扩展方法到底做了什么

扩展方法是.net3.5以后出现的一个比较优雅的写法，昨天讨论中有同学提出这也是通过反射实现的，不过我并不这么认为。

扩展方法的定义必须是静态类的静态方法，对一个静态类的静态方法进行反射明显不明智。而且linq to object中大量使用扩展方法，如果是用发射实现方法调用，明显过于笨拙。

那么事实到底如何

先定义一个扩展方法和一个同样功能的非扩展方法

扩展方法定义

public   static   class  StringHelp
    {

        ///   <summary>
         ///  裁减字符串（扩展方法写法）
         ///   </summary>
         ///   <param name="source"></param>
         ///   <param name="length"></param>
         ///   <returns></returns>
         public   static   string  CutStr( this   string  source,  int  length,  string  lastString)
        {
             if  ( string .IsNullOrEmpty(source))
                 return   "" ;
             if  (source.Length  >  length)
            {
                 string  result  =   string .Empty;
                source.ToCharArray().Take(length).ToList().ForEach(p  =>  { result  +=  p.ToString(); });
                 return  result  +  lastString;
            }
             else
                 return  source;
        }
         ///   <summary>
         ///  裁减字符串（非扩展方法写法）
         ///   </summary>
         ///   <param name="source"></param>
         ///   <param name="length"></param>
         ///   <returns></returns>
         public   static   string  CutStrNew( string  source,  int  length,  string  lastString)
        {
             if  ( string .IsNullOrEmpty(source))
                 return   "" ;
             if  (source.Length  >  length)
            {
                 string  result  =   string .Empty;
                source.ToCharArray().Take(length).ToList().ForEach(p  =>  { result  +=  p.ToString(); });
                 return  result  +  lastString;
            }
             else
                 return  source;
        }
    }

 

然后分别进行扩展方法调用，静态方法调用，反射调用三种调用 

 

代码调用和效率比较

class  Program
    {
         static   void  Main( string [] args)
        {
             string  str  =   " hello world! " ;
            Stopwatch watch  =   new  Stopwatch();
            Console.WriteLine(str.CutStr( 5 ,  " ... " ));
            Console.WriteLine(StringHelp.StringHelp.CutStrNew(str,  5 ,  " ... " ));
            Console.WriteLine( typeof (StringHelp.StringHelp).GetMethod( " CutStr " ).Invoke( null ,  new   object [] { str,  5 ,  " ... "  }));

             // 效率比较
            watch.Start();
             for  ( int  i  =   0 ; i  <   100000 ; i ++ )
            {
                str.CutStr( 5 ,  " ... " );
            }
            watch.Stop();
            
            Console.WriteLine( " 1: " + watch.ElapsedMilliseconds);

            watch.Restart();
             for  ( int  i  =   0 ; i  <   100000 ; i ++ )
            {
                StringHelp.StringHelp.CutStrNew(str,  5 ,  " ... " );
            }
            watch.Stop();
            Console.WriteLine( " 2: "   +  watch.ElapsedMilliseconds);

            watch.Restart();
             for  ( int  i  =   0 ; i  <   100000 ; i ++ )
            {
                 typeof (StringHelp.StringHelp).GetMethod( " CutStr " ).Invoke( null ,  new   object [] { str,  5 ,  " ... "  });
            }
            watch.Stop();
            Console.WriteLine( " 3: "   +  watch.ElapsedMilliseconds);

            Console.Read();
        }
    }

 

 

IL代码

IL代码

IL_0000:  nop
  IL_0001:  ldstr       " hello world! "
  IL_0006:  stloc. 0
  IL_0007:  ldloc. 0
  IL_0008:  ldc.i4. 5
  IL_0009:  ldstr       " ... "
  IL_000e:  call        string  [StringHelp]StringHelp.StringHelp::CutStr( string ,
                                                                        int32,
                                                                         string )
  IL_0013:  call        void  [mscorlib]System.Console::WriteLine( string )
  IL_0018:  nop
  IL_0019:  ldloc. 0
  IL_001a:  ldc.i4. 5
  IL_001b:  ldstr       " ... "
  IL_0020:  call        string  [StringHelp]StringHelp.StringHelp::CutStrNew( string ,
                                                                           int32,
                                                                            string )
  IL_0025:  call        void  [mscorlib]System.Console::WriteLine( string )
  IL_002a:  nop
  IL_002b:  ldtoken    [StringHelp]StringHelp.StringHelp
  IL_0030:  call        class  [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
  IL_0035:  ldstr       " CutStr "
  IL_003a:  call       instance  class  [mscorlib]System.Reflection.MethodInfo [mscorlib]System.Type::GetMethod( string )
  IL_003f:  ldnull
  IL_0040:  ldc.i4. 3
  IL_0041:  newarr     [mscorlib]System.Object
  IL_0046:  stloc. 1
  IL_0047:  ldloc. 1
  IL_0048:  ldc.i4. 0
  IL_0049:  ldloc. 0
  IL_004a:  stelem. ref
  IL_004b:  ldloc. 1
  IL_004c:  ldc.i4. 1
  IL_004d:  ldc.i4. 5
  IL_004e:  box        [mscorlib]System.Int32
  IL_0053:  stelem. ref
  IL_0054:  ldloc. 1
  IL_0055:  ldc.i4. 2
  IL_0056:  ldstr       " ... "
  IL_005b:  stelem. ref
  IL_005c:  ldloc. 1
  IL_005d:  callvirt   instance  object  [mscorlib]System.Reflection.MethodBase::Invoke( object ,
                                                                                       object [])
  IL_0062:  call        void  [mscorlib]System.Console::WriteLine( object )

 

三种调用的方式是这样的：

1、string [StringHelp]StringHelp.StringHelp::CutStr(string,                                                                        int32,                                                                        string)

2、string [StringHelp]StringHelp.StringHelp::CutStrNew(string,
                                                                           int32,
                                                                           string)

3、instance object [mscorlib]System.Reflection.MethodBase::Invoke(object,
                                                                                      object[])

 

 

很明显，并没有使用反射，而是编译器的语法糖，使得调用方法更为简便。在编译时期，编译器会对代码作用域范围内可见的所有静态类，且带this 特征参数的方法进行搜索，

然后透明调用符合特征的方法。

 

那么效率如何呢

 

 

 1、86  2、84  3、313

 效率基本没有，差别，因为其实调用的方式都没有什么区别，而且性能比反射自然要好很多。