An exception is an object (derived from System.Exception) that contains information about a problematic event. The framework supports throwing exceptions to stop processing and catching events to handle the problem and resume processing.

The stack is unwound until a handler is found, or else the exception is handled by the CLR, which terminates the program.

You create a try/catch block; the catch part is the exception handler.

The syntax is:

throw new Sytem.Arg'umentNullException(  )

You can write multiple exception handlers to handle different exceptions; the first handler that catches the thrown exception will prevent further handling. Beware of inheritance complications in the ordering of your handlers.

If you have code that must run whether or not an exception is thrown (to close a file, for example), place that code in the finally block. You must have a try for the finally, but a catch is optional.

Create a Cat class with one int property: Age. Write a program that creates a List of Cat objects in a try block. Create multiple catch statements to handle an ArgumentOutOfRangeException, and an unknown exception, and a finally block to simulate deallocating the Cat objects. Write test code to throw an exception that you will catch and handle.

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{

   class Cat
   {
      private int age;

      public int Age
      {
         get { return age; }
         set { age = value; }
      }
      public Cat( int age )
      {
         this.age = age;
      }

   }



   class Tester
   {

      private void CatManager(Cat kitty)
      {
         Console.WriteLine ("Managing a cat who is " + kitty.Age +
                            " years old");
      }

      public void Run(  )
      {
         try
         {
            Console.WriteLine(
              "Allocate resource that must be deallocated here" );

            List<Cat> cats = new List<Cat>(  );
            cats.Add( new Cat( 5 ) );
            cats.Add( new Cat( 7 ) );

            CatManager( cats[1] ); // pass in the second cat
            CatManager( cats[2] ); // pass in the non-existent third cat

            Console.WriteLine(
            "This line may or may not print" );
         }

         catch ( System.ArgumentOutOfRangeException )
         {
            Console.WriteLine(
           "I've often seen a cat without a smile, 
but this is " +
           "the first time I've seen a smile without a cat" );
         }

         catch (Exception e)
         {
            Console.WriteLine( "Unknown exception caught"  + e.Message);
         }

         finally
         {
            Console.WriteLine( "Deallocation of resource here." );
         }

      }


      static void Main(  )
      {
         Console.WriteLine( "Enter Main..." );
         Tester t = new Tester(  );
         t.Run(  );
         Console.WriteLine( "Exit Main..." );
      }
   }
}

Modify Exercise 16-1 so that it does not throw an error. Create a custom error type CustomCatError that derives from System.ApplicationException, and create a handler for it. Add a method to CatManager that checks the cat’s age, and throws an error of type CustomCatError if the age is less than or equal to 0, with an appropriate message.

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{

    class Cat
    {
        private int age;

        public int Age
        {
            get { return age; }
            set { age = value; }
        }
        public Cat(int age)
        {
            this.age = age;
        }

    }

    // custom exception class
    public class CustomCatException :
    System.ApplicationException
    {
        public CustomCatException(string message)
            :
        base(message) // pass the message up to the base class
        {

        }
    }


    class Tester
    {

        private void CheckCat(Cat testCat)
        {
            if (testCat.Age <= 0)
            {
                // create a custom exception instance
                CustomCatException e =
                new CustomCatException("Your cat does not exist");
                e.HelpLink =
                "http://www.libertyassociates.com/NoZeroDivisor.htm";
                throw e;
            }
        }


        private void CatManager(Cat kitty)
        {
            CheckCat(kitty);
            Console.WriteLine("Managing a cat who is " + kitty.Age +
                              " years old");
        }

        public void Run(  )
        {
            try
            {
                Console.WriteLine("Allocate resource that must be deallocated
                                   here");

                List<Cat> cats = new List<Cat>(  );
                cats.Add(new Cat(7));
                cats.Add(new Cat(-2));

                CatManager(cats[0]); // pass in the first cat
                CatManager(cats[1]); // pass in the second cat

                Console.WriteLine(
                "This line may or may not print");
            }

            // catch custom exception
            catch (CustomCatException e)
            {
                Console.WriteLine(
                "
CustomCatException! Msg: {0}",
                e.Message);
                Console.WriteLine(
                "
HelpLink: {0}
", e.HelpLink);
            }

            catch (System.ArgumentOutOfRangeException)
            {
                Console.WriteLine(
               "I've often seen a cat without a smile, 
but this is " +
               "the first time I've seen a smile without a cat");
            }

            catch (Exception e)
            {
                Console.WriteLine("Unknown exception caught" + e.Message);
            }

            finally
            {
                Console.WriteLine("Deallocation of resource here.");
            }

        }

        static void Main(  )
        {
            Console.WriteLine("Enter Main...");
            Tester t = new Tester(  );
            t.Run(  );
            Console.WriteLine("Exit Main...");
        }
    }
}