11.3 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 11-7 illustrates the creation of a custom exception.

Example 11-7. Creating a custom exception

namespace Programming_CSharp
{
   using System;

   public class MyCustomException : 
         System.ApplicationException
   {
      public MyCustomException(string message):
         base(message)
      {
            
      }
   }

   public class Test
   {
      public static void Main( )
      {
         Test t = new Test( );
         t.TestFunc( );
      }

      // try to divide two numbers
      // handle possible exceptions
      public void TestFunc( )
      {
         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 (MyCustomException e)
         {
            Console.WriteLine(
               "\nMyCustomException! Msg: {0}",
               e.Message);
            Console.WriteLine(
               "\nHelpLink: {0}\n", e.HelpLink);
         }
         catch 
         {
            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)
         {
            MyCustomException e = 
               new MyCustomException(
                  "Can't have zero divisor");
            e.HelpLink =
            "http://www.libertyassociates.com/NoZeroDivisor.htm";
            throw e;
         }
         return a/b;
      }
   }
}

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, as described in Chapter 4. In this case, 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.