异常用于指示运行程序时发生了错误。
如果将null引用传递给不接受其作为有效参数的方法，则会抛出ArgumentNulLexception。

C# ArgumentNulLException异常示例

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace application

{

    class MainClass

    {

        static void a(string s)

        {

            if (s == null)

            {

                throw new ArgumentNullException();

            }

        }

         static void Main()

        {

            try

            {

                string s = null;

                a(s);

            }          

            //catch (ArgumentNullException e)

            //{

            //   Console.WriteLine("{0} exception caught.", e);

            //}

                  catch (ArgumentNullException ex)

            {

                Console.WriteLine(ex);

                Console.WriteLine("HelpLink = {0}", ex.HelpLink);

                Console.WriteLine("Message = {0}", ex.Message);

                Console.WriteLine("Source = {0}", ex.Source);

                Console.WriteLine("StackTrace = {0}", ex.StackTrace);

                Console.WriteLine("TargetSite = {0}", ex.TargetSite);

            }

            finally

            {

                Console.WriteLine("Task completed");

            }

            Console.ReadLine();

        }

    }

}