every file in the project must go through this process.
Architect review – the architect of the team must review the core modules of the project to ensure that they adhere to the design and there is no “big” mistakes that can affect the project in the long run.
Group review – randomly select one or more files and conduct a group review once in a week. Distribute a printed copy of the files to all team members 30 minutes before the meeting. Let them read and come up with points for discussion. In the group review meeting, use a projector to display the file content in the screen. Go through every sections of the code and let every member give their suggestions on how could that piece of code can be written in a better way. (Don’t forget to appreciate the developer for the good work and also make sure he does not get offended by the “group attack”!)
4. Naming Conventions and Standards
Note :
The terms Pascal Casing and Camel Casing are used throughout this document.
Pascal Casing - First character of all words are Upper Case and other characters are lower case.
Example: BackColor
Camel Casing - First character of all words, except the first word are Upper Case and other characters are lower case.
Example: backColor
1. Use Pascal casing for Class names
public class HelloWorld
{
...
}
2. Use Pascal casing for Method names
void SayHello(string name)
{
...
}
3. Use Camel casing for variables and method parameters
int totalCount = 0;
void SayHello(string name)
{
string fullMessage = "Hello " + name;
...
}
4. Use the prefix “I” with Camel Casing for interfaces ( Example: IEntity )
5. Do not use Hungarian notation to name variables.
In earlier days most of the programmers liked it - having the data type as a prefix for the variable name and using m_ as prefix for member variables. Eg:
string m_sName;
int nAge;
However, in .NET coding standards, this is not recommended. Usage of data type and m_ to represent member variables should not be used. All variables should use camel casing.
Some programmers still prefer to use the prefix m_ to represent member variables, since there is no other easy way to identify a member variable.
6. Use Meaningful, descriptive words to name variables. Do not use abbreviations.
Good:
string address
int salary
Not Good:
string nam
string addr
int sal
7. Do not use single character variable names like i, n, s etc. Use names like index, temp
One exception in this case would be variables used for iterations in loops:
for ( int i = 0; i < count; i++ )
{
...
}
If the variable is used only as a counter for iteration and is not used anywhere else in the loop, many people still like to use a single char variable (i) instead of inventing a different suitable name.
8. Do not use underscores (_) for local variable names.
9. All member variables must be prefixed with underscore (_) so that they can be identified from other local variables.
10. Do not use variable names that resemble keywords.
11. Prefix boolean variables, properties and methods with “is” or similar prefixes.
Ex: private bool _isFinished
12. Namespace names should follow the standard pattern
...
13. Use appropriate prefix for the UI elements so that you can identify them from the rest of the variables.
There are 2 different approaches recommended here.
a. Use a common prefix ( ui_ ) for all UI elements. This will help you group all of the UI elements together and easy to access all of them from the intellisense.
b. Use appropriate prefix for each of the ui element. A brief list is given below. Since .NET has given several controls, you may have to arrive at a complete list of standard prefixes for each of the controls (including third party controls) you are using.
Control
Prefix
Label
lbl
TextBox
txt
DataGrid
dtg
Button
btn
ImageButton
imb
Hyperlink
hlk
DropDownList
ddl
ListBox
lst
DataList
dtl
Repeater
rep
Checkbox
chk
CheckBoxList
cbl
RadioButton
rdo
RadioButtonList
rbl
Image
img
Panel
pnl
PlaceHolder
phd
Table
tbl
Validators
val
14. File name should match with class name.
For example, for the class HelloWorld, the file name should be helloworld.cs (or, helloworld.vb)
15. Use Pascal Case for file names.
5. Indentation and Spacing
1. Use TAB for indentation. Do not use SPACES. Define the Tab size as 4.
2. Comments should be in the same level as the code (use the same level of indentation).
Good:
// Format a message and display
string fullMessage = "Hello " + name;
DateTime currentTime = DateTime.Now;
string message = fullMessage + ", the time is : " + currentTime.ToShortTimeString();
MessageBox.Show ( message );
Not Good:
// Format a message and display
string fullMessage = "Hello " + name;
DateTime currentTime = DateTime.Now;
string message = fullMessage + ", the time is : " + currentTime.ToShortTimeString();
MessageBox.Show ( message );
3. Curly braces ( {} ) should be in the same level as the code outside the braces.
4. Use one blank line to separate logical groups of code.
Good:
bool SayHello ( string name )
{
string fullMessage = "Hello " + name;
DateTime currentTime = DateTime.Now;
string message = fullMessage + ", the time is : " + currentTime.ToShortTimeString();
MessageBox.Show ( message );
if ( ... )
{
// Do something
// ...
return false;
}
return true;
}
Not Good:
bool SayHello (string name)
{
string fullMessage = "Hello " + name;
DateTime currentTime = DateTime.Now;
string message = fullMessage + ", the time is : " + currentTime.ToShortTimeString();
MessageBox.Show ( message );
if ( ... )
{
// Do something
// ...
return false;
}
return true;
}
5. There should be one and only one single blank line between each method inside the class.
6. The curly braces should be on a separate line and not in the same line as if, for etc.
Good:
if ( ... )
{
// Do something
}
Not Good:
if ( ... ) {
// Do something
}
7. Use a single space before and after each operator and brackets.
Good:
if ( showResult == true )
{
for ( int i = 0; i < 10; i++ )
{
//
}
}
Not Good:
if(showResult==true)
{
for(int i= 0;i<10;i++)
{
//
}
}
8. Use #region to group related pieces of code together. If you use proper grouping using #region, the page should like this when all definitions are collapsed.
9. Keep private member variables, properties and methods in the top of the file and public members in the bottom.
6. Good Programming practices
1. Avoid writing very long methods. A method should typically have 1~25 lines of code. If a method has more than 25 lines of code, you must consider re factoring into separate methods.
2. Method name should tell what it does. Do not use mis-leading names. If the method name is obvious, there is no need of documentation explaining what the method does.
Good:
void SavePhoneNumber ( string phoneNumber )
{
// Save the phone number.
