public override bool Equals(object obj)
{
  // From the book http://www.amazon.co.uk/Pro-2010-NET-4-0-Platform/dp/1430225491
  // Page 254!
  if (obj is Person && obj != null)
...
}

基于我的理解,我认为代码应该重写如下：

public override bool Equals(object obj)
{
  if (obj is Person)
...
}

那是对的吗？

基于http://msdn.microsoft.com/en-us/library/scekt9xw%28v=vs.80%29.aspx

如果提供的表达式为非空,则表达式计算结果为true,并且可以将提供的对象转换为提供的类型,而不会引发异常.

我认为没有必要额外检查null.换句话说,该代码“obj！= null”根本不应该被打.

谢谢

// 更新 //

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication2
{
    class Employee
    {
        public static void CheckIsEmployee(object obj)
        {
            if (obj is Employee)
            {
                Console.WriteLine("this is an employee");
            }
            else if (obj == null)
            {
                Console.WriteLine("this is null");
            }
            else
            {
                Console.WriteLine("this is Not an employee");
            }
        }   
    }

    class NotEmployee
    { }

    class Program
    {
        static void Main(string[] args)
        {
            Employee e = new Employee();

            Employee.CheckIsEmployee(e);

            Employee f = null;
            Employee.CheckIsEmployee(f);

            NotEmployee g = new NotEmployee();
            Employee.CheckIsEmployee(g);
        }
    }
}

输出结果：

this is an employee
this is null
this is Not an employee