'Why do I get "The response ended prematurely" when using the HttpClient in C#?

My first code of HTTP Client in .Net Core. The server is running on my PC as well, so I want to send it to localhost.

I've added printing of the inner exception after someone recommended it here but it's not enough for me to understand the problem.

namespace HTTPClient
{
    // The object I want to send
    public class Msg
    {
        public string ID { get; set; }
        public string Data { get; set; }
        public int RSSIR { get; set; }
        public int RSSIT { get; set; }
        public int Slot { get; set; }
    }

    class Program
    {
        static Msg msg = new Msg
        {
            ID = "00CC",
            Data = "9F4CC",
            RSSIR = 123, 
            RSSIT = 321,
            Slot = 1
        };

        static void Main(string[] args)
        {
            string url = "http://127.0.0.1:5001";
            Console.WriteLine("result = " , callURL(url)); 
            Console.ReadKey();
        }


        public static bool callURL(string url)
        {
            ServicePointManager.ServerCertificateValidationCallback += (sender, cert, chain, sslPolicyErrors) => true;
            bool Result = false;
            HttpClient client = new HttpClient();
            client.BaseAddress = new Uri(url);
            client.DefaultRequestHeaders.Accept.Clear();
            HttpResponseMessage response = null;
            try
            {
                client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
                HttpContent content = new StringContent(JsonConvert.SerializeObject(msg), Encoding.UTF8, "application/json");
                response = client.PostAsync("", content).Result;
            }
            catch (AggregateException err)
            {
                foreach (var errInner in err.InnerExceptions)
                {   // for printing the inner execption
                    Console.WriteLine("ERROR");
                    Console.WriteLine(errInner); 
                }
            }


            if (response.IsSuccessStatusCode)
            {
                Result = true;
            }
            else
            {
                Result = false;
            }
            return Result;
        }
        
    }

}

The error I received from the loop inside the catch is:

System.Net.Http.HttpRequestException: An error occurred while sending the request.
 ---> System.IO.IOException: The response ended prematurely.
   at System.Net.Http.HttpConnection.FillAsync()
   at System.Net.Http.HttpConnection.ReadNextResponseHeaderLineAsync(Boolean foldedHeadersAllowed)
   at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithNtConnectionAuthAsync(HttpConnection connection, HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)

How can I figure out why can't I send a message to the server? Does the content of the message can disrupt the connection generation?

I checked the server with another client and it's working so the problem has to be in my code.

EDIT: The server is using https so I changed my URL to https as well and this is the error I received:

System.Net.Http.HttpRequestException: The SSL connection could not be established, see inner exception.
 ---> System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.
   at System.Net.Security.SslStream.StartSendAuthResetSignal(ProtocolToken message, AsyncProtocolRequest asyncRequest, ExceptionDispatchInfo exception)
   at System.Net.Security.SslStream.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.PartialFrameCallback(AsyncProtocolRequest asyncRequest)
--- End of stack trace from previous location where exception was thrown ---
   at System.Net.Security.SslStream.ThrowIfExceptional()
   at System.Net.Security.SslStream.InternalEndProcessAuthentication(LazyAsyncResult lazyResult)
   at System.Net.Security.SslStream.EndProcessAuthentication(IAsyncResult result)
   at System.Net.Security.SslStream.EndAuthenticateAsClient(IAsyncResult asyncResult)
   at System.Net.Security.SslStream.<>c.<AuthenticateAsClientAsync>b__65_1(IAsyncResult iar)
   at System.Threading.Tasks.TaskFactory`1.FromAsyncCoreLogic(IAsyncResult iar, Func`2 endFunction, Action`1 endAction, Task`1 promise, Boolean requiresSynchronization)
--- End of stack trace from previous location where exception was thrown ---
   at System.Net.Http.ConnectHelper.EstablishSslConnectionAsyncCore(Stream stream, SslClientAuthenticationOptions sslOptions, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.ConnectHelper.EstablishSslConnectionAsyncCore(Stream stream, SslClientAuthenticationOptions sslOptions, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean allowHttp2, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.CreateHttp11ConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.GetHttpConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)

Do I need to add some SSL commands in order to make a secure connection?

