18.9 Custom Exceptions

The intrinsic exception types the CLR provides, coupled with the custom messages shown in the previous example, will often be all you need to provide extensive information to a catch block when an exception is thrown.

There will be times, however, when you want to provide more extensive information or need special capabilities in your exception. It is a trivial matter to create your own custom exception class; the only restriction is that it must derive (directly or indirectly) from System.ApplicationException. Example 18-7 illustrates the creation of a custom exception.

Example 18-7. A custom exception

using System;

namespace ExceptionHandling
{
    // custom exception class
    public class MyCustomException : 
        System.ApplicationException
    {
        public MyCustomException(string message):
            base(message) // pass the message up to the base class
        {
            
        }
    }

   class Tester
   {
      public void Run()
      {
          try
          {
              Console.WriteLine("Open file here");
              double a = 0;
              double b = 5;             
              Console.WriteLine ("{0} / {1} = {2}",
                  a, b, DoDivide(a,b));
              Console.WriteLine (
                  "This line may or may not print");
          }

              // most derived exception type first
          catch (System.DivideByZeroException e)
          {
              Console.WriteLine(
                  "\nDivideByZeroException! Msg: {0}",
                  e.Message);
              Console.WriteLine(
                  "\nHelpLink: {0}\n", e.HelpLink);
          }

          // catch custom exception 
          catch (MyCustomException e)
          {
              Console.WriteLine(
                  "\nMyCustomException! Msg: {0}",
                  e.Message);
              Console.WriteLine(
                  "\nHelpLink: {0}\n", e.HelpLink);
          }
          catch     // catch any uncaught exceptions
          {
              Console.WriteLine(
                  "Unknown exception caught");
          } 
          finally
          {
              Console.WriteLine ("Close file here.");
          }
      }

       // do the division if legal
       public double DoDivide(double a, double b)
       {
           if (b == 0)
           {
               DivideByZeroException e = 
                   new DivideByZeroException();
               e.HelpLink=
                   "http://www.libertyassociates.com";
               throw e;
           }
           if (a == 0)
           {
               // create a custom exception instance
               MyCustomException e = 
                   new MyCustomException(
                   "Can't have zero divisor");
               e.HelpLink =
                   "http://www.libertyassociates.com/NoZeroDivisor.htm";
               throw e;
           }
           return a/b;
       }

       static void Main()
       {
           Console.WriteLine("Enter Main...");
           Tester t = new Tester();
           t.Run();
           Console.WriteLine("Exit Main...");           
       }
   }
}
Output:
Enter Main...
Open file here

MyCustomException! Msg: Can't have zero divisor

HelpLink: http://www.libertyassociates.com/NoZeroDivisor.htm

Close file here.
Exit Main...

MyCustomException is derived from System.ApplicationException and consists of nothing more than a constructor that takes a string message that it passes to its base class.

The advantage of creating this custom exception class is that it better reflects the particular design of the Test class, in which it is not legal to have a zero divisor. Using the ArithmeticException rather than a custom exception would work as well, but it might confuse other programmers because a zero divisor wouldn't normally be considered an arithmetic error.