Short Version

I'm adding the response header:

Connection: keep-alive

but it's not in the resposne.

Long Version

I am trying to add a header to an HttpResponse in ASP.net:

public void ProcessRequest(HttpContext context)
{
    context.Response.CacheControl = "no-cache";
    context.Response.AppendHeader("Connection", "keep-alive");
    context.Response.AppendHeader("AreTheseWorking", "yes");
    context.Response.Flush();
}

And when the response comes back to the client (e.g. Chrome, Edge, Internet Explorer, Postman), the Connection header is missing:

HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Transfer-Encoding: chunked
Expires: -1
Server: Microsoft-IIS/10.0
AreTheseWorking: yes
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Sat, 26 Feb 2022 16:29:17 GMT

What am I doing wrong?

Bonus Chatter

In addition to trying AppendHeader:

context.Response.AppendHeader("Connection", "keep-alive"); //preferred

I also tried AddHeader (which exists "for compatibility with earlier versions of ASP"):

context.Response.AddHeader("Connection", "keep-alive"); // legacy

I also tried Headers.Add:

context.Response.Headers.Add("Connection", "keep-alive"); //requires IIS 7 and integrated pipeline

What am i doing wrong?

Bonus: hypothetical motivation for the question

By default keep-alive is not allowed in ASP.net.

In order to allow it, you need to add an option to your web.config:

web.config:

<configuration>
    <system.webServer>
        <httpProtocol allowKeepAlive="true" />
    </system.webServer>
</configuration>

This is especially important for Server-Send Events:

public void ProcessRequest(HttpContext context)
{
   if (context.Request.AcceptTypes.Any("text/event-stream".Contains))
   {
      //Startup the HTTP Server Send Event - broadcasting values every 1 second.
      SendSSE(context);
      return;
   }
}

private void SendSSE(HttpContext context)
{
   //Don't worry about it.
   string sessionId = context.Session.SessionID; //https://stackoverflow.com/a/1966562/12597

   //Setup the response the way SSE needs to be
   context.Response.ContentType = "text/event-stream";
   context.Response.CacheControl = "no-cache";
   context.Response.AppendHeader("Connection", "keep-alive");
   context.Response.Flush();

   while (context.Response.IsClientConnected)
   {
      System.Threading.Thread.Sleep(1000);
 
      String data = DateTime.Now.ToString();
      context.Response.Write("data: " + data + "\n\n");
      context.Response.Flush();
   }
}