如何在 Web API 控制器中等待 Task<T> 完成

Question

我正在尝试使用返回异步任务的方法。方法是：

    public async Task<DataResponse[]> GetGridDataAsync(DataRequest[] requestCollection, object state)
    {
        return await Task.Factory.FromAsync<DataResponse[]>(BeginGetDataAsync(requestCollection, ar => EndGetDataAsync(ar), state), 
            EndGetDataAsync);
    }

在 Web API 控制器中使用它时，90% 的时间都会出现以下错误：

异步模块或处理程序在异步完成时完成 操作仍在等待中。

我的尝试是：

    [HttpGet]
    [Route("GetGridDataAsync")]
    public IEnumerable<DataResponse> GetGridDataAsync()
    {
        var proxy = new Proxy();
        var task = proxy.GetGridDataAsync(dataRequest, new object());

        if (!task.IsCompleted)
        {
            task.Start();
            task.Wait();                
        }                        

        //var x = (IAsyncResult)task; //i also tried this
        //task.Start();
        //x.AsyncWaitHandle.WaitOne();

        return task.Result;
    }

如何在请求完成（或至少指定超时）之前阻塞线程？我将从 Angular 使用这个端点。

这是我上一个问题的后续：Have Web API controller wait for IAsyncResult before completing?

更新 1： 返回 proxy.GetGridDataAsync(dataRequest, new object()) 引发了异常。我可以单步执行 BeginXXX 函数，但 EndXXX 中的断点永远不会被击中。不知道为什么。

有时我没有收到上面的“异步模块或处理程序...”错误，并在堆栈跟踪中收到不同的错误。但我不确定这意味着什么：

{  
"Message":"An error has occurred.",
"ExceptionMessage":"Object cannot be null.",
"ExceptionType":"System.ArgumentNullException",
"StackTrace":" at 
System.Threading.ThreadPool.RegisterWaitForSingleObjectNative(WaitHandle waitHandle, Object state, UInt32 timeOutInterval, Boolean executeOnlyOnce, RegisteredWaitHandle registeredWaitHandle, StackCrawlMark& stackMark, Boolean compressStack)
at System.Threading.ThreadPool.RegisterWaitForSingleObject(WaitHandle waitObject, WaitOrTimerCallback callBack, Object state, UInt32 millisecondsTimeOutInterval, Boolean executeOnlyOnce, StackCrawlMark& stackMark, Boolean compressStack)
at System.Threading.ThreadPool.RegisterWaitForSingleObject(WaitHandle waitObject, WaitOrTimerCallback callBack, Object state, Int32 millisecondsTimeOutInterval, Boolean executeOnlyOnce)
at System.Threading.Tasks.TaskFactory`1.FromAsyncImpl(IAsyncResult asyncResult, Func`2 endFunction, Action`1 endAction, TaskCreationOptions creationOptions, TaskScheduler scheduler, StackCrawlMark& stackMark)
at System.Threading.Tasks.TaskFactory.FromAsync[TResult](IAsyncResult asyncResult, Func`2 endMethod)
at proxy.d__1.MoveNext() in proxy.cs:line 30
--- End of stack trace from previous location where exception was thrown ---
}

Answer 1

更改您的控制器以返回像这样的Task<IEnumerable<DataResponse>>...

[HttpGet]
[Route("GetGridDataAsync")]
public async Task<IEnumerable<DataResponse>> GetGridDataAsync()
{
    var proxy = new Proxy();
    return await proxy.GetGridDataAsync(dataRequest, new object());
}

Answer 2

当你调用一个async方法时，推荐await它。在您的方法中，您实际上是阻塞直到任务完成，这与同步运行它完全一样。

如果你只想在GetGridDataAsync完成时返回，你需要标记你的方法async，让它返回一个Task<IEnumerable<DataResponse>>和await它：

[HttpGet]
[Route("GetGridDataAsync")]
public async Task<IEnumerable<DataResponse>> GetGridDataAsync()
{
    var proxy = new Proxy();
    return await proxy.GetGridDataAsync(dataRequest, new object());
}

如果您不需要等到GetGridDataAsync 完成，您可以自己保存状态机生成并简单地返回Task：

[HttpGet]
[Route("GetGridDataAsync")]
public Task<IEnumerable<DataResponse>> GetGridDataAsync()
{
    return proxy.GetGridDataAsync(dataRequest, new object());
}

另外，请注意您的 GetGridDataAsync 还会创建一个冗余状态机，因为它对等待的结果没有任何作用。你可以简单地返回没有await的结果，让调用者await就可以了：

public async Task<DataResponse[]> GetGridDataAsync(DataRequest[] requestCollection, object state)
{
    return Task.Factory.FromAsync<DataResponse[]>(BeginGetDataAsync(requestCollection, ar => EndGetDataAsync(ar), state), 
        EndGetDataAsync);
}