C#重载运算符详解

本文较为详细的描述了重载运算符的方法。一般来说，重载运算符在实际的项目开发中会经常的用到，但如果某些自定义类型通过简短几行代码重载一些常用的运算符（如：+-*/），就能让编程工作带来方便；重载运算符就是告诉编译器+-*/等运算符对于自定义类型进行什么样的操作，在代码中需要注意几点。

一、尽可能的不要改变运算符本身的含义

二、所有的运算符重载都必须声明为public和static

三、不同于扩展方法，所重载的方法必须是在被重载的类型内部，且用关键字operator

c#中的两个字符串相加，实际上是连接两个字符串，假如有两个employeedetail类型相加得到一个employeecollection集合，如：

employeedetail a,b;

....

employeecollection collection = a+b;

当编译器遇到上面的代码时就会自动调用employeedetail类上标有operator +的静态方法，并将两个操作数a和b作为参数传递给对于的方法，该方法需要方法一个值赋给collection，假设employeedetail类有三个属性分别是firstname,middlename,lastname，还重写了tostring方法返回一个连接这三个名称的字符串，代码如：

[serializable]
 public class employeedetail
 {
   public string firstname { get; set; }
   public string middlename { get; set; }
   public string lastname { set;get; }
   public override string tostring()
   {
     return string.format("{0}{1}{2}{3}{4}", firstname, string.isnullorwhitespace(middlename) ? null : "."
       , middlename
       , string.isnullorwhitespace(lastname) ? null : ".",
       lastname).trim();
   }
 }

下面的代码为“+”运算符提供支持的运算符重载：

public static employeecollection operator +(employeedetail a, employeedetail b)
{
  return new employeecollection() { a, b };
}

ok，给employeedetail类加上这样的一个方法之后，我们就可以像下面那个写代码了：

employeecollection collection = new employeedetail(){firstname="jackson",lastname="bruce"} + new employeedetail(){firstname="michael",lastname="jackson"} ;

但是这样还不够完美，假设a，b，c都是employeedetail类型，下面的代码会抛出一个编译错误：

employeecollection collection = a + b + c;

为什么编译不通过呢？大家都知道除了赋值运算符外表达式是从左到右执行的，a+b返回的是employeecollection类型，employeecollection类型并没有重载“+”运算符，编译器不知道要执行什么操作，所以我们还有下面的两个方法：

public static employeecollection operator +(employeecollection collection, employeedetail a)
{
  collection.add(a);
  return collection;
}
public static employeecollection operator +(employeedetail a, employeecollection collection)
{
  return collection + a;
}

这看起来似乎已经很完美了，但我们还可以做得更好一些，比如要将字符串“jackson.bruce”直接隐式转换为employeedetail类型，也就是说可以将“jackson.bruce"这种格式的字符串直接赋给employeedetail类型的对象，如：employeedetail employee= “jackson.bruce"，那么就需要重载隐式类型转换运算符了，代码如下：

/// <summary>
/// 隐式类型转换
/// </summary>
/// <param name="name"></param>
/// <returns></returns>
public static implicit operator employeedetail(string name)
{
　　　　　　　/// 其实在这里可以写一个正则表达式检查name的字符串格式是否合法，如果不合法就抛出异常
　　　　　　　///
  string[] arr;
  return string.isnullorwhitespace(name) ? null :
 new employeedetail()
 {
   firstname = (arr = name.trim().split('.'))[0]
   ,
   lastname = arr.length > 1 ? arr[arr.length > 2 ? 2 : 1] : null,
   middlename = arr.length > 2 ? arr[1] : null
 };
}
public static employeecollection operator +(employeedetail a, string b)
{
  return new employeecollection() { a, b };
}

看到这里您是不是迫不及待地想试试看，ok写个控制台程序来测试一下：

static void main(string[] args)
{
  employeedetail employee = "jackson.bruce";
  console.writeline("firstname={0},middlenam={1},lastname={2}", employee.firstname, employee.middlename, employee.lastname);
  console.writeline("tostring={0}", employee);
  console.writeline();

  employeecollection collection = "michael.jackson" + employee;

  collection += "bruce.lee";
  foreach (var e in collection)
  {
 console.writeline(e);
  }
  console.writeline();

  collection -= employee;

  foreach (var e in collection)
  {
 console.writeline(e);
  }
  console.writeline("===end===");
  console.read();
}

运行结果如下图所示：

全部代码，里面还包含其他运算符的重载，这里就不再介绍了，赶紧动手测试一下吧：

using system;
using system.collections.generic;
using system.linq;
using system.text;

namespace 重载运算符
{
  [serializable]
  public class employeedetail
  {
    public string firstname { get; set; }
    public string middlename { get; set; }
    public string lastname { set;get; }
    public static employeecollection operator +(employeedetail a, employeedetail b)
    {
      return new employeecollection() { a, b };
    }

   
    
    public static employeecollection operator +(employeecollection collection, employeedetail a)
    {
      collection.add(a);
      return collection;
    }
    public static employeecollection operator +(employeedetail a, employeecollection collection)
    {
      return collection + a;
    }
    /// <summary>
    /// 隐式类型转换
    /// </summary>
    /// <param name="name"></param>
    /// <returns></returns>
    public static implicit operator employeedetail(string name)
    {
      string[] arr;
      return string.isnullorwhitespace(name) ? null :
        new employeedetail()
        {
          firstname = (arr = name.trim().split('.'))[0]
          ,
          lastname = arr.length > 1 ? arr[arr.length > 2 ? 2 : 1] : null,
          middlename = arr.length > 2 ? arr[1] : null
        };
    }
    public static employeecollection operator +(employeedetail a, string b)
    {
      return new employeecollection() { a, b };
    }
    public override string tostring()
    {
      return string.format("{0}{1}{2}{3}{4}", firstname, string.isnullorwhitespace(middlename) ? null : "."
        , middlename
        , string.isnullorwhitespace(lastname) ? null : ".",
        lastname).trim();
    }
  }
  public class employeecollection : list<employeedetail>
  {
    public static employeecollection operator +(employeecollection a, string b)
    {
      a.add(b);
      return a;
    }
    public static employeecollection operator +(string b, employeecollection a)
    {
      return a + b;
    }

    public static employeecollection operator -(employeecollection a, employeedetail b)
    {
      a.remove(b);
      return a;
    }
  }

  class program
  {
    static void main(string[] args)
    {
      employeedetail employee = "jackson.bruce";
      console.writeline("firstname={0},middlenam={1},lastname={2}", employee.firstname, employee.middlename, employee.lastname);
      console.writeline("tostring={0}", employee);
      console.writeline();

      employeecollection collection = "michael.jackson" + employee;

      collection += "bruce.lee";
      foreach (var e in collection)
      {
        console.writeline(e);
      }
      console.writeline();

      collection -= employee;

      foreach (var e in collection)
      {
        console.writeline(e);
      }


      console.writeline("===end===");
      console.read();
    }
  }
}