Invoke a javascript function with VB.net through COM Interop

Question

I have a VB.net class registered for COM interop which I'm instantiating within an HTML page using the following code:

<script type="text/javascript">
var MyClass = new ActiveXObject("Namespace.TestClass");
</script>

I can call methods on it just fine, but suppose I want to set a javascript function as a property, like so:

MyClass.TestFunction = function () { alert("It worked!"); }

How would I set my vb.net code up to be able to fire that function? This is how MSXML works in javascript for XMLHttpRequest objects, you can set

XHR.onreadystatechange = function () {}

I'm looking for a similar implementation in my class.

Answer 1

You have to expose a COM event, and assign the JavaScript method to that event. This way, when you invoke the event in your code, the JavaScript method will be called.

Example -

C# Code

[ComVisible(false)]
public delegate void OperationCompleted(string message); //No need to expose this delegate

public event OperationCompleted OnOperationCompleted;

if(OnOperationCompleted != null)
    OnOperationCompleted("Hello World!");

JavaScript

comObject.OnOperationCompleted = function(message) { alert(message); }

Note: I have done this before. And I guess there was some COM related error. To resolve it I had to attach some attribute somewhere in the code (I don't remember it exactly right now). But you'll be able to figure it out or google it.

Answer 2

After trying for a while, we managed to figure out a solution that worked pretty well. Since we're setting a javascript function to the property, all the properties and methods on that function are made available to the VB.net, including the javascript standard methods, call and apply:

(function () { alert("Hello"); }).call();

The solution was to just invoke the call method in the VB.net code and it seems to work pretty well.