Calling C# BHO methods from Javascript

I'm trying to figure out how to call C# methods in my BHO object from Javascript within the page. I found numerous pages on how to do it in C++/ATL/Com such as:

Browser Helper Objects and Scripts Options

Calling BHO method from Javascript?

I have tried to follow along and implement it correctly in C#, but I can't get it to work probably due to some obvious COM problems that I have which I don't fully understand.

I am using C# 4.0.

Here are the relevant parts of the code:

using SHDocVw;
using mshtml;
using System.Runtime.InteropServices;

 ProgId("My Extension")]
public class BrowserHelperObject : IObjectWithSite
    const int DISPATCH_PROPERTYPUT = 4;
    const int FDEX_NAME_ENSURE = 2;
    const uint LOCALE_USER_DEFAULT = 0x0400;

    WebBrowser browser;

    public void OnDocumentComplete(dynamic frame, ref dynamic url)
        var window = browser.Document.parentWindow;

        int pid = 0;
        window.GetDispId("myExtension", FDEX_NAME_ENSURE, ref pid);

        System.Runtime.InteropServices.ComTypes.DISPPARAMS dispParms = new System.Runtime.InteropServices.ComTypes.DISPPARAMS();
        dispParms.cArgs = 1;
        dispParms.cNamedArgs = 0;
        dispParms.rgvarg = ???;
        dispParms.rgdispidNamedArgs = IntPtr.Zero;
        System.Runtime.InteropServices.ComTypes.EXCEPINFO einfo = new System.Runtime.InteropServices.ComTypes.EXCEPINFO();
        window.Invoke(pid, LOCALE_USER_DEFAULT, DISPATCH_PROPERTYPUT, ref dispParms, this, ref einfo);            


I hate to answer my own question, but I really feel like the community ought to know the answer because it is short, simple, and beautiful with C# 4.0 and SO many people seem to have this problem.

Make sure that you correctly expose the Browser Helper Object:

 ClassInterface(ClassInterfaceType.None), ProgId("MyExtension"),
public class BrowserHelperObject : IObjectWithSite, IExtension
    public int Foo(string s) { ... }
    public void OnDocumentComplete(dynamic frame, ref dynamic url)
        dynamic window = browser.Document.parentWindow;
        IExpando windowEx = (IExpando)window;
        window.myExtension = this;

And you will need a definition for your extensions:

public interface IExtension
    int Foo(string s);

You can access your Browser Helper Object in javascript thus:

var result = window.myExtension.Foo("bar");

or just

var result = myExtension.Foo("bar");

That's it. Stop banging your head against the wall and go celebrate!

Also, after you add property you need to make sure to release COM references of window and windowEx

Need Your Help

c# looping through datatable, changing data

c# .net data-binding datatable

i need to loop through one of the columns of my datatable and do some string manipulation to it. can someone please give me an example of how i would loop through the table and update some data?

Any way to access Gearman administration?

php gearman

I want to be able to query a gearman server to determine how many instances of a worker I have running (basically I want to make sure that RunTaskA is available and RunTaskB is available if there a...