How to Build a Simple HTTP Proxy in CSharp in just 25 lines of code

It's easy to build a basic proxy server in C# using the .NET framework. The key is using the HttpListener and WebClient classes.

First we need to add the necessary namespaces:

using System;
using System.Net;
using System.IO;

Then we create an HttpListener to listen for incoming requests:

HttpListener listener = new HttpListener();
listener.Prefixes.Add("<http://localhost:9097/>");
listener.Start();

Next we need an event handler when a request comes in:

listener.BeginGetContext(new AsyncCallback(HandleRequest), listener);

The handler method will fetch the target URL and stream it back to the client:

static void HandleRequest(IAsyncResult result) {
  HttpListener listener = (HttpListener)result.AsyncState;

  HttpListenerContext context = listener.EndGetContext(result);

  string url = context.Request.Url.AbsolutePath.Substring(1);

  WebClient client = new WebClient();
  context.Response.OutputStream.Write(client.DownloadData(url), 0, client.DownloadData(url).Length);

  context.Response.OutputStream.Close();
}

We slice off the leading slash and use WebClient to download and stream back the response.

Putting it all together:

using System;
using System.Net;
using System.IO;

class Proxy {

  static void Main() {
    HttpListener listener = new HttpListener();
    listener.Prefixes.Add("<http://localhost:9097/>");
    listener.Start();

    listener.BeginGetContext(new AsyncCallback(HandleRequest), listener);

    Console.ReadLine();
    listener.Close();
  }

  static void HandleRequest(IAsyncResult result) {
    //...handle request
  }

}

This gives you a simple 25 line proxy server in C#. Of course there is still a lot more you would want to add for a robust production proxy. But it shows how easy the .NET framework makes building basic network apps.

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