}
Not Good:
// This method will save the phone number.
void SaveDetails ( string phoneNumber )
{
// Save the phone number.
}
3. A method should do only 'one job'. Do not combine more than one job in a single method, even if those jobs are very small.
Good:
// Save the address.
SaveAddress ( address );
// Send an email to the supervisor to inform that the address is updated.
SendEmail ( address, email );
void SaveAddress ( string address )
{
// Save the address.
// ...
}
void SendEmail ( string address, string email )
{
// Send an email to inform the supervisor that the address is changed.
// ...
}
Not Good:
// Save address and send an email to the supervisor to inform that
// the address is updated.
SaveAddress ( address, email );
void SaveAddress ( string address, string email )
{
// Job 1.
// Save the address.
// ...
// Job 2.
// Send an email to inform the supervisor that the address is changed.
// ...
}
4. Use the c# or VB.NET specific types (aliases), rather than the types defined in System namespace.
int age; (not Int16)
string name; (not String)
object contactInfo; (not Object)
Some developers prefer to use types in Common Type System than language specific aliases.
5. Always watch for unexpected values. For example, if you are using a parameter with 2 possible values, never assume that if one is not matching then the only possibility is the other value.
Good:
If ( memberType == eMemberTypes.Registered )
{
// Registered user… do something…
}
else if ( memberType == eMemberTypes.Guest )
{
// Guest user... do something…
}
else
{
// Un expected user type. Throw an exception
throw new Exception (“Un expected value “ + memberType.ToString() + “’.”)
// If we introduce a new user type in future, we can easily find
// the problem here.
}
Not Good:
If ( memberType == eMemberTypes.Registered )
{
// Registered user… do something…
}
else
{
// Guest user... do something…
// If we introduce another user type in future, this code will
// fail and will not be noticed.
}
6. Do not hardcode numbers. Use constants instead. Declare constant in the top of the file and use it in your code.
However, using constants are also not recommended. You should use the constants in the config file or database so that you can change it later. Declare them as constants only if you are sure this value will never need to be changed.
7. Do not hardcode strings. Use resource files.
8. Convert strings to lowercase or upper case before comparing. This will ensure the string will match even if the string being compared has a different case.
if ( name.ToLower() == “john” )
{
//…
}
9. Use String.Empty instead of “”
Good:
If ( name == String.Empty )
{
// do something
}
Not Good:
If ( name == “” )
{
// do something
}
10. Avoid using member variables. Declare local variables wherever necessary and pass it to other methods instead of sharing a member variable between methods. If you share a member variable between methods, it will be difficult to track which method changed the value and when.
11. Use enum wherever required. Do not use numbers or strings to indicate discrete values.
Good:
enum MailType
{
Html,
PlainText,
Attachment
}
void SendMail (string message, MailType mailType)
{
switch ( mailType )
{
case MailType.Html:
// Do something
break;
case MailType.PlainText:
// Do something
break;
case MailType.Attachment:
// Do something
break;
default:
// Do something
break;
}
}
Not Good:
void SendMail (string message, string mailType)
{
switch ( mailType )
{
case "Html":
// Do something
break;
case "PlainText":
// Do something
break;
case "Attachment":
// Do something
break;
default:
// Do something
break;
}
}
12. Do not make the member variables public or protected. Keep them private and expose public/protected Properties.
13. The event handler should not contain the code to perform the required action. Rather call another method from the event handler.
14. Do not programmatically click a button to execute the same action you have written in the button click event. Rather, call the same method which is called by the button click event handler.
15. Never hardcode a path or drive name in code. Get the application path programmatically and use relative path.
16. Never assume that your code will run from drive "C:". You may never know, some users may run it from network or from a "Z:".
17. In the application start up, do some kind of "self check" and ensure all required files and dependancies are available in the expected locations. Check for database connection in start up, if required. Give a friendly message to the user in case of any problems.
18. If the required configuration file is not found, application should be able to create one with default values.
19. If a wrong value found in the configuration file, application should throw an error or give a message and also should tell the user what are the correct values.
20. Error messages should help the user to solve the problem. Never give error messages like "Error in Application", "There is an error" etc. Instead give specific messages like "Failed to update database. Please make sure the login id and password are correct."
21. When displaying error messages, in addition to telling what is wrong, the message should also tell what should the user do to solve the problem. Instead of message like "Failed to update database.", suggest what should the user do: "Failed to update database. Please make sure the login id and password are correct."
22. Show short and friendly message to the user. But log the actual error with all possible information. This will help a lot in diagnosing problems.
23. Do not have more than one class in a single file.
24. Have your own templates for each of the file types in Visual Studio. You can include your company name, copy right information etc in the template. You can view or edit the Visual Studio file templates in the folder C:\Program Files\Microsoft Visual Studio 8\Common7\IDE\ItemTemplatesCache\CSharp\1033. (This folder has the templates for C#, but you can easily find the corresponding folders or any other language)
25. Avoid having very large files. If a single file has more than 1000 lines of code, it is a good candidate for refactoring. Split them logically into two or more classes.
26. Avoid public methods and properties, unless they really need to be accessed from outside the class. Use “internal” if they are accessed only within the same assembly.
27. Avoid passing too many parameters to a method. If you have more than 4~5 parameters, it is a good candidate to define a class or structure.
28. If you have a method returning a collection, return an empty collection instead of null, if you have no data to return. For example, if you have a method returning an ArrayList, always return a valid ArrayList. If you have no items to return, then return a valid ArrayList with 0 items. This will make it easy for the calling application to just check for the “count” rather than doing an additional check for “null”.
29. Use the AssemblyInfo file to fill information like version number, description, company name, copyright notice etc.
30. Logically organize all your files within appropriate folders. Use 2 level folder hierarchies. You can have up to 10 folders in the root folder and each folder can have up to 5 sub folders. If you have too many folders than cannot be accommodated with the above mentioned 2 level hierarchy, you may need re factoring into multiple assemblies.
