为什么我的 C# winforms 应用程序中的第一个 HttpClient.PostAsync 调用非常慢？

Question

I have an httpclient like this:

var client = new HttpClient();

I post to it like this:

var result = client.PostAsync(
                endpointUri,
                requestContent);

And get the response like this:

HttpResponseMessage response = result.Result;

I understand this call will block the thread, thats how its supposed to work (just building a tool for myself, no async threads needed)

The first time I run this call, it takes about 2 minutes to get a result. Meanwhile, if I do the exact same call elsewhere its done in 200ms. Even if I hit google, it takes 2 minutes. But, after the first call, as long as I keep the app open any additional calls are good. Its just the first cal when I open the application. What could be causing this?

Answer 1

The problem was that it was hanging for a very long time trying to resolve a proxy for the client. Initializing the HttpClient like this did the trick:

var client = new HttpClient(new HttpClientHandler
            {
                UseProxy = false
            });

Answer 2

In my case I was trying to access a service on localhost . Apparently the HTTP client tries to connect to the IPv6 localhost first before connecting to its IPv4 equivalent (source: https://github.com/jchristn/restwrapper ), which causes slowdown.

Changing localhost to 127.0.0.1 in my case cut off 2000ms of delay, although there is still maybe ~120ms delay on the first request.