c# - 为什么在 GetCurrent 之前调用 IEnumerator.MoveNext？

Question

我是 IL 的新手，但据我所知，MoveNext 应该在 Current 之前调用，假设我们有这样的 foreach 语句:

foreach (var i in Enumerable.Empty<string>())
{          
}

如果我们查看生成的 IL，我们将看到 Current 实际上首先被调用:

IL_0014:  br.s       IL_001f
IL_0016:  ldloc.1
IL_0017:  callvirt   instance !0 class [mscorlib]System.Collections.Generic.IEnumerator`1<string>::get_Current()
IL_001c:  stloc.0
IL_001d:  nop
IL_001e:  nop
IL_001f:  ldloc.1
IL_0020:  callvirt   instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
IL_0025:  stloc.2
IL_0026:  ldloc.2
IL_0027:  brtrue.s   IL_0016
IL_0029:  leave.s    IL_003b

问题是为什么？

Answer 1

IL_0014:  br.s       IL_001f

根据 MSDN，br.s

Unconditionally transfers control to a target instruction (short form).

引用的地址是调用MoveNext的指令。

IL_001f:  ldloc.1
IL_0020:  callvirt   instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()