16. Make sure you have a good logging class which can be configured to log errors, warning or traces. If you configure to log errors, it should only log errors. But if you configure to log traces, it should record all (errors, warnings and trace). Your log class should be written such a way that in future you can change it easily to log to Windows Event Log, SQL Server, or Email to administrator or to a File etc without any change in any other part of the application. Use the log class extensively throughout the code to record errors, warning and even trace messages that can help you trouble shoot a problem.
17. If you are opening database connections, sockets, file stream etc, always close them in the finally block. This will ensure that even if an exception occurs after opening the connection, it will be safely closed in the finally block.
18. Declare variables as close as possible to where it is first used. Use one variable declaration per line.
19. Use StringBuilder class instead of String when you have to manipulate string objects in a loop. The String object works in weird way in .NET. Each time you append a string, it is actually discarding the old string object and recreating a new object, which is a relatively expensive operations.
Consider the following example:
public string ComposeMessage (string[] lines)
{
string message = String.Empty;
for (int i = 0; i < lines.Length; i++)
{
message += lines [i];
}
return message;
}
In the above example, it may look like we are just appending to the string object ‘message’. But what is happening in reality is, the string object is discarded in each iteration and recreated and appending the line to it.
If your loop has several iterations, then it is a good idea to use StringBuilder class instead of String object.
See the example where the String object is replaced with StringBuilder.
public string ComposeMessage (string[] lines)
{
StringBuilder message = new StringBuilder();
for (int i = 0; i < lines.Length; i++)
{
message.Append( lines[i] );
}
return message.ToString();
}
7. Architecture
1. Always use multi layer (N-Tier) architecture.
2. Never access database from the UI pages. Always have a data layer class which performs all the database related tasks. This will help you support or migrate to another database back end easily.
3. Use try-catch in your data layer to catch all database exceptions. This exception handler should record all exceptions from the database. The details recorded should include the name of the command being executed, stored proc name, parameters, connection string used etc. After recording the exception, it could be re thrown so that another layer in the application can catch it and take appropriate action.
4. Separate your application into multiple assemblies. Group all independent utility classes into a separate class library. All your database related files can be in another class library.
8. ASP.NET
1. Do not use session variables throughout the code. Use session variables only within the classes and expose methods to access the value stored in the session variables. A class can access the session using System.Web.HttpCOntext.Current.Session
2. Do not store large objects in session. Storing large objects in session may consume lot of server memory depending on the number of users.
3. Always use style sheet to control the look and feel of the pages. Never specify font name and font size in any of the pages. Use appropriate style class. This will help you to change the UI of your application easily in future. Also, if you like to support customizing the UI for each customer, it is just a matter of developing another style sheet for them
9. Comments
Good and meaningful comments make code more maintainable. However,
1. Do not write comments for every line of code and every variable declared.
2. Use // or /// for comments. Avoid using /* … */
3. Write comments wherever required. But good readable code will require very less comments. If all variables and method names are meaningful, that would make the code very readable and will not need many comments.
4. Do not write comments if the code is easily understandable without comment. The drawback of having lot of comments is, if you change the code and forget to change the comment, it will lead to more confusion.
5. Fewer lines of comments will make the code more elegant. But if the code is not clean/readable and there are less comments, that is worse.
6. If you have to use some complex or weird logic for any reason, document it very well with sufficient comments.
7. If you initialize a numeric variable to a special number other than 0, -1 etc, document the reason for choosing that value.
8. The bottom line is, write clean, readable code such a way that it doesn't need any comments to understand.
9. Perform spelling check on comments and also make sure proper grammar and punctuation is used.
10. Exception Handling
1. Never do a 'catch exception and do nothing'. If you hide an exception, you will never know if the exception happened or not. Lot of developers uses this handy method to ignore non significant errors. You should always try to avoid exceptions by checking all the error conditions programmatically. In any case, catching an exception and doing nothing is not allowed. In the worst case, you should log the exception and proceed.
2. In case of exceptions, give a friendly message to the user, but log the actual error with all possible details about the error, including the time it occurred, method and class name etc.
3. Always catch only the specific exception, not generic exception.
Good:
void ReadFromFile ( string fileName )
{
try
{
// read from file.
}
catch (FileIOException ex)
{
// log error.
// re-throw exception depending on your case.
throw;
}
}
Not Good:
void ReadFromFile ( string fileName )
{
try
{
// read from file.
}
catch (Exception ex)
{
// Catching general exception is bad... we will never know whether
// it was a file error or some other error.
// Here you are hiding an exception.
// In this case no one will ever know that an exception happened.
return "";
}
}
4. No need to catch the general exception in all your methods. Leave it open and let the application crash. This will help you find most of the errors during development cycle. You can have an application level (thread level) error handler where you can handle all general exceptions. In case of an 'unexpected general error', this error handler should catch the exception and should log the error in addition to giving a friendly message to the user before closing the application, or allowing the user to 'ignore and proceed'.
5. When you re throw an exception, use the throw statement without specifying the original exception. This way, the original call stack is preserved.
Good:
catch
{
// do whatever you want to handle the exception
throw;
}
Not Good:
catch (Exception ex)
{
// do whatever you want to handle the exception
throw ex;
}
6. Do not write try-catch in all your methods. Use it only if there is a possibility that a specific exception may occur and it cannot be prevented by any other means. For example, if you want to insert a record if it does not already exists in database, you should try to select record using the key. Some developers try to insert a record without checking if it already exists. If an exception occurs, they will assume that the record already exists. This is strictly not allowed. You should always explicitly check for errors rather than waiting for exceptions to occur. On the other hand, you should always use exception handlers while you communicate with external systems like network, hardware devices etc. Such systems are subject to failure anytime and error checking is not usually reliable. In those cases, you should use exception handlers and try to recover from error.
7. Do not write very large try-catch blocks. If required, write separate try-catch for each task you perform and enclose only the specific piece of code inside the try-catch. This will help you find which piece of code generated the exception and you can give specific error message to the user.
8. Write your own custom exception classes if required in your application. Do not derive your custom exceptions from the base class SystemException. Instead, inherit from ApplicationException.
