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C# HttpListener

last modified July 4, 2022

In this article, we show how to create simple HTTP servers in C# using HttpListener class.

The Hypertext Transfer Protocol (HTTP) is an application protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web.

HttpListener is a simple, programmatically controlled HTTP protocol listener. It can be used to create HTTP servers. It is located in the System.Net namespace.

An HTTP server uses HTTP (Hypertext Transfer Protocol) to respond to client requests made by browsers, crawlers, or tools such as curl or wget. It responds with a web resource (such as HTML page, images, or multimedia) or an HTTP error message.

C# HttpListener status

In the first example, the server responds with a status code. HTTP response status codes indicate whether a specific HTTP request has been successfully completed

Program.cs
using System.Net;

using var listener = new HttpListener();
listener.Prefixes.Add("http://localhost:8001/");

listener.Start();

Console.WriteLine("Listening on port 8001...");

while (true)
{
    HttpListenerContext ctx = listener.GetContext();
    using HttpListenerResponse resp = ctx.Response;

    resp.StatusCode = (int) HttpStatusCode.OK;
    resp.StatusDescription = "Status OK";
}

In the example, the listener responds with HttpStatusCode.OK.

using var listener = new HttpListener();
listener.Prefixes.Add("http://localhost:8001/");

We create an instance of HttpListener and add an URL and port on which the listener listens.

listener.Start();

With Start, we begin to receive requests.

while (true)
{
    HttpListenerContext ctx = listener.GetContext();
    using HttpListenerResponse resp = ctx.Response;

Inside the loop, call GetContext which waits for an incoming request and returns when one is received. From the context we get the response object. This is the object that is sent back to the client.

resp.StatusCode = (int) HttpStatusCode.OK;
resp.StatusDescription = "Status OK";

We write the stats code and description to the response object.

$ curl localhost:8001 -i
HTTP/1.1 200 Status OK
Server: Microsoft-NetCore/2.0
Date: Mon, 04 Jul 2022 12:38:24 GMT
Transfer-Encoding: chunked

C# HttpListener send text

The next example sends text data to the client.

Program.cs
using System.Net;
using System.Text;

using var listener = new HttpListener();
listener.Prefixes.Add("http://localhost:8001/");

listener.Start();

Console.WriteLine("Listening on port 8001...");

while (true)
{
    HttpListenerContext context = listener.GetContext();
    HttpListenerRequest req = context.Request;

    Console.WriteLine($"Received request for {req.Url}");

    using HttpListenerResponse resp = context.Response;
    resp.Headers.Set("Content-Type", "text/plain");

    string data = "Hello there!";
    byte[] buffer = Encoding.UTF8.GetBytes(data);
    resp.ContentLength64 = buffer.Length;

    using Stream ros = resp.OutputStream;
    ros.Write(buffer, 0, buffer.Length);
}

The server sends a text message.

HttpListenerRequest req = context.Request;

Console.WriteLine($"Received request for {req.Url}");

On the server side, we also log the request URL.

using HttpListenerResponse resp = context.Response;
resp.Headers.Set("Content-Type", "text/plain");

In the response headers, we set the Content-Type to text/plain to hint the client what kind of data to expect.

string data = "Hello there!";
byte[] buffer = Encoding.UTF8.GetBytes(data);

We transform the message into bytes.

resp.ContentLength64 = buffer.Length;

We set the content length.

using Stream ros = resp.OutputStream;
ros.Write(buffer, 0, buffer.Length);

Finally, we write the bytes to the output stream.

$ curl localhost:8001
Hello there!

C# HttpListener user agent

The User-Agent request header is a string that lets servers and network peers identify the application, operating system, vendor, and/or version of the requesting user agent.

