Home > Uncategorized > Intercepting #Ajax requests in #CEFSharp (Chrome for C#)

Intercepting #Ajax requests in #CEFSharp (Chrome for C#)


The webbrowser control for .NET is a version of IE. That version of IE can be configured via the registry, but it’s still IE, and Chrome is the daddy of browsers. So, if you want to embed a Chrome browser in a C# application, then using CEFSharp is the way to go.

Firstly, you need to install the NUGET package as follows;

Install-Package CefSharp.WinForms

You then will have to change your project to either x86 or x64, since this won’t work as AnyCPU.

Define a reference to ChromiumWebBrowser in your app as follows;

private static ChromiumWebBrowser browser;

Then initialise it using the code;

CefSharpSettings.LegacyJavascriptBindingEnabled = true;
var settings = new CefSettings();
var strInitialUrl = “http://www.yourwebsite.com”;
browser = new ChromiumWebBrowser(strInitialUrl)
RenderProcessMessageHandler = new RenderHandler()
InteractionHandler.browser = browser;
var jsInterface = new jsMapInterface
onHookEvent = InteractionHandler.HookEventHandler
browser.JavascriptObjectRepository.Settings.LegacyBindingEnabled = true;
browser.JavascriptObjectRepository.Register(“jsInterface”, jsInterface, isAsync: true, options: BindingOptions.DefaultBinder);
browser.Dock = DockStyle.Fill;

Now, I have introduced a few new classes here, that haven’t been defined yet, but we’ll get to them

  • RenderHandler
  • jsMapInterface
  • InteractionHandler

So, let’s start with RenderHandler. This will be used to inject Javascript into the page once it is loaded. The Javascript will call out to jsMapInterface to record the Ajax event.

class RenderHandler : IRenderProcessMessageHandler
public void OnContextReleased(IWebBrowser browserControl, IBrowser browser, IFrame frame)

public void OnFocusedNodeChanged(IWebBrowser browserControl, IBrowser browser, IFrame frame, IDomNode node)

public void OnUncaughtException(IWebBrowser browserControl, IBrowser browser, IFrame frame, JavascriptException exception)

void IRenderProcessMessageHandler.OnContextCreated(IWebBrowser browserControl, IBrowser browser, IFrame frame)
var strScript = File.ReadAllText(@”inject.js”);

This class does nothing apart for waiting for OnContextCreated to fire, and read the content of Inject.js and put it into the page.

Inject.js contains the following

function () {
(function(open) {
XMLHttpRequest.prototype.open = function(method, url, async, user, pass) {
function() {
if (this.readyState === 4) {
window.jsInterface.hook(this.responseText, method, url, async, user, pass, “Context”);
open.call(this, method, url, async, user, pass);

This file should be set to “Copy Always”. Note the window.jsInterface.hook – this is the call back to the C# Code.

Next we define jsInterface

public class jsMapInterface
public Action<hookEvent> onHookEvent = null;

public void hook(string message, string method, string url, bool async, string user, string pass, string context) // Must be lowercase!
if (onHookEvent != null)
onHookEvent(new hookEvent()
message = message,
url = url,
async = async,
user = user,
pass = pass,
context = context

public class hookEvent
public string message {get; set;}
public string url {get; set;}
public bool async {get; set;}
public string user {get; set;}
public string pass {get; set;}
public string context { get; set; }

A very important point to note here that the method MUST be lowercase!

And Finally, we define InteractionHandler as follows;

class InteractionHandler
public static ChromiumWebBrowser browser;

public static void HookEventHandler(jsMapInterface.hookEvent hookEventArgs)

// And your AJAX response will be caught here!


You can set a breakpoint on the comment above, to log, or otherwise handle your ajax response that you capture.

Categories: Uncategorized
  1. david
    September 5, 2019 at 10:57 pm

    very useful tutorial
    i have question, how can get the method of that ajax requet and how resend that captured request.
    thanks for your attention.


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