C# Coding Standards and Best Programming Practices
Monday, May 3, 2010
Web Services
Webservices are services exposed over the internet. Typically, webservice is just like any other class library, written in any language. What make it a 'web service' is, it can be accessed across internet.
Eventhough webservice is a new technology with a wide range of usage, it is a pretty simple concept. It doesn't require much additional knowledge to create web services if you are already familiar with C# or VB.NET. (Web services are not specific to .NET. Even Java has web service applications, but here we are discussing only the .NET web services.)
As we mentioned, web services are exposed over internet. To make this happen, it has to be hosted with a web site. Othen than hosted as part of a web site to make this visible across internet, web services have no web pages or UI. It is just a set of classes with public/private methods and properties.
Web services are a group of [web] methods. Consumer applications can treat this as just another class library. Regular class libraries are located along with the same application and we can call any methods in the class libraries (assemblies). But in case of web services, the assembly is located in the internet server. The consumer application will not have direct access to the assembly. Consumer applications have to add a reference to the webservice URL and then call methods in it. The method call goes across the internet using SOAP protocal and results are returned across internet in the form of XML. The communication across internet happens transparently and the application need not know anything about this communication.
Creating a simple web service
If you are using VS.NET, it won't take more than 2-3 minutes to create your first webservice. Follow the steps below to create a web service:
//[WebMethod]
//public string HelloWorld()
//{
//return "Hello World";
//}Note that this method is pretty much same as any other regular method.The only difference is, it has an attribute [WebMethod]. This attribute make
the method visible across internet. If you remove this attribute, your
application will still compile, but the consumer applications cannot call this
method across internet.
Ok, thats all you have to do to create a simple webservice. Build your solution and your web service is ready to be called by other applications.
Create a consumer application
A wide range of applications can consume the web services, including web page, other web services, windows applications etc. let us create a sample windows application and call our web service.
Type the URL of your web service in the space provided for 'URL'. If you have created the web service in the local machine's default web site, the URL might be like this :
http://localhost/MyWebService/Service1.asmxIf you have given a different name fro the project or if you have changed the default service name, then your URL may differ. Type the URL and press enter. It will attempt to connect with the web service and if succeeded it will retrieve the public web methods and display. Once you get this success screen, press the 'Add Reference' button on the screen to add the web reference to your application. There is a space to specify the 'web reference name', which will have the default value 'localhost'. Leave it as it is.
Now you are ready to make calls to the webservice. Just use the following code in your application:
localhost.Service1 service = new localhost.Service1(); string result = service.HelloWorld(); MessageBox.Show( result );Let us analyze our code. The first line creates an instance of the Service1 class, just like any regular class. Only thing is it uses the namespace 'localhost'. This is same as the 'web reference name' we specified while adding the web reference. We can use any web reference name when we add the reference and have to use the same name as namespace while making calls to the web service class.
In the second line, we are actually calling the web method 'HelloWorld()' and it returns the result as a string. Note this is just like any other method call. So, your application makes the call to web service as if it is calling any local class library calls!! The communication with web service, wrapping your calls in SOAP/XML, retrieve results from web service, unwrap the SOAP/XML to get the actual result etc are done behind the scene by the framework and you need not do any additional work to make all this work.
Now you can go back to your web service, add more methods with different parameters and play around with that. You will be able to call methods in the web service just like you use any local class. Note that when you make any changes to the method name or parameters, or if you add new methods, you will need to refresh your web reference to get access to the updated web service methods. To do this, go to the Server Explorer, right click on the web reference name 'localhost' under the 'Web References'. Then choose 'Refresh'. This will communicate with the web service again and retrieve the latest web service data.
How does all this work?
To make calls to any classes, your application need to know the details of the class. To successfully compile your code, it has to have information about the class, what are the public methods and properties etc. But in your sample code which calls the web service, the actual Service1class resides in the web (in your case, in your local web server). So how does the application compile without having the 'Service1' class in your application?
This is where the framework help you. When you add a 'web reference' to the web service URL, VS.NET actually creates local proxy in your application. A local proxy is a minimal version of the actual class. The proxy has just the sufficient information about the 'real class', which is required for your application to successfully compile. The proxy class has the list of all public web methods in the real web service class and it knows the parameters and their data types of each method.
The most important thing is, your application is actually using this proxy class and not the real web service class.
localhost.Service1 service = new localhost.Service1(); string result = service.HelloWorld(); MessageBox.Show( result );In the above code, localhost.Service1is actually the proxy class (a representative of a real class), which is created automatically by VS.NET when you added the web reference. You are creating an instance of the proxy class and calling the 'HelloWorld()' method of this proxy. Since the proxy class knows that it is 'only a proxy of the real class', it redirects all calls to the 'real web service class'. The method 'HelloWorld()' in the proxy class takes care of converting the call to a web request using SOAP/XML. After retrieving the result of the web request to the real web service class, this proxy class takes the responsibility of parsing the SOAP/XML response and returning only the result string to the calling applicationn. So the proxy class does all the dirty job for you.
The proxy class has no implementation of the functionality. It just redirects the call to the web service. So, if you make any changes to the implementation in the web service, still you will get the 'updated result' when you make calls to web service. But, if you change the method name, or change the parameter types in the web service and do not 'refresh the web reference', then your application will compile with the old proxy. But when the proxy redirect the call to the web service, it will find that the parameters or method names do not match and it will fail. So, if there is any change in the method names or parameters, you must refresh your web reference.
We will explore more features of web services in another chapter. Please complete the following feedback, if this article is incomplete or misleading. Please let us know if there is anyway we can improve this article.
You can download web service samples from our projects section.
Introduction to XML
XML stands for eXtended Markup Language. XML has some similarities to HTML, which is also another Markup Language (HyperText Markup Language).
An important difference between XML and HTML is, XML is used to define data, while HTML is basically used to display/format text. Main use of HTML is to make web pages. But XML is used in a wide variety of applications now and is becoming a standard for data exchange between different paltforms.
