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Exception Handling in C#

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An exception is a problem that arises during the execution of a program. A C# exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero.

Exceptions provide a way to transfer control from one part of a program to another. C# exception handling is built upon four keywords: try, catch, finally, and throw.

  • try − A try block identifies a block of code for which exceptions is activated. It is followed by one or more catch blocks.
  • catch − A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The catch keyword indicates the catching of an exception.
  • finally − The finally block is used to execute a given set of statements, whether an exception is thrown or not thrown. For example, if you open a file, it must be closed whether an exception is raised or not.
  • throw − A program throws an exception when a problem shows up. This is done using a throw keyword.

Syntax

Assuming a block raises an exception, a method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following −

try {
   // statements causing exception
} catch( ExceptionName e1 ) {
   // error handling code
} catch( ExceptionName e2 ) {
   // error handling code
} catch( ExceptionName eN ) {
   // error handling code
} finally {
   // statements to be executed
}

You can list down multiple catch statements to catch different type of exceptions in case your try block raises more than one exception in different situations.

Here's the basic list of .NET exception types, along with their base type that they inherit from:

EXCEPTION TYPE

BASE TYPE

Exception

Object

SystemException

Exception

IndexOutOfRangeException

SystemException

NullReferenceException

SystemException

AccessViolationException

SystemException

InvalidOperationException

SystemException

ArgumentException

SystemException

ArgumentNullException

ArgumentException

ArgumentOutOfRangeException

ArgumentException

ExternalException

SystemException

COMException

ExternalException

SEHException

ExternalException

Below we'll examine each of these core exception types provided by .NET, looking at why and when they might be raised during normal execution.

Exception

The Exceptionclass is the base class from which all other exceptions inherit. As with many exception handlers in other programming languages, the Exception class provides several useful properties that assist with exception handling:

• StackTrace: A stack trace to see where exactly the exception occurred.

• InnerException: Useful when exceptions chain offs one another. This allows one type of exception to throw another type of exception, ad infinitum.

• Message: The detailed message indicating what happened.

• Data: A dictionary that can be used to store arbitrary data associated with this particular exception instance.

SystemException

SystemException houses all exceptions related to, well, the system. These include all runtime-generated errors, such as System.IO.IOException, which is thrown when an I/O error occurs.

In short, SystemException is the base class that all non-application errors inherit from. If your application code screws up somehow, that's handled by another exception type, but if the system screws up, it's on SystemExceptionto sort it out.

IndexOutOfRangeException

The IndexOutOfRangeException is thrown anytime an array or collection is accessed using an index outside its bounds.

NullReferenceException

If you attempt to use an object that is considered a null object, the NullReferenceException will be thrown.

AccessViolationException

An AccessViolationException is thrown when unmanaged code attempts to access memory which is unallocated. For many .NET applications, this will never occur, due to how .NET handles managed vs unmanaged code.

Managed code is code that .NET compiles and executes using the common language runtime . Conversely, unmanagedcode compiles into machine code, which is notexecuted within the safety of the CLR.

InvalidOperationException

InvalidOperationException is somewhat intelligent exception. It is typically thrown when the state of an object cannot support the particular method call being attempted for that object instance.

ArgumentException

As the name implies, ArgumentException is thrown when a call is made to a method using an invalid argument.

One interesting note is that since an executing program is not intelligent enough to deduce whether or not a provided argument is valid for the context of the current execution, most of the time an ArgumentException is thrown, it's because a developer placed it there intentionally, to be thrown in a particular situation.

ArgumentNullException

ArgumentNullException is inherited from ArgumentException but is thrown specifically when a method is called that doesn't allow an argument to be null.

ArgumentOutOfRangeException

Another child of ArgumentException, the ArgumentOutOfRangeException error is thrown when a method expects argument values within a specified range, yet the provided argument falls outside those bounds.

ExternalException

ExternalException is the base exception used for any error occurring externally, outside the bounds of your own application. Typically, this means that it's used when there's a problem communicating with a web address, database, and the like.

COMException

COM handled exceptions through a property called HResult (or result handler), which is a 32-bit value that combines a few fields together into a simple string. The encoded HResult would tell the application which exception was thrown, and the application could respond appropriately.

SEHException

Also inherited from ExternalException, anSEHException acts as a sort of "catch-all" for unmanaged code exceptions which have not already been mapped to an existing .NET exception class. SEH, in this instance, stands for structured exception handling , which is a mechanism used in .NET for handling both hardware and software exceptions.

Exception Classes in C#

C# exceptions are represented by classes. The exception classes in C# are mainly directly or indirectly derived from the System.Exception class. Some of the exception classes derived from the System.Exception class are the System.ApplicationException and System.SystemException classes.

The System.ApplicationException class supports exceptions generated by application programs. Hence the exceptions defined by the programmers should derive from this class.

The System.SystemException class is the base class for all predefined system exception.

The following table provides some of the predefined exception classes derived from the Sytem.SystemException class −

Sr.No.

Exception Class & Description

1

System.IO.IOException

Handles I/O errors.

2

System.IndexOutOfRangeException

Handles errors generated when a method refers to an array index out of range.

3

System.ArrayTypeMismatchException

Handles errors generated when type is mismatched with the array type.

4

System.NullReferenceException

Handles errors generated from referencing a null object.

5

System.DivideByZeroException

Handles errors generated from dividing a dividend with zero.

6

System.InvalidCastException

Handles errors generated during typecasting.

7

System.OutOfMemoryException

Handles errors generated from insufficient free memory.

8

System.StackOverflowException

Handles errors generated from stack overflow.

Handling Exceptions

C# provides a structured solution to the exception handling in the form of try and catch blocks. Using these blocks the core program statements are separated from the error-handling statements.

These error handling blocks are implemented using the try, catch, and finally keywords. Following is an example of throwing an exception when dividing by zero condition occurs −

using System;

    
namespace ErrorHandlingApplication {
   class DivNumbers {
      int result;
      
      DivNumbers() {
         result = 0;
      }
      public void division(int num1, int num2) {
         try {
            result = num1 / num2;
         } catch (DivideByZeroException e) {
            Console.WriteLine("Exception caught: {0}", e);
         } finally {
            Console.WriteLine("Result: {0}", result);
         }
      }
      static void Main(string[] args) {
         DivNumbers d = new DivNumbers();
         d.division(25, 0);
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Exception caught: System.DivideByZeroException: Attempted to divide by zero. 
at ...
Result: 0

Creating User-Defined Exceptions

You can also define your own exception. User-defined exception classes are derived from the Exception class. The following example demonstrates this −

using System;

    
namespace UserDefinedException {
   class TestTemperature {
      static void Main(string[] args) {
         Temperature temp = new Temperature();
         try {
            temp.showTemp();
         } catch(TempIsZeroException e) {
            Console.WriteLine("TempIsZeroException: {0}", e.Message);
         }
         Console.ReadKey();
      }
   }
}
public class TempIsZeroException: Exception {
   public TempIsZeroException(string message): base(message) {
   }
}
public class Temperature {
   int temperature = 0;
   
   public void showTemp() {
      
      if(temperature == 0) {
         throw (new TempIsZeroException("Zero Temperature found"));
      } else {
         Console.WriteLine("Temperature: {0}", temperature);
      }
   }
}

When the above code is compiled and executed, it produces the following result −

TempIsZeroException: Zero Temperature found