foreach是否在每次迭代时都评估数组？

我只是以此来嘲笑你的代码

foreach(var v in Enumerable.Range(1,10).Skip(1))    v.Dump();

这是生成的IL。

IL_0001:  nop         IL_0002:  ldc.i4.1    IL_0003:  ldc.i4.s    0A IL_0005:  call        System.Linq.Enumerable.RangeIL_000A:  ldc.i4.1    IL_000B:  call        System.Linq.Enumerable.Skip//Call to SkipIL_0010:  callvirt    System.Collections.Generic.IEnumerable<System.Int32>.GetEnumeratorIL_0015:  stloc.1     // CS
Skip
00IL_0016:  br.s        IL_0026IL_0018:  ldloc.1     // CS
IEnumerator<int> e = ((IEnumerable<int>)values).GetEnumerator();//Get the enumeratortry{  int m;//This variable is here prior to c#5.0  while(e.MoveNext())  {//int m; is declared here starting from c#5.0    m = (int)(int)e.Current;    //Your pre here  }}finally{  if (e != null) ((IDisposable)e).Dispose();}
00IL_0019:  callvirt    System.Collections.Generic.IEnumerator<System.Int32>.get_CurrentIL_001E:  stloc.0     // vIL_001F:  ldloc.0     // vIL_0020:  call        LINQPad.Extensions.DumpIL_0025:  pop         IL_0026:  ldloc.1     // CS
VeryLongRunningMethodThatReturnsEnumerable
00IL_0027:  callvirt    System.Collections.IEnumerator.MoveNextIL_002C:  stloc.2     // CS
foreach(var obj in VeryLongRunningMethodThatReturnsEnumerable()){   //Do something with that obj}
01IL_002D:  ldloc.2     // CS01IL_002E:  brtrue.s    IL_0018IL_0030:  leave.s     IL_0042IL_0032:  ldloc.1     // CS00IL_0033:  ldnull      IL_0034:  ceq         IL_0036:  stloc.2     // CS01IL_0037:  ldloc.2     // CS01IL_0038:  brtrue.s    IL_0041IL_003A:  ldloc.1     // CS00IL_003B:  callvirt    System.IDisposable.DisposeIL_0040:  nop         IL_0041:  endfinally

如您所见，仅被调用一次。

等效的C＃代码看起来像这样

考虑下面的代码，如果foreach
在每次迭代中调用，那将是一场噩梦。语言设计中的巨大缺陷。幸运的是它没有那样做。