Unlike HTML, XML has no pre-defined set of tags. You can define your own tags to structure and organize your data. Let us see some sample XML :
The above piece of XML is quite self explanatory. It is used to represent the data related to a company. One thing to note here is, the tags like
Need for XML
XML is a platform neutral standard. Since it is represented in plain text, any platform can understand it. You can use XML to exchange data between Unix and Windows, Mainframes and windows applications and virtually any other platforms. It is a simple, easy to use *language* to exchange/store data.
In real world, if you have to store data, you have several choices. One of the most common mechanism is databases. Databases are the best choice if you have lot of data to be stored/manipulated. But if you have to send data to another application or platform, databases will not be much helpful. Suppose you have your data stored in an SQL Server database. What if your manager ask you to give the list of Employees in the Sales department ? How will you give the data to him? You cannot give your SQL Server database to him. He may not have a SQL Server software or he may not have the technical expertise to view data from SQL Server. XML is very helpful in such scenarios. You just need to generate an XML from the SQL Server database with the required records and hand over the XML as a print out or email. XML is so simple so that any one can simply read it using any text editor.
There are several XML viewers and editors available, which will help you read XML in the form of a tree view. Internet Explorer is a good XML viewer. Simply copy the above XML and save into a file called 'sample.xml'. Then open the xml file in internet explorer and see how it works!
You can define your own XML tags based on your needs. You can have nested tags too, which help you organize the data any way you want it. See a slightly different version of the above XML:
The XML data altogether is called an 'XML Document'. .NET gives several classes to read and manipulate XML Documents. We will explore some of them in upcoming chapters.
You might have seen our Tutorial Index, which lists all chapters. You can click on any chapter title and navigate to the corresponding page. This index is populated from an XML file.
Click here to view the XML file, from which we load our tutorial chapters.
The reason we load it from an XML file is, our chapters are continuously changing and we add new chapters frequently. All our chapters have a link to Next Chapter. If we hard-code these links in all chapters, we have to make changes in several places when we change the order of some chapters or when we add new chapters. There is all possibility that we might make some mistakes and some of the chapters may lead to 'wrong next chapter'. In our current implementation, we programmatically read the XML file and display the Tutorial Index. Also, each page has a piece of code which read the current page URL and find the Next Page Url from the XML file based on the current page URL. This mechanism helps us add new chapters without making any changes to existing pages. All we have to do is, just insert one line of entry in the XML file and the changes will be automatically reflected in all other relevant pages. Even if we want to re-order some of the chapters, we just need to re-arrange the entries in the XML file.
We use the following code to load our Chapter Index from the XML file:
public string GetChapters( string filePath )
{
// Create an xml document.
XmlDocument doc = new XmlDocument();
// Load the XML from the file.
doc.Load(filePath);
// Retrieve all categories from the xml.
XmlNodeList categories = doc.SelectNodes("DotNetSpider/tutorials/Category");
string chapterString = "";
int chapter = 0;
// Iterate through all the categories
foreach ( XmlNode categoryNode in categories )
{
// Add category name to the string.
chapterString += "
" + categoryNode.Attributes["Name"].Value + "
";
// Get all chapters in the current category.
XmlNodeList chapters = categoryNode.SelectNodes("Chapter");
// Loop through all chapters in the current category and add to the string.
foreach ( XmlNode chapterNode in chapters )
{
++chapter;
chapterString += "
XML Specification
We gave a very brief introduction to XML in this chapter. This chapter is meant to give a very brief introduction and we haven't covered many important aspects of XML here. We will explain most of them in some other chapters.
XML has become a standard now and you can read the rules and specification for this standard here : http://www.w3.org/TR/2004/REC-xml-20040204
Custom Exceptions
The Microsoft .NET team has done a good job by providing us a rich set of Exception classes. Most of the time, these exception classes are sufficient enough to handle any common error situation. However, it may be required that our application would need some additional exception classes, that are not available in .NET Framework.
The .NET Framework exception handling mechanism allows us to create our own custom exception classes and use it in our applications.
Custom Exceptions
All exceptions in .NET are derived from the root exception class - System.Exception. There are two other
exception classes immediately derived from the System.Exception :
All exceptions thrown by the .NET Framework is derived from the SystemException class. If we raise any exception from our application, it should be derived from the System.ApplicationException. This will differentiate our exceptions from the system generated exceptions.
It is easy to create a custom exception class. Just create a new class and mark it as derived from System.ApplicationException
public class UserNotExistsException : System.ApplicationException
{
} Now we have derived a custom exception class and we can use it in ourr application. See the sample code, showing how we
can throw ourr custom exception.
public void UpdateUser( User userObj )
{
if ( some error condition )
{
throw new UserNotExistsException();
}
}The 'throw' statement allows us to raise an exception programmatically. Here we are creating a new object of type 'UserNotExistsException' and throwing it. Another class or method which calls our method should handle this exception.
try
{
UpdateUser( userObj );
}
catch ( UserNotExistsException ex )
{
MessageBox.Show ( ex.Message );
}
In this sample, we are calling the method UpdateUser(...), which might throw an exception of type UserNotExistsException. If this exception is raised, it will go to our catch block and we are showing a message to the user.
Enhancing our custom exception
In the custom exception we defined above, we haven't specified any constructor. But since we derive it from the ApplicationException, it will use the public constructors available in AppicationException. It is a good idea to provide at least one constructor, which takes an error message as a parameter.
public class UserNotExistsException : System.ApplicationException
{
public UserNotExistsException ( string message ) : base ( message )
{
}
} Now, when we throw this exception, we have to pass an error message to the constructor. The error handlers will be able to retrieve this message from the Exception object and show a better message to the user.
public void UpdateUser( User userObj )
{
if ( some error condition )
{
throw new UserNotExistsException( "The user you are trying to update does not exists." );
}
}See the exception handler code :
try
{
UpdateUser( userObj );
}
catch ( UserNotExistsException ex )
{
MessageBox.Show ( ex.Message );
}Now the user will see the more friendly error message which we passed as part of the exception, while throwing it.
