Httplistener request always processed twice

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1.00/5 (1 vote)

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Dear experts
I have no clue what I'm doing wrong, I hope you can enlighten me.
I'm playing with HttpListener to learn about it.

In the code shown in 'What have you tried' I experience following when I do a request 'http://localhost:8001/' by e.g. chrome:
a.) In the log I see for one browser request, I get two log entries
b.) In the browser I see incrementing the counter always by 2 for each request.

Why one and the same request is processed twice?
Btw. same happens if I take the asynch approach using httpListener.BeginGetContext and httpListener.EndGetContext

Thank you very much in advance.

What I have tried:

namespace WFATestApp
{
    public partial class FormMain : Form
    {
        BackgroundWorker bgWorker = new BackgroundWorker();
        HttpListener httpListener = new HttpListener();


        public FormMain()
        {
            InitializeComponent();

            httpListener.Prefixes.Add("http://localhost:8001/");

            bgWorker.DoWork += new DoWorkEventHandler(BgWorker_DoWork);
            bgWorker.RunWorkerAsync();
        }

        void LogMsg(String Msg)
        {
            listBoxLog.Items.Add(Msg);
            listBoxLog.SelectedIndex = listBoxLog.Items.Count - 1;
        }

        int requestCount = 0;

        private void BgWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker bgw = sender as BackgroundWorker;
            bool cancel = false;

            httpListener.Start();

            while (!cancel)
            {
                var context = httpListener.GetContext();

                requestCount++;

                // The request
                // HttpListenerRequest request = context.Request;

                // Get response object.
                HttpListenerResponse response = context.Response;

                // Construct a response. 
                string responseString = $"<HTML><BODY> It Works! {requestCount} </BODY></HTML>";
                byte[] buffer = System.Text.Encoding.UTF8.GetBytes(responseString);
                
                // Write to response stream.
                response.ContentLength64 = buffer.Length;
                System.IO.Stream output = response.OutputStream;
                output.Write(buffer, 0, buffer.Length);
                
                // Close the output stream.
                output.Close();

                this.Invoke((MethodInvoker)delegate
                {
                    LogMsg("http request count: " + requestCount.ToString());
                });


                cancel = bgw.CancellationPending;
            }
            httpListener.Stop();
        }

    }
}

Posted 26-Mar-23 3:21am

0x01AA

Updated 26-Mar-23 3:56am

v2

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Graeme_Grant · Accepted Answer · 2023-03-26T03:56:00

Solution 1

Browser requires a FavIcon. If you open your Web browser tools > Network tab, and make a request, you can clearly see it.

In you app use:

C#

LogMsg($"http request {context.Request.RawUrl}  | count: {requestCount}");

Now you can see it. 😉

Posted 26-Mar-23 3:56am

Graeme_Grant

Comments

0x01AA 26-Mar-23 9:58am

Hundred thousand thanks. I can only give my little 5 :)

Graeme_Grant 26-Mar-23 10:02am

All good. Another way is to set a breakpoint on

requestCount++;

Then, when the breakpoint is hit, go to your immediate window and post:

context.Request

Then you will see something like:

{System.Net.HttpListenerRequest}
    AcceptTypes: {string[6]}
    ClientCertificateError: 'context.Request.ClientCertificateError' threw an exception of type 'System.InvalidOperationException'
    ContentEncoding: {System.Text.SBCSCodePageEncoding}
    ContentLength64: 0
    ContentType: null
    Cookies: {System.Net.CookieCollection}
    HasEntityBody: false
    Headers: {sec-ch-ua: "Google Chrome";v="111", "Not(A:Brand";v="8", "Chromium";v="111"
DNT: 1
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: no-cors
Sec-Fetch-Dest: image
Connection: keep-alive
Accept: image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Host: localhost:8001
Referer: http://localhost:8001/
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36

}
    HttpListenerContext: {System.Net.HttpListenerContext}
    HttpMethod: "GET"
    InputStream: {System.IO.Stream.NullStream}
    IsAuthenticated: false
    IsLocal: true
    IsSecureConnection: false
    IsWebSocketRequest: false
    KeepAlive: true
    LocalEndPoint: {[::1]:8001}
    OriginalBlobAddress: 0x02bc6958
    ProtocolVersion: {1.1}
    QueryString: {System.Collections.Specialized.NameValueCollection}
    RawUrl: "/favicon.ico"
    RemoteEndPoint: {[::1]:63541}
    RequestBuffer: {byte[4096]}
    RequestId: 18230571318439313419
    RequestScheme: "http"
    RequestTraceIdentifier: {00000000-0000-0000-0b00-0040060000fd}
    RequestUri: {http://localhost:8001/favicon.ico}
    ServiceName: null
    TransportContext: {System.Net.HttpListenerRequestContext}
    Url: {http://localhost:8001/favicon.ico}
    UrlReferrer: {http://localhost:8001/}
    UserAgent: "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36"
    UserHostAddress: "[::1]:8001"
    UserHostName: "localhost:8001"
    UserLanguages: {string[2]}
    m_BoundaryType: ContentLength
    m_ClientCertState: NotInitialized
    m_ClientCertificate: null
    m_ClientCertificateError: 0
    m_ConnectionId: 18230571318170877960
    m_ContentLength: 0
    m_CookedUrlHost: "localhost:8001"
    m_CookedUrlPath: "/favicon.ico"
    m_CookedUrlQuery: null
    m_Cookies: {System.Net.CookieCollection}
    m_HttpContext: {System.Net.HttpListenerContext}
    m_HttpMethod: "GET"
    m_IsDisposed: false
    m_KeepAlive: True
    m_LocalEndPoint: {[::1]:8001}
    m_Lock: {object}
    m_MemoryBlob: {System.Net.SyncRequestContext}
    m_RawUrl: "/favicon.ico"
    m_RemoteEndPoint: {[::1]:63541}
    m_RequestId: 18230571318439313419
    m_RequestStream: {System.IO.Stream.NullStream}
    m_RequestUri: {http://localhost:8001/favicon.ico}
    m_ServiceName: null
    m_SslStatus: Insecure
    m_TokenBindingVerifyMessageStatus: 0
    m_TokenBindings: null
    m_Version: {1.1}
    m_WebHeaders: {sec-ch-ua: "Google Chrome";v="111", "Not(A:Brand";v="8", "Chromium";v="111"
DNT: 1
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: no-cors
Sec-Fetch-Dest: image
Connection: keep-alive
Accept: image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Host: localhost:8001
Referer: http://localhost:8001/
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWeb

or go to the Locals window and expand the context variable and view the branches ... the joys of debugging

0x01AA 26-Mar-23 10:12am

Thank you very much again.

0x01AA 26-Mar-23 10:26am

I'm aware now I'm becoming something off topic to ask a question in a comment.
Nevertheless I do it ;) :
Is it a bad praxis to 'POST' e.g. a zip file as content type 'octet stream' to a rest service?

Graeme_Grant 26-Mar-23 13:27pm

No ... use ContentType "application/octet-stream"
But yes, now off topic....

0x01AA 26-Mar-23 13:34pm

Thanks ;)