C# 在IHTTPassynchandler中同步调用Web服务

我试图调用IHttpAsyncHandler中的Web服务，我发现有这样一个答案

我对答案有疑问。如果有人能帮助我，我将不胜感激

它有

Task webClientDownloadTask = webClientDownloadCompletionSource.Task;

我的问题是

Webclient DownloadCompletionSource与Webclient（客户端）对象无关，因此执行此操作的目的是什么：

//获取TCS的任务，以便我们可以附加一些continuation Task webClientDownloadTask=webClientDownloadCompletionSource.Task

这里的“taskCompletionSource”是什么：

为什么我要在ContinueWith（）回调中dispose（）WebClient，为什么不在设置taskCompletionSource.SetResult（true）

//工作完成后，务必处置客户 webClientDownloadTask.ContinueWith( _ => { client.Dispose（）； }, TaskContinuationOptions.ExecuteSynchronously）

以下是完整的代码：

    public class MyAsyncHandler : IHttpAsyncHandler
{
    public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
    {
        // NOTE: the result of this operation is void, but TCS requires some data type so we just use bool
        TaskCompletionSource<bool> webClientDownloadCompletionSource = new TaskCompletionSource<bool>();

        WebClient webClient = new WebClient())
        HttpContext currentHttpContext = HttpContext.Current;

        // Setup the download completed event handler
        client.DownloadDataCompleted += (o, e) =>
        {
            if(e.Cancelled)
            {
                // If it was canceled, signal the TCS is cacnceled
                // NOTE: probably don't need this since you have nothing canceling the operation anyway
                webClientDownloadCompletionSource.SetCanceled();
            }
            else if(e.Error != null)
            {
                // If there was an exception, signal the TCS with the exception
                webClientDownloadCompletionSource.SetException(e.Error);
            }
            else
            {
                // Success, write the response
                currentHttpContext.Response.ContentType = "text/xml";
                currentHttpContext.Response.OutputStream.Write(e.Result, 0, e.Result.Length);

                // Signal the TCS that were done (we don't actually look at the bool result, but it's needed)
                taskCompletionSource.SetResult(true);
            }
        };

        string url = "url_web_service_url";

        // Kick off the download immediately
        client.DownloadDataAsync(new Uri(url));

        // Get the TCS's task so that we can append some continuations
        Task webClientDownloadTask = webClientDownloadCompletionSource.Task;

        // Always dispose of the client once the work is completed
        webClientDownloadTask.ContinueWith(
            _ =>
            {
                client.Dispose();
            },
            TaskContinuationOptions.ExecuteSynchronously);

        // If there was a callback passed in, we need to invoke it after the download work has completed
        if(cb != null)
        {
            webClientDownloadTask.ContinueWith(
               webClientDownloadAntecedent =>
               {
                   cb(webClientDownloadAntecedent);
               },
               TaskContinuationOptions.ExecuteSynchronously);
         }

        // Return the TCS's Task as the IAsyncResult
        return webClientDownloadTask;
    }

    public void EndProcessRequest(IAsyncResult result)
    {
        // Unwrap the task and wait on it which will propagate any exceptions that might have occurred
        ((Task)result).Wait();
    }

    public bool IsReusable
    {
        get 
        { 
            return true; // why not return true here? you have no state, it's easily reusable!
        }
    }

