c#索引器

1.索引器概述

c#中的索引器提供了语法的简洁方便的特性，它允许你访问对象元素如同访问数组那样，通常我们会在实现索引器的类的内部维护一个内部的集合或数组，通过索引器来实现对集合中的元素的存取操作。例如，我定义了一个row对象，内部维护了column数组，我们访问通过数组的方式访问row就相当于访问了对应的column元素，一目了然。代码如下：

   public class Row
    {
        private string[] _column;
        public int Age { get; set; }

        public Row(int len)
        {
            _column = new string[len];
            
        }
        public string this[int index]
        {
            get { return _column[index]; }
            set { _column[index] = value; }
        }

    }

//测试代码：



  class Program
    {
        static void Main(string[] args)
        {
            Row r = new Row(10);
            for (int i = 0; i < 10; i++)
            {
                r[i] = i.ToString();
            }
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine("row["+i+"]="+r[i]);
            }
        }


    }

程序运行的结果：

2.索引器的背后机制：

通过上面的代码的例子我们需要知道访问row[0]是究竟编译器帮我们在背后做了什么，同ildasm可以看到编译器为我们多生成了两个方法，一个是set_Item,一个是get_Item,如下图所示:

我们在看main方法生成的il中间码如下所示：

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       117 (0x75)
  .maxstack  4
  .locals init ([0] class Test1.Row r,
           [1] int32 i,
           [2] bool CS$4$0000,
           [3] object[] CS$0$0001)
  IL_0000:  nop
  IL_0001:  ldc.i4.s   10
  IL_0003:  newobj     instance void Test1.Row::.ctor(int32)
  IL_0008:  stloc.0
  IL_0009:  ldc.i4.0
  IL_000a:  stloc.1
  IL_000b:  br.s       IL_0022
  IL_000d:  nop
  IL_000e:  ldloc.0
  IL_000f:  ldloc.1
  IL_0010:  ldloca.s   i
  IL_0012:  call       instance string [mscorlib]System.Int32::ToString()
22   IL_0017:  callvirt   instance void Test1.Row::set_Item(int32,
23                                                          string)
  IL_001c:  nop
  IL_001d:  nop
  IL_001e:  ldloc.1
  IL_001f:  ldc.i4.1
  IL_0020:  add
  IL_0021:  stloc.1
  IL_0022:  ldloc.1
  IL_0023:  ldc.i4.s   10
  IL_0025:  clt
  IL_0027:  stloc.2
  IL_0028:  ldloc.2
  IL_0029:  brtrue.s   IL_000d
  IL_002b:  ldc.i4.0
  IL_002c:  stloc.1
  IL_002d:  br.s       IL_006b
  IL_002f:  nop
  IL_0030:  ldc.i4.4
  IL_0031:  newarr     [mscorlib]System.Object
  IL_0036:  stloc.3
  IL_0037:  ldloc.3
  IL_0038:  ldc.i4.0
  IL_0039:  ldstr      "row["
  IL_003e:  stelem.ref
  IL_003f:  ldloc.3
  IL_0040:  ldc.i4.1
  IL_0041:  ldloc.1
  IL_0042:  box        [mscorlib]System.Int32
  IL_0047:  stelem.ref
  IL_0048:  ldloc.3
  IL_0049:  ldc.i4.2
  IL_004a:  ldstr      "]="
  IL_004f:  stelem.ref
  IL_0050:  ldloc.3
  IL_0051:  ldc.i4.3
  IL_0052:  ldloc.0
  IL_0053:  ldloc.1
  IL_0054:  callvirt   instance string Test1.Row::get_Item(int32)
  IL_0059:  stelem.ref
  IL_005a:  ldloc.3
  IL_005b:  call       string [mscorlib]System.String::Concat(object[])
  IL_0060:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0065:  nop
  IL_0066:  nop
  IL_0067:  ldloc.1
  IL_0068:  ldc.i4.1
  IL_0069:  add
  IL_006a:  stloc.1
  IL_006b:  ldloc.1
  IL_006c:  ldc.i4.s   10
  IL_006e:  clt
  IL_0070:  stloc.2
  IL_0071:  ldloc.2
  IL_0072:  brtrue.s   IL_002f
  IL_0074:  ret
} // end of method Program::Main

可以看到代码的22行和60行分别对应的是源码中的为row[i]赋值和取值的语句。我们不难猜测在set_Item和get_Item的代码内部是对Row内部的字符串数组_column进行操作。

从中我们可以总结出，索引器的背后原理实际是调用了对象的set方法和get方法，这就是索引器为我们带来的便利性，简化了我们变成的方式。