System.Net.Http.HttpRequestException

It's probably caused by a local network connectivity issue (but also a DNS error is possible). Unfortunately HResult is generic, however you can determine the exact issue catching HttpRequestException and then inspecting InnerException: if it's a WebException then you can check the WebException.Status property, for example WebExceptionStatus.NameResolutionFailure should indicate a DNS resolution problem.

It may happen, there isn't much you can do.

What I'd suggest to always wrap that (network related) code in a loop with a try/catch block (as also suggested here for other fallible operations). Handle known exceptions, wait a little (say 1000 msec) and try again (for say 3 times). Only if failed all times then you can quit/report an error to your users. Very raw example like this:

private const int NumberOfRetries = 3;
private const int DelayOnRetry = 1000;

public static async Task<HttpResponseMessage> GetFromUrlAsync(string url) {
    using (var client = new HttpClient()) {
        for (int i=1; i <= NumberOfRetries; ++i) {
            try {
                return await client.GetAsync(url); 
            }
            catch (Exception e) when (i < NumberOfRetries) {
                await Task.Delay(DelayOnRetry);
            }
        }
    }
}

You just need to keep digging. The exception "The response ended prematurely" isn't the root cause. Keep digging into the inner exceptions until you find the last one. You'll find this:

System.IO.IOException: Authentication failed because the remote party has closed the transport stream.

So it's not about your code. It seems the server you're hitting either can't handle the load, or is intentionally dropping your requests because you're hitting it too hard.

As you are using HttpClient, try to use response.EnsureSuccessStatusCode();

Now HttpClient will throw exception when response status is not a success code.

try
{
    HttpResponseMessage response = await client.GetAsync("http://www.ajshdgasjhdgajdhgasjhdgasjdhgasjdhgas.tk/");
    response.EnsureSuccessStatusCode();    // Throw if not a success code.

    // ...
}
catch (HttpRequestException e)
{
    // Handle exception.
}

ORIGINAL SOURCE OF THE CODE: http://www.asp.net/web-api/overview/advanced/calling-a-web-api-from-a-net-client

we resolved this problem with 2 code changes:

1. Dispose of the httpResponseMessage and just work with a simple DTO

   using (var httpResponseMessage = await httpClient.SendAsync(httpRequestMessage))
   {
       return await CreateDto(httpResponseMessage);
   }
   

2. Downgrade the version of HTTP to v1.0

   var httpRequestMessage = new HttpRequestMessage(HttpMethod.Post, new Uri(url))
   {
       Version = HttpVersion.Version10,
       Content = httpContent
   };
   
   await client.SendAsync(httpRequestMessage);
   

which has the effect of adding this Http header

Connection: close 

rather than this

Connection: keep-alive