C# 4.0 Pocket Reference, 3rd Edition

try Statements and Exceptions

A try statement specifies a code block subject to error-handling or cleanup code. The try block must be followed by a catch block, a finally block, or both. The catch block executes when an error occurs in the try block. The finally block executes after execution leaves the try block (or if present, the catch block), to perform cleanup code, whether or not an error occurred.

A catch block has access to an Exception object that contains information about the error. You use a catch block to either compensate for the error or rethrow the exception. You rethrow an exception if you merely want to log the problem, or if you want to rethrow a new, higher-level exception type.

A finally block adds determinism to your program, by always executing no matter what. It’s useful for cleanup tasks such as closing network connections.

A try statement looks like this:

try
{
  ... // exception may get thrown within execution of
      // this block
}
catch (ExceptionA ex)
{
  ... // handle exception of type ExceptionA
}
catch (ExceptionB ex)
{
  ... // handle exception of type ExceptionB
}
finally
{
  ... // clean-up code
}

Consider the following code:

int x = 3, y = 0;
Console.WriteLine (x / y);

Because y is zero, the runtime throws a DivideByZeroException, and our program terminates. We can prevent this by catching the exception as follows:

try
{
  int x = 3, y = 0;
  Console.WriteLine (x / y);
}
catch (DivideByZeroException ex)
{
  Console.Write ("y cannot be zero. ");
} // Execution resumes ...