C# 迭代ToList方法与存储在内存中的ToList数据

当前的.NET编译器是否以不同的方式处理这些问题

List<string> stuff = GetSomeStuff();
foreach(string thing in stuff) { ... }

---

如果启用编译器优化，则两组代码将编译为相同的IL。例如，这是使用LinqPad时主方法的IL输出：

IL_0000:  ldarg.0     
IL_0001:  call        UserQuery.GetSomeStuff
IL_0006:  callvirt    System.Collections.Generic.List<System.String>.GetEnumerator
IL_000B:  stloc.0     
IL_000C:  br.s        IL_0016
IL_000E:  ldloca.s    00 
IL_0010:  call        System.Collections.Generic.List<System.String>.get_Current
IL_0015:  pop         
IL_0016:  ldloca.s    00 
IL_0018:  call        System.Collections.Generic.List<System.String>.MoveNext
IL_001D:  brtrue.s    IL_000E
IL_001F:  leave.s     IL_002F
IL_0021:  ldloca.s    00 
IL_0023:  constrained. System.Collections.Generic.List<>.Enumerator
IL_0029:  callvirt    System.IDisposable.Dispose
IL_002E:  endfinally  
IL_002F:  ret  

IL_0000:ldarg.0
IL_0001:调用UserQuery.GetSomeStuff
IL_0006:callvirt System.Collections.Generic.List.GetEnumerator
IL_000B:stloc.0
IL_000C:br.s IL_0016
IL_000E:ldloca.s 00
IL_0010:调用System.Collections.Generic.List.get_Current
IL_0015：流行音乐
IL_0016:ldloca.s 00
IL_0018:调用System.Collections.Generic.List.MoveNext
IL_001D:brtrue.s IL_000E
IL_001F：离开s IL_002F
IL_0021:ldloca.s 00
IL_0023：受约束。System.Collections.Generic.List.Enumerator
IL_0029:callvirt System.IDisposable.Dispose
IL_002E：最后结束
IL_002F:ret

前者的唯一好处是，如果您想使用

stuff

变量执行其他操作，则前者可能会重复。对于您的代码，您最好使用后者。

IL_0000:  ldarg.0     
IL_0001:  call        UserQuery.GetSomeStuff
IL_0006:  callvirt    System.Collections.Generic.List<System.String>.GetEnumerator
IL_000B:  stloc.0     
IL_000C:  br.s        IL_0016
IL_000E:  ldloca.s    00 
IL_0010:  call        System.Collections.Generic.List<System.String>.get_Current
IL_0015:  pop         
IL_0016:  ldloca.s    00 
IL_0018:  call        System.Collections.Generic.List<System.String>.MoveNext
IL_001D:  brtrue.s    IL_000E
IL_001F:  leave.s     IL_002F
IL_0021:  ldloca.s    00 
IL_0023:  constrained. System.Collections.Generic.List<>.Enumerator
IL_0029:  callvirt    System.IDisposable.Dispose
IL_002E:  endfinally  
IL_002F:  ret