在try-finally块中等待

Question

我一直在使用Visual Studio 14 CTP 2.这个版本的C#vNext允许在awaitfinally块中使用关键字.

我想弄清楚这是如何实现的.我知道这是一个实现细节,在RTM发布之前可能会发生变化,但我仍然不得不选择这个功能.

为了尝试理解底层编译器生成的代码,我创建了这个示例代码:

private async void button1_Click(object sender, EventArgs e)
{
    try
    {
    }
    finally
    {
        await MyFinallyTest();
    }
}

private async Task MyFinallyTest()
{
    await Task.Delay(1000);
}

这是编译器生成的类:

[CompilerGenerated]
private sealed class <button1_Click>d__1 : IAsyncStateMachine
{
    public int <>1__state;
    public Form1 <>4__this;
    public object <>7__wrap1;
    public int <>7__wrap2;
    public AsyncVoidMethodBuilder <>t__builder;
    public TaskAwaiter <>u__$awaiter0;

    private void MoveNext()
    {
        int num = this.<>1__state;
        try
        {
            TaskAwaiter awaiter;
            switch (num)
            {
                case 1:
                    break;

                default:
                {
                    this.<>7__wrap1 = null;
                    this.<>7__wrap2 = 0;

                    try
                    {
                    }
                    catch (object obj2)
                    {
                        this.<>7__wrap1 = obj2;
                    }

                    awaiter = this.<>4__this.MyFinallyTest().GetAwaiter();
                    if (awaiter.IsCompleted)
                    {
                        goto Label_0096;
                    }

                    this.<>1__state = num = 1;
                    this.<>u__$awaiter0 = awaiter;

                    Form1.<button1_Click>d__1 stateMachine = this;
                    this.<>t__builder.AwaitUnsafeOnCompleted<TaskAwaiter, Form1.<button1_Click>d__1>(ref awaiter, ref stateMachine);
                    return;
                }
            }

            awaiter = this.<>u__$awaiter0;
            this.<>u__$awaiter0 = new TaskAwaiter();
            this.<>1__state = num = -1;

        Label_0096:
            awaiter.GetResult();
            awaiter = new TaskAwaiter();
            object obj3 = this.<>7__wrap1;
            if (obj3 != null)
            {
                Exception source = obj3 as Exception;
                if (source <= null)
                {
                    throw obj3;
                }
                ExceptionDispatchInfo.Capture(source).Throw();
            }

            int num1 = this.<>7__wrap2;
            this.<>7__wrap1 = null;
        }
        catch (Exception exception2)
        {
            this.<>1__state = -2;
            this.<>t__builder.SetException(exception2);
            return;
        }
        this.<>1__state = -2;
        this.<>t__builder.SetResult();
    }

    [DebuggerHidden]
    private void SetStateMachine(IAsyncStateMachine stateMachine)
    {
    }
}

据我所知,编译器正在采用finally代码块并将其移动到编译器生成的catch块之后.类似于我们想要await捕捉的东西 - 最终直到C#6.0.

我看到了一些我不明白的事情:

1. 编译器正在以obj2异常catch的形式添加生成的块(在我的方法中不存在)catch (object obj2)并将其内部设置object为obj2异常.我不明白为什么要这样做.

2. finally我创建的块不再存在.这是否意味着块awaited内的任何代码finally都不能"享受"我们从实际将代码置于这样的块中所获得的保证？

Answer 1

编译器正在转向:

try
{
    Foo();
}
finally
{
    Bar();
}

变成这样的东西:

Exception caught = null;
try
{
    Foo();
}
catch (Exception e)
{
    caught = e;
}
Bar();
if (caught != null)
{
    throw caught;
}

......但是以异步方式.它最终会得到相同的结果 - finally无论是否抛出异常,你的块仍然会执行,它只是使用"catch all然后执行"而不是IL版本finally.

我建议你考虑在各种情况下执行流程会是什么样的(例如,是否在try块中抛出异常)并说服自己在每种情况下结果都是预期的结果.

关于为什么不在C#5中,Mads Torgersen在C#6 CTP文档中写道:

在C#5.0中,我们不允许使用await关键字in catch和finallyblocks,因为我们以某种方式确信自己无法实现.现在我们已经弄明白了,显然这毕竟不是不可能的.