从异步HttpWebRequest的捕获异常的任务调用 [英] Catching exceptions from Asynchronous HttpWebRequest Calls in a Task

问题描述

我怎么会赶在下面这种方法的例外？

How would i catch the exception in this method below?

    private static Task<string> MakeAsyncRequest(string url)
    {
        if (!url.Contains("http"))
            url = "http://" + url;

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";
        request.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
        request.Method = "GET";
        request.KeepAlive = false;
        request.ProtocolVersion = HttpVersion.Version10;

        Task<WebResponse> task = Task.Factory.FromAsync(
        request.BeginGetResponse,
        asyncResult => request.EndGetResponse(asyncResult),
        (object)null);

        return task.ContinueWith(t => FinishWebRequest(t.Result));

    }

具体的地方我得到404，403等错误是：

The specific place i am getting 404, 403, etc errors is:

Task<WebResponse> task = Task.Factory.FromAsync(
            request.BeginGetResponse,
            asyncResult => request.EndGetResponse(asyncResult),
            (object)null);

我无法弄清楚如何处理它们。

I cant figure out how to handle them

推荐答案

您错误可能发生在你的委托调用 request.EndGetResponse（asyncResult）。

Your error is probably happening in your delegate calling request.EndGetResponse(asyncResult).

但是，您可以创建一个使用任务：

However you can create the task using:

Task<WebResponse> task = Task.Factory.FromAsync<WebResponse>(request.BeginGetResponse, request.EndGetResponse, null);

这应该传播任何异常的任务。

which should propagate any exceptions to the task.

您可以检查你的 ContinueWith 委托错误：

You can check for errors in your ContinueWith delegate:

return task.ContinueWith(t =>
{
    if (t.IsFaulted)
    {
        //handle error
        Exception firstException = t.Exception.InnerExceptions.First();
    }
    else
    {
        return FinishWebRequest(t.Result);
    }
});

另外，如果你使用C＃5，那么你可以使用异步/的await创建 MakeAsyncRequest 。这将解开从异常的 AggregateException 为您提供：

Alternatively if you're using C#5 then you can use async/await to create your MakeAsyncRequest. This will unwrap the exception from the AggregateException for you:

private static async Task<string> MakeAsyncRequest(string url)
{
    if (!url.Contains("http"))
        url = "http://" + url;

    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
    request.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";
    request.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
    request.Method = "GET";
    request.KeepAlive = false;
    request.ProtocolVersion = HttpVersion.Version10;

    Task<WebResponse> task = Task.Factory.FromAsync<WebResponse>(request.BeginGetResponse, request.EndGetResponse, null);
    WebResponse response = await task;
    return FinishWebRequest(response);
}

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