C#异步编程之async/await详解

class Program
{
    static void Main(string[] args)
    {
        var task =  IntTask();
		Console.WriteLine("等待中...");
        Console.WriteLine($"算完了？ 让我康康！ result = {task.Result}");
    }

    static async Task<int> IntTask()
    {
        Console.WriteLine("等3秒吧");
        await Task.Delay(3000);
        return 1;
    }
}

class StockPrices
{
    private Dictionary<string, decimal> _stockPrices;
    public async Task<decimal> GetStockPriceForAsync(string companyId)
    {
        await InitializeMapIfNeededAsync();
        _stockPrices.TryGetValue(companyId, out var result);
        return result;
    }
 
    private async Task InitializeMapIfNeededAsync()
    {
        if (_stockPrices != null)
            return;
 
        await Task.Delay(42);
        // Getting the stock prices from the external source and cache in memory.
        _stockPrices = new Dictionary<string, decimal> { { "MSFT", 42 } };
    }
}

class GetStockPriceForAsync_StateMachine
{
    enum State { Start, Step1, }
    private readonly StockPrices @this;
    private readonly string _companyId;
    private readonly TaskCompletionSource<decimal> _tcs;
    private Task _initializeMapIfNeededTask;
    private State _state = State.Start;

    public GetStockPriceForAsync_StateMachine(StockPrices @this, string companyId)
    {
        this.@this = @this;
        _companyId = companyId;
    }

    public void Start()
    {
        try
        {
            if (_state == State.Start)
            {
                // The code from the start of the method to the first 'await'.

                if (string.IsNullOrEmpty(_companyId))
                    throw new ArgumentNullException();

                _initializeMapIfNeededTask = @this.InitializeMapIfNeeded();

                // Update state and schedule continuation
                _state = State.Step1;
                _initializeMapIfNeededTask.ContinueWith(_ => Start());
            }
            else if (_state == State.Step1)
            {
                // Need to check the error and the cancel case first
                if (_initializeMapIfNeededTask.Status == TaskStatus.Canceled)
                    _tcs.SetCanceled();
                else if (_initializeMapIfNeededTask.Status == TaskStatus.Faulted)
                    _tcs.SetException(_initializeMapIfNeededTask.Exception.InnerException);
                else
                {
                    // The code between first await and the rest of the method

                    @this._store.TryGetValue(_companyId, out var result);
                    _tcs.SetResult(result);
                }
            }
        }
        catch (Exception e)
        {
            _tcs.SetException(e);
        }
    }

    public Task<decimal> Task => _tcs.Task;
}

public Task<decimal> GetStockPriceForAsync(string companyId)
{
    var stateMachine = new GetStockPriceForAsync_StateMachine(this, companyId);
    stateMachine.Start();
    return stateMachine.Task;
}

static void Main()
{
	TaskCompletionSource<int> tcs1 = new TaskCompletionSource<int>();
	Task<int> t1 = tcs1.Task;

	// Start a background task that will complete tcs1.Task
	Task.Factory.StartNew(() =>
	{
		Thread.Sleep(1000);
		tcs1.SetResult(15);
	});
}

class Program
{
    public class Result<T>
    {
        public T V;
    }

    static void Main(string[] args)
    {
        int a = 1;
        int b = 2;
        int c = 3;
        int d = 4;

        Result<int> ar = new() { V = a};
        Calc3(ar, b, c, d, Calc2);

        Console.WriteLine($"a = {ar.V}");
    }

    static void Calc3(Result<int> ar, int b, int c, int d, Action<Result<int>, int, int> continues)
    {
        int tmp = b + c;
        continues(ar, tmp, d);
    }

    static void Calc2(Result<int> ar, int tmp, int d)
    {
        ar.V = tmp + d;
    }
}

struct _GetStockPriceForAsync_d__1 : IAsyncStateMachine
{
    public StockPrices __this;
    public string companyId;
    public AsyncTaskMethodBuilder<decimal> __builder;
    public int __state;
    private TaskAwaiter __task1Awaiter;

    public void MoveNext()
    {
        decimal result;
        try
        {
            TaskAwaiter awaiter;
            if (__state != 0)
            {
                // State 1 of the generated state machine:
                if (string.IsNullOrEmpty(companyId))
                    throw new ArgumentNullException();

                awaiter = __this.InitializeLocalStoreIfNeededAsync().GetAwaiter();

                // Hot path optimization: if the task is completed,
                // the state machine automatically moves to the next step
                if (!awaiter.IsCompleted)
                {
                    __state = 0;
                    __task1Awaiter = awaiter;

                    // The following call will eventually cause boxing of the state machine.
                    __builder.AwaitUnsafeOnCompleted(ref awaiter, ref this);
                    return;
                }
            }
            else
            {
                awaiter = __task1Awaiter;
                __task1Awaiter = default(TaskAwaiter);
                __state = -1;
            }

            // GetResult returns void, but it'll throw if the awaited task failed.
            // This exception is catched later and changes the resulting task.
            awaiter.GetResult();
            __this._stocks.TryGetValue(companyId, out result);
        }
        catch (Exception exception)
        {
            // Final state: failure
            __state = -2;
            __builder.SetException(exception);
            return;
        }

        // Final state: success
        __state = -2;
        __builder.SetResult(result);
    }

    void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine)
    {
        __builder.SetStateMachine(stateMachine);
    }
}

[AsyncStateMachine(typeof(_GetStockPriceForAsync_d__1))]
public Task<decimal> GetStockPriceFor(string companyId)
{
    _GetStockPriceForAsync_d__1 _GetStockPriceFor_d__;
    _GetStockPriceFor_d__.__this = this;
    _GetStockPriceFor_d__.companyId = companyId;
    _GetStockPriceFor_d__.__builder = AsyncTaskMethodBuilder<decimal>.Create();
    _GetStockPriceFor_d__.__state = -1;
    var __t__builder = _GetStockPriceFor_d__.__builder;
    __t__builder.Start<_GetStockPriceForAsync_d__1>(ref _GetStockPriceFor_d__);
    return _GetStockPriceFor_d__.__builder.Task;
}

[SecuritySafeCritical]
internal void Run()
{
    if (this.m_context != null)
    {
        try
        {
            // 我们并未给 s_invokeMoveNext 赋值，所以 callback == null
            ContextCallback callback = s_invokeMoveNext;
            if (callback == null)
            {
                // 将回调设置为下方的 InvokeMoveNext 方法
                s_invokeMoveNext = callback = new
                ContextCallback(AsyncMethodBuilderCore.MoveNextRunner.InvokeMoveNext);
            }
            ExecutionContext.Run(this.m_context, callback, this.m_stateMachine, true);
            return;
        }
        finally
        {
            this.m_context.Dispose();
        }
    }
    this.m_stateMachine.MoveNext();
}

[SecurityCritical]
private static void InvokeMoveNext(object stateMachine)
{
    ((IAsyncStateMachine) stateMachine).MoveNext();
}