SendAsync(httpclient) fails with InnerException - "The underlying connection was closed: An unexpected error occurred on a send.", but Fiddler not

Question

I saw these post, but they did not work out:

• C# HttpWebRequest The underlying connection was closed: An unexpected error occurred on a send
• The underlying connection was closed: An unexpected error occurred on a send.--- NuGet

You can find the entire code here :https://github.com/te2020/GoEuroAPiClassLibrary/blob/master/GOEuroApi_Requests.cs

Only the relevant part of it(what I think):

httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8");
            httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Accept-Encoding", "gzip, deflate");
            httpClient.DefaultRequestHeaders.TryAddWithoutValidation("User-Agent", "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0");

            httpClient.BaseAddress = new Uri("https://www.goeuro.com/");


            HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, "/GoEuroAPI/rest/api/v5/searches");
            request.Content = new StringContent(jsonString,
                                                Encoding.UTF8,
                                                "application/json");//CONTENT-TYPE header

My fiddler looks like this and it works.

I get the eror after sending that request:

 var response = await httpClient.SendAsync(request);

What did I miss ?

Answer 1

If you use HTTP for your request (instead of HTTPS), you can see a more helpful response from the server:

{"message":"Search options are not correct!","statusCode":400}

This would seem to indicate that the searchOptions JSON you're sending is invalid for some reason. Without knowing the goeuro API it's impossible for me to say what the exact problem is. Unfortunately I don't have time to probe it.

What I would do though is use the Fiddler 'compare' tool to compare your working request against the request coming from your program. Just select both requests in Fiddler and right click -> compare. Then try to make your request exactly the same as the working one. I notice there are cookies in the working request that you probably aren't sending, you should add those.