Program.cs
using System.Net;
using System.Text;

using var listener = new HttpListener();
listener.Prefixes.Add("http://localhost:8001/");

listener.Start();

Console.WriteLine("Listening on port 8001...");

while (true)
{
    HttpListenerContext ctx = listener.GetContext();
    HttpListenerRequest req = ctx.Request;

    string? ua = req.Headers.Get("User-Agent");

    using HttpListenerResponse resp = ctx.Response;
    resp.Headers.Set("Content-Type", "text/plain");

    string data = ua ?? "unknown";
    byte[] buffer = Encoding.UTF8.GetBytes(data);
    resp.ContentLength64 = buffer.Length;

    using Stream ros = resp.OutputStream;
    ros.Write(buffer, 0, buffer.Length);
}

The listener retrieves the User-Agent header from the client request and sends it back.

string? ua = req.Headers.Get("User-Agent");

From the request headers, we get the User-Agent field.

string data = ua ?? "unknown";
byte[] buffer = Encoding.UTF8.GetBytes(data);
resp.ContentLength64 = buffer.Length;

using Stream ros = resp.OutputStream;
ros.Write(buffer, 0, buffer.Length);

We turn the text value into bytes and write the bytes into the response output stream.


Program.cs
var url = "http://localhost:8001";

using var client = new HttpClient();
client.DefaultRequestHeaders.Add("User-Agent", "C# program");

var res = await client.GetStringAsync(url);
Console.WriteLine(res);

We create an HttpClient which sends a request to the server; it also sets the User-Agent header field.

$ dotnet run
C# program

C# HttpListener send image

In the next example, the server sends an image.

Program.cs
using System.Net;
using System.Text;

using var listener = new HttpListener();
listener.Prefixes.Add("http://localhost:8001/");

listener.Start();

Console.WriteLine("Listening on port 8001...");

while (true)
{
    HttpListenerContext ctx = listener.GetContext();
    HttpListenerRequest req = ctx.Request;

    string? path = ctx.Request.Url?.LocalPath;

    if (path == "/image")
    {
        sendImage(ctx);
    }
    else
    {
        notFound(ctx);
    }
}

void notFound(HttpListenerContext ctx)
{
    using HttpListenerResponse resp = ctx.Response;
    resp.Headers.Set("Content-Type", "text/plain");

    using Stream ros = resp.OutputStream;

    ctx.Response.StatusCode = (int)HttpStatusCode.NotFound;
    string err = "404 - not found";

    byte[] ebuf = Encoding.UTF8.GetBytes(err);
    resp.ContentLength64 = ebuf.Length;

    ros.Write(ebuf, 0, ebuf.Length);
}

void sendImage(HttpListenerContext ctx)
{
    using HttpListenerResponse resp = ctx.Response;
    resp.Headers.Set("Content-Type", "image/png");

    byte[] buf = File.ReadAllBytes("public/img/sid.png");
    resp.ContentLength64 = buf.Length;

    using Stream ros = resp.OutputStream;
    ros.Write(buf, 0, buf.Length);
}

Inside the public/img directory, we have a PNG file.

string? path = ctx.Request.Url?.LocalPath;

From the request, we get the local path value.

if (path == "/image")
{
    sendImage(ctx);
}
else
{
    notFound(ctx);
}

If the path is equal to /image, we send the image with sendImage. Otherwise, we send the not found error.

void notFound(HttpListenerContext ctx)
{
    using HttpListenerResponse resp = ctx.Response;
    resp.Headers.Set("Content-Type", "text/plain");

    using Stream ros = resp.OutputStream;

    ctx.Response.StatusCode = (int)HttpStatusCode.NotFound;
    string err = "404 - not found";

    byte[] ebuf = Encoding.UTF8.GetBytes(err);
    resp.ContentLength64 = ebuf.Length;

    ros.Write(ebuf, 0, ebuf.Length);
}

The notFound method sends 404 error message back to the client. It is a standard HTTP error message indicating that the resource was not found.

void sendImage(HttpListenerContext ctx)
{
    using HttpListenerResponse resp = ctx.Response;
    resp.Headers.Set("Content-Type", "image/png");

    byte[] buf = File.ReadAllBytes("public/img/sid.png");
    resp.ContentLength64 = buf.Length;

    using Stream ros = resp.OutputStream;
    ros.Write(buf, 0, buf.Length);
}

Inside sendImage, we read the image with File.ReadAllBytes and write the array of bytes to the output stream with Write. We also set the appropriate image/png content type.

In this article, we have used HttpListener to create simple HTTP servers.

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