    public void ProcessRequest(HttpContext context)
    {
    }
}

公共类MyAsyncHandler:IHTTPassynchandler
{
公共IAsyncResult BeginProcessRequest（HttpContext上下文、AsyncCallback cb、object extraData）
{
//注意：这个操作的结果是void，但是TCS需要一些数据类型，所以我们只使用bool
TaskCompletionSource webClientDownloadCompletionSource=新TaskCompletionSource（）；
WebClient WebClient=新的WebClient（）
HttpContext currentHttpContext=HttpContext.Current；
//设置下载完成的事件处理程序
client.DownloadDataCompleted+=（o，e）=>
{
如果（如已取消）
{
//如果取消，则向TCS发送取消信号
//注意：可能不需要这个，因为您没有任何取消操作的方法
webClientDownloadCompletionSource.setCancelled（）；
}
否则如果（例如错误！=null）
{
//如果出现异常，则向TCS发送异常信号
webClientDownloadCompletionSource.SetException（e.Error）；
}
其他的
{
//成功，写下回应
currentHttpContext.Response.ContentType=“text/xml”；
currentHttpContext.Response.OutputStream.Write（e.Result，0，e.Result.Length）；
//向已完成的TCS发送信号（我们实际上没有查看bool结果，但这是必需的）
taskCompletionSource.SetResult（true）；
}
};
string url=“url\u web\u service\u url”；
//立即开始下载
DownloadDataAsync（新Uri（url））；
//获取TCS的任务，以便我们可以附加一些continuation
Task webClientDownloadTask=webClientDownloadCompletionSource.Task；
//工作完成后，务必处置客户
webClientDownloadTask.ContinueWith(
_ =>
{
client.Dispose（）；
},
TaskContinuationOptions.ExecuteSynchronously）；
//如果传入了回调，我们需要在下载工作完成后调用它
如果（cb！=null）
{
webClientDownloadTask.ContinueWith(
webClientDownloadAntecedent=>
{
cb（网络客户端下载先行项）；
},
TaskContinuationOptions.ExecuteSynchronously）；
}
//将TCS的任务作为IAsyncResult返回
返回webClientDownloadTask；
}
公共无效EndProcessRequest（IAsyncResult结果）
{
//展开任务并等待它，这将传播可能发生的任何异常
（（任务）结果）。等待（）；
}
公共布尔可重用
{
得到
{ 
return true；//为什么不在这里返回true？您没有状态，它很容易重用！
}
}
公共void ProcessRequest（HttpContext上下文）
{
}
}

查看此项了解其用途。最后还有一个例子

在许多情况下，启用任务来表示 外部异步操作TaskCompletionSource{TResult} 为此目的而提供。它允许创建一个可以 分发给消费者，这些消费者可以使用 这项任务和其他任务一样重要。但是，与大多数任务不同的是 TaskCompletionSource创建的任务的状态受控制 由TaskCompletionSource上的方法显式执行。这使得 完成要传播到的外部异步操作 基本任务。这种分离还确保消费者 无法在不访问相应服务器的情况下转换状态 TaskCompletionSource

    public class MyAsyncHandler : IHttpAsyncHandler
{
    public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
    {
        // NOTE: the result of this operation is void, but TCS requires some data type so we just use bool
        TaskCompletionSource<bool> webClientDownloadCompletionSource = new TaskCompletionSource<bool>();

        WebClient webClient = new WebClient())
        HttpContext currentHttpContext = HttpContext.Current;

        // Setup the download completed event handler
        client.DownloadDataCompleted += (o, e) =>
        {
            if(e.Cancelled)
            {
                // If it was canceled, signal the TCS is cacnceled
                // NOTE: probably don't need this since you have nothing canceling the operation anyway
                webClientDownloadCompletionSource.SetCanceled();
            }
            else if(e.Error != null)
            {
                // If there was an exception, signal the TCS with the exception
                webClientDownloadCompletionSource.SetException(e.Error);
            }
            else
            {
                // Success, write the response
                currentHttpContext.Response.ContentType = "text/xml";
                currentHttpContext.Response.OutputStream.Write(e.Result, 0, e.Result.Length);

                // Signal the TCS that were done (we don't actually look at the bool result, but it's needed)
                taskCompletionSource.SetResult(true);
            }
        };

        string url = "url_web_service_url";

        // Kick off the download immediately
        client.DownloadDataAsync(new Uri(url));

        // Get the TCS's task so that we can append some continuations
        Task webClientDownloadTask = webClientDownloadCompletionSource.Task;

        // Always dispose of the client once the work is completed
        webClientDownloadTask.ContinueWith(
            _ =>
            {
                client.Dispose();
            },
            TaskContinuationOptions.ExecuteSynchronously);

        // If there was a callback passed in, we need to invoke it after the download work has completed
        if(cb != null)
        {
            webClientDownloadTask.ContinueWith(
               webClientDownloadAntecedent =>
               {
                   cb(webClientDownloadAntecedent);
               },
               TaskContinuationOptions.ExecuteSynchronously);
         }

        // Return the TCS's Task as the IAsyncResult
        return webClientDownloadTask;
    }

    public void EndProcessRequest(IAsyncResult result)
    {
        // Unwrap the task and wait on it which will propagate any exceptions that might have occurred
        ((Task)result).Wait();
    }

    public bool IsReusable
    {
        get 
        { 
            return true; // why not return true here? you have no state, it's easily reusable!
        }
    }

    public void ProcessRequest(HttpContext context)
    {
    }
}