You can customize the custom exception classes such a way that it passes any information you want including the date and time the exception occurred, the method name in which the exception was first raised, user name of the current user etc. You may also provide a Save() method for your custom exception class so that when a caller catch our custom exception, he can call the Save() method, which will record the error information to the disk.
Exception classes in .NET
NET Framework provides several classes to work with exceptions. When there is an exception, the .NET framework creates an object of type 'Exception' and 'throws' it. This Exception object contains all information about the 'error'.
If you enclose your code within the try-catch block, you will receive the exception object in the 'catch' block when the exception occurs. You can use this object to retrieve the information regarding the error and take appropriate action.
try
{
// Code which can cause an exception.
}
catch(Exception ex)
{
// Code to handle exception
MessageBox.Show ( ex.Message );
}
Within the catch block, you can use the Exception object to get more information about the error. The exception object exposes a property called 'Message', which gives a description about the error. This may not be a very friendly message for the end user. But you can use it to log the error and show another friendly message to the user.
The System.Exception class
In .NET, all exceptions are derived from the Exception class. The Exception class is defined inside the System namespace.Other derived exception classes are spread across many other namespaces.
Since all other exceptions are derived from the System.Exception class, if you catch System.Exception, that would cover all exceptions derived from System.Exception also. So, the statement catch (Exception) would catch all exceptions of type System.Exception and all derived exceptions. In .NET, all exceptions are derived from System.Exception. So,catch (Exception) will catch all posibble exceptions in your .NET application.
Specify what exception type you want to catch
See the catch block in the above sample code. catch ( Exception ex ) - specifies that we want to catch all exceptions of Type Exception. So, if there is an exception of Type Exception, it will be caught by the catch block.
If you specify an exception Type in the catch statement, it will catch all exceptions of the specified type and all types derived from it.
See the following sample :
try
{
// Code which can cause an exception.
}
catch(System.Web.HttpException ex)
{
// Code to handle exception
MessageBox.Show ( ex.Message );
}
The above example will only catch the exception of type System.Web.HttpException and any other exception derived from it. So, if there is any exception of typeSystem.ArithmeticException, it will not be handled and it may lead to program termination.
Handling specific Exception types
In my previous article, I mentioned few examples from real life - "When you ride a bike, you may wear a helmet. When you go for boating, you might use a life jacket. A car driver might use seat belts while driving at high speed in a high way. What is the purpose ? To handle exceptions (accidents), right ? We do not know when it might happen, so we are prepared to 'handle' such situations any time. "
Yes, a helmet helps a bike rider when he meet with an accident. A life jacket may be helpful in water and a seat belt would help a car driver. But what if some one ride a bike with a helpmet, life jacket and a seat belt ? That may not look good, right ?
Similarly, in Exception handling, you do not need to catch all exceptions. You need to catch only the 'expected' exceptions. Which means,if you are doing an arithmetic calculation, you must handle ArithmeticException and DivideByZeroException. When you are accessing the web from your code, you must handle HttpException.
The bottom line is, depending on the nature of the code, you must handle the appropriate, specific exceptions, instead of catching the father of all exceptions (System.Exception).
Multiple catch blocks
You can have any number of catch blocks for each try block. For example, if you are doing some operation which involve web access and also some arithmetic operations, you can handle both System.Web.HttpException and System.ArithmeticException. See the sample below:
try
{
// Code which can cause a web exception or arithmetic exception.
}
catch(System.Web.HttpException ex)
{
MessageBox.Show ( "A web exception occurred." );
}
catch(System.ArithmeticException ex)
{
MessageBox.Show ( "An arithmetic exception occurred." );
}
Program Flow
When an exception occurs within a try block, the program control will jump to the first catch block and compare if the exception type is same the as the type specified in the catch block. If the type matches, it will execute the catch block. If the types do not match, it will jump to the next catch block and compare. Like this, it will compare against all catch blocks until a match is found. If there is no catch block found which matches the exception type, it will become an unhandled exception and will lead to program termination
Catch derived exceptions first and base exception last
A base exception type will match the derived exceptions also. If there is a catch block with the type 'Exception', it will catch all types of exceptions. So, you have multiple catch blocks, you must specify the derived exception types first and the base types last. See the following code:
try
{
// Code which can cause a web exception or arithmetic exception.
}
catch (System.Exception ex)
{
MessageBox.Show ( "An exception occurred." );
}
catch (System.Web.HttpException ex)
{
MessageBox.Show ( "A web exception occurred." );
}
In the above code, assume there is a web exception occurred. The exception type will be compared against the first catch block. Since the WebException is derived from System.Exception or oneof it's derived classes, the types will match and the first catch block will be executed. So, you might be expecting that the catch (System.Web.HttpException)block will be executed, but it would never get called because of the catch(System.Exception ex) before that.
You must change the above code as shown below :
try
{
// Code which can cause a web exception or arithmetic exception.
}
catch (System.Web.HttpException ex)
{
MessageBox.Show ( "A web exception occurred." );
}
catch(System.Exception ex)
{
MessageBox.Show ( "An exception occurred." );
}
Now, if there is a web exception, it will go to the catch (System.Web.HttpException ex) block. All other exceptions will go to catch (System.Exception ex).
Some commonly used .NET Exception classes
System.ArithmeticException - This is the base class for exceptions that occur during arithmetic operations, such as System.DivideByZeroException and System.OverflowException.
System.ArrayTypeMismatchException - ArrayTypeMismatchException is thrown when a an incompatible object is attpemted to store into an Array.
System.DivideByZeroException - This exception is thrown when an attempt to divide a number by zero.
System.IndexOutOfRangeException - IndexOutOfRangeException is thrown when attempted to access an array using an index that is less than zero or outside the bounds of the array.
System.InvalidCastException - Thrown when an explicit type conversion from a base type or interface to a derived type fails at run time.
System.NullReferenceException - This exception is thrown when an object is accessed but it is null.
