调试易

下面是MSDN的例程。但当我将运行它时，它却报错。
在  MethodInfo m = assem.GetType("Example").GetMethod("SampleMethod");     
一行出现“未处理NullReferenceException”的错误请问，是什么原因？应该怎样解决？
谢谢using System;
using System.Configuration;
using System.Security.Permissions;
using System.Reflection;
namespace ConsoleApplication1
{    [assembly: AssemblyVersionAttribute("1.0.2000.0")]    public class Example
    {
        private int factor;
        public Example(int f)
        {
            factor = f;
        }        public int SampleMethod(int x)
        {
            Console.WriteLine("\nExample.SampleMethod({0}) executes.", x);
            return x * factor;
        }        public static void Main()
        {
            Assembly assem = Assembly.GetExecutingAssembly();            Console.WriteLine("Assembly Full Name:");
            Console.WriteLine(assem.FullName);
            // The AssemblyName type can be used to parse the full name.
            AssemblyName assemName = assem.GetName();
            Console.WriteLine("\nName: {0}", assemName.Name);
            Console.WriteLine("Version: {0}.{1}",
                assemName.Version.Major, assemName.Version.Minor);            Console.WriteLine("\nAssembly CodeBase:");
            Console.WriteLine(assem.CodeBase);            // Create an object from the assembly, passing in the correct number
            // and type of arguments for the constructor.
            Object o = assem.CreateInstance("Example", false,
                BindingFlags.ExactBinding,
                null, new Object[] { 2 }, null, null);                // Make a late-bound call to an instance method of the object.    
            MethodInfo m = assem.GetType("Example").GetMethod("SampleMethod");     // 错误的地方            Object ret = m.Invoke(o, new Object[] { 42 });
            Console.WriteLine("SampleMethod returned {0}.", ret);
            Console.WriteLine("\nAssembly entry point:");
            Console.WriteLine(assem.EntryPoint);
            Console.ReadLine();
        }
    }   
}