c# – 为什么async关键字在编译时生成枚举器和其他结构体？

如果我创建一个简单的类,如下所示：

public class TestClass
{
    public Task TestMethod(int someParameter)
    {
        return Task.Fromresult(someParameter);
    }

    public async Task TestMethod(bool someParameter)
    {
        await Task.Fromresult(someParameter);
    }
}

并在NDepend中检查它,它显示了使用bool和async Task的TestMethod具有使用枚举器生成的结构体,枚举器状态机和一些附加的东西.

为什么编译器使用async方法的枚举器生成一个名为TestClass&TestMethod> d__0的结构体？

它似乎比实际的方法产生更多的IL.在本示例中,编译器为我的类生成35行IL,同时为该结构生成81行IL.这也增加了编译代码的复杂性,并导致NDepend将其标记为违规违规.

解决方法

这是因为异步和等待关键字只是语法糖,称为 coroutines.

没有特殊的IL指令来支持创建异步方法.相反,异步方法可以以某种方式被看作是一种状态机.

我会尽量使这个例子尽可能的简短：

[TestClass]
public class asynctest
{
    [TestMethod]
    public async Task RunTest_1()
    {
        var result = await GetStringAsync();
        Console.WriteLine(result);
    }

    private async Task AppendLineAsync(StringBuilder builder,string text)
    {
        await Task.Delay(1000);
        builder.AppendLine(text);
    }

    public async Task<string> GetStringAsync()
    {
        // Code before first await
        var builder = new StringBuilder();
        var secondLine = "Second Line";

        // First await
        await AppendLineAsync(builder,"First Line");

        // Inner synchronous code
        builder.AppendLine(secondLine);

        // Second await
        await AppendLineAsync(builder,"Third Line");

        // Return
        return builder.ToString();
    }
}

这是一些异步代码,您可能习惯了：我们的GetStringAsync方法首先同步创建一个StringBuilder,然后等待一些异步方法,最后返回结果.如果没有等待关键字,将如何实现？

将以下代码添加到asynctest类中：

[TestMethod]
public async Task RunTest_2()
{
    var result = await GetStringAsyncWithoutAwait();
    Console.WriteLine(result);
}

public Task<string> GetStringAsyncWithoutAwait()
{
    // Code before first await
    var builder = new StringBuilder();
    var secondLine = "Second Line";

    return new StateMachine(this,builder,secondLine).CreateTask();
}

private class StateMachine
{
    private readonly asynctest instance;
    private readonly StringBuilder builder;
    private readonly string secondLine;
    private readonly taskcompletionsource<string> completionSource;

    private int state = 0;

    public StateMachine(asynctest instance,StringBuilder builder,string secondLine)
    {
        this.instance = instance;
        this.builder = builder;
        this.secondLine = secondLine;
        this.completionSource = new taskcompletionsource<string>();
    }

    public Task<string> CreateTask()
    {
        DoWork();
        return this.completionSource.Task;
    }

    private void DoWork()
    {
        switch (this.state)
        {
            case 0:
                goto state_0;
            case 1:
                goto state_1;
            case 2:
                goto state_2;
        }

        state_0:
            this.state = 1;

            // First await
            var firstAwaiter = this.instance.AppendLineAsync(builder,"First Line")
                                        .GetAwaiter();
            firstAwaiter.OnCompleted(DoWork);
            return;

        state_1:
            this.state = 2;

            // Inner synchronous code
            this.builder.AppendLine(this.secondLine);

            // Second await
            var secondAwaiter = this.instance.AppendLineAsync(builder,"Third Line")
                                            .GetAwaiter();
            secondAwaiter.OnCompleted(DoWork);
            return;

        state_2:
            // Return
            var result = this.builder.ToString();
            this.completionSource.SetResult(result);
    }
}

显然,第一个等待关键字之前的代码保持不变.一切都转换成一个状态机,它使用goto语句来分段执行你的前一个代码.每次等待完成的任务,状态机进入下一步.

这个例子过于简单,以澄清幕后发生的情况.在异步方法中添加错误处理和一些foreach-Loops,状态机变得复杂得多.

顺便说一下,C#中有另外一个构造就是这样做：yield关键字.这也产生一个状态机,代码看起来与等待产生的相似.

要进一步阅读,请查看this CodeProject,深入了解生成的状态机.