Exception Management

Introduction

Exception management is one of the key area for all kinds of application development .You should adopt an appropriate strategy for exception management to build high quality and robust application .It is a very powerful concept and makes the development work very easy if its used efficiently. Inappropriate way of handling exception can degrade the performance. Before dig into this its very important to know what is an exception? The general meaning of "an exception is the breaching of predefined assumption of the application". Remember exception and error are not the same. To explain this let me explain couple of examples.
Example1Lets say you are try to log data to a file and your application assumes the file is present in the target path , but if is not then exception would be raised .On the other hand if your job is to trace the file and if it is present then log the data in this scenario raising an exception is a bad coding practice. It should be handled through validation code.
Example 2
Lets say in a normal ASP.NET application you are trying to update all necessary fields to the database , and your application assume the database connection is available suppose the connection is not available then raising an exception is a ideal solution. On the other hand while update the mandatory fields in database and few of them having the null values then raising an exception is not necessary it should be handled through validation code.

How to Handle
As a developer you must take the advantage of structured exception handling mechanism through try, catch and finally block .The .NET framework provides a broad hierarchy of exception classes to handle the different types of exception and all these classes derived from Exception class (base class). The developer can extend exception mechanism through inheritance , it helps to implement the custom error handling or provide a proper gateway to handle the complex error handling for the application. Unfortunately, many developers misuse this architecture capability .One very important thing you should keep in mind is how to react when a exception occur at runtime. The good approach to react the exception is

• Just ignore the exception or implement different possible catch blocks to catch the exception.
  
• Catch the exception and perform required action for your application and if you can not recover from the exception rethrow the exception.
  
• Catch the exception and wrap with another exception which is more relevant for your application. Exception wrapping used to avoid breaking the layer abstraction due to exception. For preserving the original exception you can use the InnerException property of the exception class this allows the original exception to be wrapped inside a new exception ( which is more relevant for your application) to understand the wrapping of exception lets look at this example inside a method of your application caught a lower level exception called IOException , you can wrap this original exception with a application level exception called LoadingException or FailtoLoadInfo exception or something else which is more relevant for your application rather then alerting the lower level exception to the user .

The exception management architecture of a application capable to

• Detect Exception
• Perform code clean up
• Wrap one exception inside another
• Replace one exception with another
• Logging and reporting error information
• Generating events that can be monitored externally to assist system operation

At the beginning of the design you must plan for a consistency and robust exception management architecture and it should be well encapsulated and abstract the details of logging and reporting throughout all architecture layer of your application .Lets discuss some of the best practices of exception
Best Practices
The following list contains tips/suggestions to be consider while handling the exceptions:

• Exception is a expensive process , for this reason, you should use exceptions only in exceptional situations and not to control regular logic flow. For example
  void EmpExits ( string EmpId)
  {//... search for employeeif ( dr.Read(EmpId) ==0 ) // no record found, ask to create{throw( new Exception("Emp Not found"));
  }
  } 
  The best practice is :
  
  bool EmpExits ( string EmpId)
  {//... search for Productif ( dr.Read(EmpId) ==0 ) // no record found, ask to create{return false}
  }
  
• Avoid exception handling inside loops, if its really necessary implement try/catch block surround the loop
  
• Adopt the standard way of handling the exception through try, catch and finally block .This is the recommended approach to handle exceptional error conditions in managed code, finally blocks ensure that resources are closed even in the event of exceptions. For example
  
  SqlConnection conn = new SqlConnection("...");try{
  conn.Open();//.some operation 
  // ... some additional operations}catch(…)
  {// handle the exception}finally{if (conn.State==ConnectionState.Open)
  conn.Close(); // closing the connection}
  
• Where ever possible use validation code to avoid unnecessary exceptions .If you know that a specific avoidable condition can happen, precisely write code to avoid it. For example, before performing any operations checking for null before an object/variable can significantly increase performance by avoiding exceptions. For example
  
  double result = 0;try{
  result = firstVal/secondVal;
  }catch( System.Exception e)
  {//handling the zero divided exception }
  This is better then the above code
  
  double result = 0;if(secondVal >0)
  result = firstVal/secondVal;elseresult = System.Double.NaN;
  
• Do not rethrow exception for unnecessary reason because cost of using throw to rethrow an existing exception is approximately the same as creating a new exception and rethrow exception also makes very difficult to debug the code. For example
  
  try{// Perform some operations ,in case of throw an exception…}catch (Exception e)
  {// Try to handle the exception with ethrow;
  }
  
• The recommended way to handle different error in different way by implement series of catch statements this is nothing but ordering your exception from more specific to more generic for example to handle file related exception its better to catch FileNotFoundException, DirectoryNotFoundException, SecurityException,IOException, UnauthorizedAccessException and at last Exception .
  
• ADO.NET errors should be capture through SqlException or OleDbException
  
  
  • Use the ConnectionState property for checking the connection availability instead of implementing an exception 
     
  • Use Try/Finally more often, finally provides option to close the connection or the using statement provides the same functionality.
     
  • Use the specific handler to capture specific exception, in few scenarios if you know that there is possibility for a specific error like database related error it can be catch through SqlException or OleDbException as below
    
    try{ ...
    }catch (SqlException sqlexp) // specific exception handler{ ...
    }catch (Exception ex) // Generic exception handler{ ...
    }
    
• Your exception management system capable to detect, wrap one exception inside another, Replace one exception with another, logging and reporting the exception information for monitoring the application.
• Its recommend to use "Exception Management Application Block" provided by Microsoft .It is a simple and extensible framework for logging exception information to the event log or you can customize to write the exception information to other data sources without affecting your application code and implemented all best practices and tested in Microsoft Lab.

For more information

1. http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnbda/html/exceptdotnet.asp
   
2. http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpconbestpracticesforhandlingexceptions.asp