When your program throws an exception, you can use a try-catch-finally block to catch the exception and examine it to determine its class. When you want to throw your own exception, however, you must know what exception classes are available so that you can pick the best one to throw. The following sections describe some of the most useful classes for throwing exceptions.

Standard Exception Classes

The following table lists some of the most useful exception classes in C#. If possible you should use one of these standard classes when you need to throw an exception.

Use the throw statement to throw an exception. The following code throws a DivideByZeroException.

throw new DivideByZeroException("No employees are defined.");

This code passes the exception class’s constructor a message describing the exception. In this case, the divide by zero exception occurred because the application did not have any employees defined. Notice that the message explains the reason for the exception, not just the fact that a division by zero occurred.

Custom Exception Classes

To define a custom exception class, make a class that inherits from the Exception class. To give developers who use the class the most flexibility, provide four constructors that delegate their work to the parent class’s corresponding constructors.

The following code shows the InvalidWorkAssignmentException class. The parameterless constructor passes the Exception class’s constructor a default error message. The other constructors simply pass their arguments to the Exception class’s other constructors.

public class InvalidWorkAssignmentException : Exception
{
    public InvalidWorkAssignmentException()
        : base("This work assignment is invalid")
    {
    }
    public InvalidWorkAssignmentException(string message)
        : base(message)
    {
    }
    public InvalidWorkAssignmentException(string message, Exception innerException)
        : base(message, innerException)
    {
    }
    public InvalidWorkAssignmentException(SerializationInfo info,
    StreamingContext context)
        : base(info, context)
    {
    }
}

For more information on custom exception classes, see the section “Custom Exceptions” in Chapter 9, “Error Handing.” Also see the online documentation for topics such as “Designing Custom Exceptions” (msdn.microsoft.com/library/vstudio/ms229064(v=vs.100).aspx) and “Design Guidelines for Exceptions” (msdn.microsoft.com/ms229014.aspx).