Exceptions and Exception Handling (C# Programming Guide)

The C# language's exception handling features help you deal with any unexpected or exceptional situations that occur when a program is running. Exception handling uses the try, catch, and finally keywords to try actions that may not succeed, to handle failures when you decide that it is reasonable to do so, and to clean up resources afterward. Exceptions can be generated by the common language runtime (CLR), by the .NET Framework or any third-party libraries, or by application code. Exceptions are created by using the throw keyword.

In many cases, an exception may be thrown not by a method that your code has called directly, but by another method further down in the call stack. When this happens, the CLR will unwind the stack, looking for a method with a catch block for the specific exception type, and it will execute the first such catch block that if finds. If it finds no appropriate catch block anywhere in the call stack, it will terminate the process and display a message to the user.

In this example, a method tests for division by zero and catches the error. Without the exception handling, this program would terminate with a DivideByZeroException was unhandled error.

class ExceptionTest
{
    static double SafeDivision(double x, double y)
    {
        if (y == 0)
            throw new System.DivideByZeroException();
        return x / y;
    }
    static void Main()
    {
        // Input for test purposes. Change values 
        // to see exception handling behavior. 
        double a = 98, b = 0;
        double result = 0;

        try
        {
            result = SafeDivision(a, b);
            Console.WriteLine("{0} divided by {1} = {2}", a, b, result);
        }
        catch (DivideByZeroException e)
        {
            Console.WriteLine("Attempted divide by zero.");
        }
    }
}

Exceptions Overview

Exceptions have the following properties:

• Exceptions are types that all ultimately derive from System.Exception.

• Use a try block around the statements that might throw exceptions.

• Once an exception occurs in the try block, the flow of control jumps to the first associated exception handler that is present anywhere in the call stack. In C#, the catch keyword is used to define an exception handler.

• If no exception handler for a given exception is present, the program stops executing with an error message.

• Do not catch an exception unless you can handle it and leave the application in a known state. If you catch System.Exception, rethrow it using the throw keyword at the end of the catch block.

• If a catch block defines an exception variable, you can use it to obtain more information about the type of exception that occurred.

• Exceptions can be explicitly generated by a program by using the throw keyword.

• Exception objects contain detailed information about the error, such as the state of the call stack and a text description of the error.

• Code in a finally block is executed even if an exception is thrown. Use a finally block to release resources, for example to close any streams or files that were opened in the try block.

• Managed exceptions in the .NET Framework are implemented on top of the Win32 structured exception handling mechanism. For more information, see Structured Exception Handling (C+) and A Crash Course on the Depths of Win32 Structured Exception Handling.

Related Sections

See the following topics for more information about exceptions and exception handling:

• Using Exceptions (C# Programming Guide)

• Exception Handling (C# Programming Guide)

• Creating and Throwing Exceptions (C# Programming Guide)

• Compiler-Generated Exceptions (C# Programming Guide)

• How to: Handle an Exception Using try/catch (C# Programming Guide)

• How to: Execute Cleanup Code Using finally (C# Programming Guide)

C# Language Specification

For more information, see the following sections in the C# Language Specification:

• 8.9.5 The throw statement

• 8.10 The try statement

• 16 Exceptions

See Also

Concepts

C# Programming Guide

Exceptions Overview

Reference

C# Keywords

throw (C# Reference)

try-catch (C# Reference)

try-finally (C# Reference)

try-catch-finally (C# Reference)

Other Resources

Design Guidelines for Exceptions

Handling and Throwing Exceptions

Writing Reliable .NET Code

Minidumps for Specific Exceptions

Change History

Date	History	Reason
July 2008	Modified code example and added text about unwinding the stack.	Content bug fix.
July 2008	Added example to explanation of the finally block.	Information enhancement.
July 2008	Added information about the Win32 structured exception handling mechanism.	Information enhancement.