System.Net.Http.HttpClient.PostAsync blocks and never returns

问题

I have a .NET framework Windows Forms application with a form that has this code:

using System;
using System.Collections.Generic;
using System.IO;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace test
{

    public partial class Main : Form
    {

        public int exitCode = 1;
        private Options opts;
        CancellationTokenSource cancellationSource = new CancellationTokenSource();


        public Main(Options opts)
        {
            InitializeComponent();
            this.opts = opts;
        }

        private void btnCancel_Click(object sender, EventArgs e)
        {
            exitCode = 1;
            cancellationSource.Cancel();
            Close();
        }

        async Task doUpload()
        {
            using (var content = new MultipartFormDataContent())
            {
                List<FileStream> streams = new List<FileStream>();
                try
                {
                    foreach (string fPath in opts.InputFiles)
                    {
                        FileStream stream = new FileStream(fPath, FileMode.Open, FileAccess.Read);
                        streams.Add(stream);
                        content.Add(new StreamContent(stream), fPath);
                    }
                    var progressContent = new ProgressableStreamContent(
                         content,
                         4096,
                         (sent, total) =>
                         {
                             double percent = 100 * sent / total;
                             progressBar.Value = (int)percent;
                         });

                    using (var client = new HttpClient())
                    {
                        using (var response = await client.PostAsync(opts.URL, progressContent, cancellationSource.Token))
                        {
                            if (response.IsSuccessStatusCode)
                            {
                                exitCode = 0;
                            }
                            else
                            {
                                MessageBox.Show(
                                    response.Content.ToString(),
                                    "Error " + response.StatusCode,
                                    MessageBoxButtons.OK, MessageBoxIcon.Error
                               );
                            }
                            Close();
                        }
                    }
                }
                finally
                {
                    foreach (FileStream stream in streams)
                    {
                        stream.Close();
                    }
                }
            }

        }

        private void Main_Load(object sender, EventArgs e)
        {
        }

        private void Main_FormClosing(object sender, FormClosingEventArgs e)
        {
            e.Cancel = !cancellationSource.IsCancellationRequested;
        }

        private void Main_Shown(object sender, EventArgs e)
        {
            doUpload();
        }
    }
}

The ProgressableStreamContent is the same that was given here: C#: HttpClient, File upload progress when uploading multiple file as MultipartFormDataContent

The problem is that the response is never returned. In other words: await for postAsync never completes. Also, the progress callback is never called back. Even if I try to use a POST URL that contains a non-exsitent domain, nothing happens. I guess it is a deadlock, but I don't see how? The async Task's result is never used anywhere and it is not awaited for.

It is different from An async/await example that causes a deadlock because .Result is not used and the method is never awaited for, and also it seems that calling ConfigureAwait(false) ha no effect.

UPDATE: I have created a new github repo for this question, so anyone can test it:

https://github.com/nagylzs/csharp_http_post_example

UPDATE: Finally it works. ConfigureAwait is not needed. All UI update operations must be placed inside Invoke. I have updated the test repo to the working version. Also added TLSv1.2 support (which is disabled by default).

回答1:

PostAsync in the code you've posted doesn't block (but it really never returns though!). It throws an exception:

System.InvalidOperationException: Cross-thread operation not valid: Control 'progressBar' accessed from a thread other than the thread it was created on.

That's the reason for the breakpoints that didn't worked for you. The right solution would be:

var progressContent = new ProgressableStreamContent(
     content,
     4096,
     (sent, total) =>
     {
         Invoke((Action) (() => {
             double percent = 100 * sent / total;
             progressBar.Value = (int) percent;
         }));
     });

(either add Invoke or BeginInvoke to the callback)

The callbacks of the HTTP client are called on a background thread, and you have to put them into your window's even queue if you want them to access your UI controls.

.ConfigureAwait(false) has nothing to do with this issue, you shouldn't use it in UI context (quite the opposite: you want it to put the continuation onto the UI thread, so you shouldn't use it).

回答2:

You need to change this:

client.PostAsync(opts.URL, progressContent, cancellationSource.Token)

to

client.PostAsync(opts.URL, progressContent, cancellationSource.Token).ConfigureAwait(false)

This is already discussed so you can find additional resources on the net, but this should be good starting point.

来源：https://stackoverflow.com/questions/58364142/system-net-http-httpclient-postasync-blocks-and-never-returns