Edit2: The new error after adding a command in callURL()

---> System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.
   at System.Net.Security.SslStream.StartSendAuthResetSignal(ProtocolToken message, AsyncProtocolRequest asyncRequest, ExceptionDispatchInfo exception)
   at System.Net.Security.SslStream.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.PartialFrameCallback(AsyncProtocolRequest asyncRequest)
--- End of stack trace from previous location where exception was thrown ---
   at System.Net.Security.SslStream.ThrowIfExceptional()
   at System.Net.Security.SslStream.InternalEndProcessAuthentication(LazyAsyncResult lazyResult)
   at System.Net.Security.SslStream.EndProcessAuthentication(IAsyncResult result)
   at System.Net.Security.SslStream.EndAuthenticateAsClient(IAsyncResult asyncResult)
   at System.Net.Security.SslStream.<>c.<AuthenticateAsClientAsync>b__65_1(IAsyncResult iar)
   at System.Threading.Tasks.TaskFactory`1.FromAsyncCoreLogic(IAsyncResult iar, Func`2 endFunction, Action`1 endAction, Task`1 promise, Boolean requiresSynchronization)
--- End of stack trace from previous location where exception was thrown ---
   at System.Net.Http.ConnectHelper.EstablishSslConnectionAsyncCore(Stream stream, SslClientAuthenticationOptions sslOptions, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.ConnectHelper.EstablishSslConnectionAsyncCore(Stream stream, SslClientAuthenticationOptions sslOptions, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean allowHttp2, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.CreateHttp11ConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.GetHttpConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)

Solution 1:^[1]

System.IO.IOException: The response ended prematurely means the server to which you're making an HTTP request closed the connection without having sent back a complete response.

It has nothing to do with your client code, as it looks to compile & send content correctly.

Yes, you should be using await client.PostAsync but that isn't related at all to your question.

Double-check that http://127.0.0.1:5001 is 100% correct - maybe it's https:// or not on port 5001.

If it is an endpoint using HTTPS, you may get System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure. as it looks like you're calling a local endpoint.

.NET does SSL certificate validation by default & will throw an exception if you are trying to call an HTTPS endpoint for which you have no/invalid SSL certificate. You probably have not set up the localhost certificate correctly on your machine.

For now, you can bypass the check by setting ServicePointManager.ServerCertificateValidationCallback to always return true using HttpClientHandler.DangerousAcceptAnyServerCertificateValidator

Initialise your HttpClient like below:

public static bool callURL(string url)
{
  bool Result = false;

  var httpClientHandler = new HttpClientHandler();
  
  #if DEBUG
  httpClientHandler.ServerCertificateCustomValidationCallback = HttpClientHandler.DangerousAcceptAnyServerCertificateValidator;
  #endif

  HttpClient client = new HttpClient(httpClientHandler);
  ...
}

However, please do note allowing all SSL certificates is a security risk if used on anything other than your local dev environment.

Read: The most dangerous code in the world: validating SSL certificates in non-browser software by Stanford University

Solution 2:^[2]

I had a similar issue when porting a .net framework project to .net core. I managed to solve it by using WinHttpHandler like this:

var handler = new WinHttpHandler();
handler.SslProtocols = System.Security.Authentication.SslProtocols.Tls12;
handler.TcpKeepAliveEnabled = true;

using (var hc = new HttpClient(handler))
{
    return wc.GetStringAsync(new Uri(apiUrl)).GetAwaiter().GetResult();
}

Sources

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Source: Stack Overflow

Solution	Source
Solution 1
Solution 2	Rickard Hagstrand

'Why do I get "The response ended prematurely" when using the HttpClient in C#?

Solution 1:[1]

Solution 2:[2]

Sources

Related Questions

Solution 1:^[1]

Solution 2:^[2]