System.OutOfMemoryException - OutOfMemoryException is thrown if the 'new' operation (creating new object) fails due to in sufficient memory.
System.OverflowException - OverflowException is thrown when an arithmetic operation overflows.
System.StackOverflowException - StackOverflowException is thrown when the execution stack is exhausted by having too many pending method calls, most probably due to infinite loop.
ArgumentException - The exception that is thrown when one of the arguments provided to a method is not valid.
Exception Handling in .NET
If you write a program, it is for sure that it will have errors and issues. You cannot avoid them. But what you can do is write the code such a way that it is easy to find the errors and issues so that you can solve them easily. You need the real skill to write 'maintainable code'.
What is maintainable code ?
If an existing application is given to a new employee in the company and ask him to fix a problem in that,most probably he will come back with the answer : "oh, it is stupid code.. I can't figure out what they have written. Instead of trouble shooting the issues in this code, I can re write the entire application with less time."
This is a common scenario in the programming world. The above answer shows the code is not a maintainable code. Over a period of time, there will be lot of changes required in existing applications to adapt to the new requirements. Only if you write 'maintainable code', you can enhance your application when new requirements come up and solve issues.
There is no specific set of rules to make an application 'maintainable'. There are lot of factors involved in it including following coding standards, writing lot of comments within code, writing self explanatory code, separating application into multiple well defined layers, having good exception handling etc.
What is an 'Exception' ?
'Exception' is an error situation that a program may encounter during runtime. For example, your program may be trying to write into a file, but your hard disk may be full. Or, the program may be trying to update a record in database, but that record may be already deleted. Such errors may happen any time and unless you handle such situation properly, your application may have un predictable behaviour.
What is the difference between 'Exception' and 'Error' ?
Error is an expected situation in an application. For example, you want to create a file in the folder "C:\Projects\Test\". If you attempt to create a file when this folder doesn't exists, it will make the operating system throw an exception. But you should not leave it to the operating system to throw the exception. This has to be handled through code. You must first check for the existence of the folder before you attempt to write the file. If the folder doesn't exists, you must first create the folder. This is how a stable application should be written.
Exception is a runtime error that the program may encounter during the execution of the program. For example, hard disk may be full when you attempt to write into a file, network connection may be lost while your application is communicating with another computer etc. There is no clear definition, but typically, Exceptions are unpredictable errors during runtime.
What is 'Exception Handling'?
When you ride a bike, you may wear a helmet. When you go for boating, you might use a life jacket. A car driver might use seat belts while driving at high speed in a high way. What is the purpose ? To handle exceptions (accidents), right ? We do not know when it might happen, so we are prepared to 'handle' such situations any time.
An exception can occur anytime during the execution of an application. Your application must be prepared to face such situations. An application will crash if an exception occurs and it is not handled.
"An exception handler is a piece of code which will be called when an exception occurs."
.NET Framework provides several classes to work with exceptions. The keywords try, catch are used to handle exceptions in .NET. You have to put the code (that can cause an exception) in a try block. If an exception occurs at any line of code inside a try block, the control of execution will be transfered to the code inside the catch block.
Syntax :
try
{
// Code which can cause an exception.
}
catch(Exception ex)
{
// Code to handle exception
}
Just like a bike rider wrap his head in a helmet to protect from accident, the above syntax will protect the code within the try block from accidents (exceptions).
If any statement within the try block raises an exception, the control of execution will be transfered to the first line within the catch block. You can write the error handling code in the catch block, like recording the error message into a log file, sending an email to the administrator about the problem occurred, showing an appropriate error message to the user etc.
See the following code :
try
{
Statement 1
Statement 2
Statement 3
}
catch(Exception ex)
{
Statement 4
Statement 5
}
Statement 6
In normal flow of program, the statements 1, 2, 3, 6 will be executed. Statements 4 and 5 will be executed only if an exception occurs.
Assume there is an exception at statement 2. In this scenario, statements 1, 2, 4, 5 and 6 will be executed. Statement 3 will not be executed at all, because an exception occurred at statement 2 and program flow was transferred to the catch block.
'finally' block
You can optionally use a 'finally' block along with the try-catch. The 'finally' block is guaranteed to be executed even if there is an exception.
Sample Code :
try
{
Statement 1
Statement 2
Statement 3
}
catch(Exception ex)
{
Statement 4
Statement 5
}
finally
{
Statement 6
Statement 7
}
Statement 8
In normal flow of program, the statements 1, 2, 3, 6, 7 and 8 will be executed. Statements 4 and 5 will be executed only if an exception occurs.
Assume there is an exception at statement 2. In this scenario, statements 1, 2, 4, 5, 6, 7 and 8 will be executed.
Note that the statements 6 and 7 are executed in both the cases. The bottom line is, the code within the 'finally' block is executed whether there is an exception or not.
You can use the finally block to do any code cleanup. For example, you are doing some database operations inside a try block.
try
{
Statement 1 - Open database
Statement 2 - Execute Query
Statement 3 - Close Database
}
catch(Exception ex)
{
Statement 4 - Show messagebox to user
}
In the above code sample, what will happen if an exception occurs when the statement 2 is executed ? The catch block will be executed and a message box will be shown to the user. But the Statement 3 is never executed in that case, which means the database will not be closed.
Here is where the finally block will be helpful.
try
{
Statement 1 - Open database
Statement 2 - Execute Query
}
catch(Exception ex)
{
Statement 3 - Show messagebox to user
}
finally
{
Statement 4 - Close Database
}
See the improved code. We have moved the code to close the database to the 'finally' block. The statement to close the database will be executed whether there is an exception or not.
Why should we catch exceptions ?
Why should you use a helmet when you ride a bike ? To protect your head from crashing when there is an accident, right ? Just like that, exception handling will protect your application from crashing when there is an exceptions.
If an exception is not 'handled' in code, the application will crash and user will see an ugly message. Instead, you can catch the exception, log the errors and show a friendly message to the user.
