C#教程之C#异步的世界【下】(2)|C#教程

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C#教程之C#教程之C#异步的世界【下】(2)

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, http = http, url = url, time = time, _builder = AsyncTaskMethodBuilder<string>.Create(), _state = -1 }; stateMachine._builder.Start(ref stateMachine); return stateMachine._builder.Task; }

方法签名完全一致，只是里面的内容变成了一个状态机 GetUrlStringAsyncdStateMachine 的调用。此状态机就是编译器自动创建的。下面来看看神秘的状态机是什么鬼：

private sealed class GetUrlStringAsyncdStateMachine : IAsyncStateMachine
{
    public int _state;
    public MyAsyncTest _this;
    private string _str1;
    public AsyncTaskMethodBuilder<string> _builder;
    private TaskAwaiter taskAwaiter1;
    private TaskAwaiter<string> taskAwaiter2;

    //异步方法的三个形参都到这里来了

    public HttpClient http;
    public int time;
    public string url;

    private void MoveNext()
    {
        string str;
        int num = this._state;
        try
        {
            TaskAwaiter awaiter;
            MyAsyncTest.GetUrlStringAsyncdStateMachine d__;
            string str2;
            switch (num)
            {
                case 0:
                    break;

                case 1:
                    goto Label_00CD;

                default:
                    //这里是异步方法 await Task.Delay(time);的具体实现
                    awaiter = Task.Delay(this.time).GetAwaiter();
                    if (awaiter.IsCompleted)
                    {
                        goto Label_0077;
                    }
                    this._state = num = 0;
                    this.taskAwaiter1 = awaiter;
                    d__ = this;
                    this._builder.AwaitUnsafeOnCompleted<TaskAwaiter, MyAsyncTest.GetUrlStringAsyncdStateMachine>(ref awaiter, ref d__);
                    return;
            }
            awaiter = this.taskAwaiter1;
            this.taskAwaiter1 = new TaskAwaiter();
            this._state = num = -1;
        Label_0077:
            awaiter.GetResult();
            awaiter = new TaskAwaiter();
            //这里是异步方法await http.GetStringAsync(url);的具体实现
            TaskAwaiter<string> awaiter2 = this.http.GetStringAsync(this.url).GetAwaiter();
            if (awaiter2.IsCompleted)
            {
                goto Label_00EA;
            }
            this._state = num = 1;
            this.taskAwaiter2 = awaiter2;
            d__ = this;
            this._builder.AwaitUnsafeOnCompleted<TaskAwaiter<string>, MyAsyncTest.GetUrlStringAsyncdStateMachine>(ref awaiter2, ref d__);
            return;
        Label_00CD:
            awaiter2 = this.taskAwaiter2;
            this.taskAwaiter2 = new TaskAwaiter<string>();
            this._state = num = -1;
        Label_00EA:
            str2 = awaiter2.GetResult();
            awaiter2 = new TaskAwaiter<string>();
            this._str1 = str2;
            str = this._str1;
        }
        catch (Exception exception)
        {
            this._state = -2;
            this._builder.SetException(exception);
            return;
        }
        this._state = -2;
        this._builder.SetResult(str);
    }

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

}