c# - 如何通过 C# 中的异常处理理解 "using"的 JITed 代码

Question

我用 C# 编写了一个非常简单的类:

class DisposableClass : IDisposable {
    public void Dispose() { }
}


static void UsingClass() {                    // line 31
    using (var dc = new DisposableClass()) {  // line 32
        DoSomething(dc);                      // line 33
    }                                         // line 34
}                                             // line 35

我在使用 WinDBG 进行 JIT 之后转储了 native 代码:

0:000> !u 000007fe87d30120 
Normal JIT generated code
SimpleConsole.Program.UsingClass()
Begin 000007fe87d30120, size 80

c:\projects\SimpleConsole\SimpleConsole\Program.cs @ 32:
>>> 000007fe`87d30120 55              push    rbp
000007fe`87d30121 4883ec30        sub     rsp,30h
000007fe`87d30125 488d6c2420      lea     rbp,[rsp+20h]
000007fe`87d3012a 48896500        mov     qword ptr [rbp],rsp
000007fe`87d3012e 48c7450800000000 mov     qword ptr [rbp+8],0
000007fe`87d30136 488d0d6b47eeff  lea     rcx,[000007fe`87c148a8]
000007fe`87d3013d e8fe24665f      call    clr+0x2640 (000007fe`e7392640) (JitHelp: CORINFO_HELP_NEWSFAST) // new DisposableClass()
000007fe`87d30142 48894508        mov     qword ptr [rbp+8],rax

c:\projects\SimpleConsole\SimpleConsole\Program.cs @ 33:
000007fe`87d30146 488b4d08        mov     rcx,qword ptr [rbp+8]
000007fe`87d3014a e8d1beeeff      call    000007fe`87c1c020 (SimpleConsole.Program.DoSomething(System.Object), mdToken: 0000000006000012)
000007fe`87d3014f 90              nop
000007fe`87d30150 90              nop

c:\projects\SimpleConsole\SimpleConsole\Program.cs @ 35:
000007fe`87d30151 488b4d08        mov     rcx,qword ptr [rbp+8]
000007fe`87d30155 4c8d1dc4feeeff  lea     r11,[000007fe`87c20020]
000007fe`87d3015c ff15befeeeff    call    qword ptr [000007fe`87c20020] // Call Dispose()
000007fe`87d30162 90              nop
000007fe`87d30163 488d6510        lea     rsp,[rbp+10h]
000007fe`87d30167 5d              pop     rbp
000007fe`87d30168 c3              ret

// I could understand the code above (without exception thrown).

c:\projects\SimpleConsole\SimpleConsole\Program.cs @ 32:
000007fe`87d30169 55              push    rbp
000007fe`87d3016a 4883ec30        sub     rsp,30h
000007fe`87d3016e 488b6920        mov     rbp,qword ptr [rcx+20h]
000007fe`87d30172 48896c2420      mov     qword ptr [rsp+20h],rbp
000007fe`87d30177 488d6d20        lea     rbp,[rbp+20h]

c:\projects\SimpleConsole\SimpleConsole\Program.cs @ 35:
000007fe`87d3017b 48837d0800      cmp     qword ptr [rbp+8],0
000007fe`87d30180 7417            je      000007fe`87d30199
000007fe`87d30182 488d1597feeeff  lea     rdx,[000007fe`87c20020]
000007fe`87d30189 488b4508        mov     rax,qword ptr [rbp+8]
000007fe`87d3018d 803800          cmp     byte ptr [rax],0
000007fe`87d30190 488b4d08        mov     rcx,qword ptr [rbp+8]
000007fe`87d30194 4c8bda          mov     r11,rdx
000007fe`87d30197 ff12            call    qword ptr [rdx]
000007fe`87d30199 90              nop
000007fe`87d3019a 4883c430        add     rsp,30h
000007fe`87d3019e 5d              pop     rbp
000007fe`87d3019f c3              ret

我可以毫无异常(exception)地理解代码(上面有评论)，但是当抛出异常时代码如何工作？代码如何进入注释下方的代码？

更新:

有些人认为我们应该从 IL 开始，所以我粘贴了下面的代码:

.method private hidebysig static 
    void UsingClass () cil managed noinlining 
{
    // Method begins at RVA 0x23bc
    // Code size 25 (0x19)
    .maxstack 1
    .locals init (
        [0] class SimpleConsole.DisposableClass dc
    )

    IL_0000: newobj instance void SimpleConsole.DisposableClass::.ctor()
    IL_0005: stloc.0
    .try
    {
        IL_0006: ldloc.0
        IL_0007: call void SimpleConsole.Program::DoSomething(object)
        IL_000c: leave.s IL_0018
    } // end .try
    finally
    {
        IL_000e: ldloc.0
        IL_000f: brfalse.s IL_0017

        IL_0011: ldloc.0
        IL_0012: callvirt instance void [mscorlib]System.IDisposable::Dispose()

        IL_0017: endfinally
    } // end handler

    IL_0018: ret
} // end of method Program::UsingClass

但我认为这没有帮助，因为 IL 几乎将所有内容都保留在 C# 中，例如完整的 try...finally 语句。我想了解的是 native 代码如何处理此处的异常。

Answer 1

抖动比反汇编窗口中看到的要多得多。首先，它生成一个描述局部变量的生命周期和存储的表。对于垃圾收集器来说非常重要，它需要该表来查找对象引用。

它会为异常生成一个展开表。它有一个非常令人满意的特性，它使try 语句免费。编写具有异常处理的代码是零成本的，不需要代码进入 try block 。所以你在反汇编中看不到任何东西。没有简单的方法可以从调试器中找到该表。对它们的相当不错的描述is here .