DataSet, DataTable, DataRow
DataSet and DataTable are the key components in ADO.NET programming. In simple words, DataSet represents an in memory representation of the database. We can load an entire database into a DataSet and manipulate the data in memory. If you aremore familiar with DataSet, you can Add, Edit and Update data in the dataset and then just call a single method 'AcceptChanges()' whichwill save all the changes back to the database.
What is DataSet ?
A DataSet is an in memory representation of data loaded from any data source. Even though the most common data sourceis database, we can use DataSet to load data from other data sources including XML files etc. In this article, we will talk about the role of DataSet in manipulating data from database.
In .NET, a DataSet is a class provided by the .NET Framework. The DataSet class exposes several proeprties and methods that can be used to retrieve, manipulate and save data from various data sources.
Just like any other classes in object oriented programming, we have to create an instance of DataSet class to work with data. Typically, we may create a new instance of a DataSet and use other classes provided by .NET Framework to populate the DataSet. See the following example:
string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\\Samples\\Employee.mdb";
OleDbConnection myConnection = new OleDbConnection( connectionString );
string query = "select * from EMPLOYEE_TABLE";
OleDbDataAdapter myAdapter = new OleDbDataAdapter( query, myConnection );
DataSet employeeData = new DataSet();
myAdapter.Fill ( employeeData );
Here we are creating a OleDbConnection object and we are just passing the object to the OleDbDataAdapterobject. Also, we pass the 'select ...' query to the OleDbDataAdapter. Next, we call the '.Fill()' method of the OleDbDataAdapter. This step will populate the dataset ( called 'employeeData' ) with the data retrieved for the sql statement 'select * from EMPLOYEE'.
As you already know, a DataSet can contain a collection of tables. But in the above case, our sql statement will retrieve data from only one table. So, our DataSet will have only one table.
Commonly used properties and methods of DataSet
Property : Tables
The Tables propertly allows us to retrieve the tables contained in the DataSet. This property returns a DataTableCollection object. The following sample code demonstrates iterating through the collection of tables in a data set and print the name of all the tables.
DataSet employeeData = new DataSet();
myAdapter.Fill( employeeData );
// Repeat for each table in the DataSet collection.
foreach ( DataTable table in employeeData.Tables )
{
MessageBox.Show ( table.TableName );
}
Or, you can use the indexer to access any specific table in the collection.
DataSet employeeData = new DataSet();
myAdapter.Fill( employeeData );
// Repeat for each table in the DataSet collection.
for ( int i = 0; i < employeeData.Tables.Count; i++ )
{
DataTable table = employeeData.Tables[i];
MessageBox.Show ( table.TableName );
}
Method : GetXml()
The GetXml() method returns the XML representation of the data from the DataSet.
DataSet employeeData = new DataSet();
myAdapter.Fill( employeeData );
string xmlData = employeeData.GetXml();
Method : WriteXml(...)
The WriteXml() method allows to save XML representation of the data from the DataSet to an XML file. There are many overloaded method available, which takes various parameters. The example shown below takes a file name as parameter and saves the data in DataSet into xml format to the file name specified as parameter. We can optionally save only the data or both data and schema.
DataSet employeeData = new DataSet();
myAdapter.Fill( employeeData );
employeeData.WriteXml( "c:\\MyData.xml" );
Method : ReadXml(...)
The ReadXml() method allows to load the DataSet from an XML representation of the data. There are many overloaded method available, which takes various parameters. The example shown below takes a file name as parameter and loads the data from XML file into the DataSet. This method can be used to load either the data only or both data and schema from the XML.
DataSet employeeData = new DataSet();
employeeData.ReadXml( "c:\\MyData.xml" );
There is another method called 'ReadXmlSchema()', which can be used to load only the schema from a file.
The methods WriteXml() and ReadXml() are useful to save the data from a database into some temporary files, transport to other places or keep it as a local file and load later. Many applications, including the SpiderAlerts tool available for download from this site, uses DataSet to manipulate data and saves/retrieves them from local disk using the WriteXml() and ReadXml() methods.
The SpiderAlerts tool communicates with webservices in our site and retrieves the alerts in the form of a DataSet. Once the Alerts are retrieved, it is saved into local computer using the WriteXml method. (This implementation may be changed soon in the future versions of this tool. We are considering saving(serializing) the DataSet into Isolated Storage (IsolatedStorage is a new feature part of the .NET Framework - it is a kind of hidden file system)
DataSet and DataTable are the key components in ADO.NET programming. While DataSet can be used to represent a database as a whole, a DataTable object can be used to represent a table in the Database/DataSet. A DataSet can contain several DataTables.
In typical database oriented applications, DataSet and DataTable are used a lot to manipulate data. DataAdapter or other classes can be used to populate a DataSet. Once a DataSet is populated, we can access the DataTables contained within the DataSet.
Just like any database table contains multiple rows (records), a DataTable can contain multiple DataRows. Each row contains multiple fields representing each column in the table.
The typical process to retrieve records from a database in ADO.NET includes the following steps:
The following sample code explains these steps. This sample code retrieves data from an MS Access database.
string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\\Samples\\Employee.mdb";
OleDbConnection myConnection = new OleDbConnection( connectionString );
string query = "select * from EMPLOYEE_TABLE";
OleDbDataAdapter myAdapter = new OleDbDataAdapter( query, myConnection );
DataSet employeeData = new DataSet();
myAdapter.Fill( employeeData );
// Repeat for each table in the DataSet collection.
foreach ( DataTable table in employeeData.Tables )
{
// Repeat for each row in the table.
foreach ( DataRow row in table.Rows )
{
MessageBox.Show( "Employee Number : " + row["EmployeeNumber"].ToString() );
MessageBox.Show( "Name : " + row["Name"].ToString() );
MessageBox.Show( "Address : " + row["Address"].ToString() );
}
}
How to create a DataTable
In most of the cases, we just access the DataTable in a DataSet. We do not need to create a new instance of the DataTable. When a DataSet is populated from database, the DataTable is created with proper schema and data.
If we explicitely create DataTable, we have to create the proper schema. It is bit confusing if you are not very familiar with the database structure and schema.
