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Exceptions and Exception Handling

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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's reasonable to do so, and to clean up resources afterward. Exceptions can be generated by the common language runtime (CLR), by .NET or 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 an exception is thrown, 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 it 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.

public class ExceptionTest
{
    static double SafeDivision(double x, double y)
    {
        if (y == 0)
            throw new DivideByZeroException();
        return x / y;
    }

    public static void Main()
    {
        // Input for test purposes. Change the values to see
        // exception handling behavior.
        double a = 98, b = 0;
        double result;

        try
        {
            result = SafeDivision(a, b);
            Console.WriteLine("{0} divided by {1} = {2}", a, b, result);
        }
        catch (DivideByZeroException)
        {
            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.
  • Don't 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 regardless of 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 .NET are implemented on top of the Win32 structured exception handling mechanism. For more information, see Structured Exception Handling (C/C++) and A Crash Course on the Depths of Win32 Structured Exception Handling.

C# Language Specification

For more information, see Exceptions in the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

See also