Question

I’m working on a project in which I’m writing a c# windows application that wants to get the current element under the cursor from a running instance of Internet Explorer.

I can get the handle of IHtmlDocument2 of the running instance of IE and I want to assign it the onmouseover event like this:

 DHTMLEventHandler myHandler = new DHTMLEventHandler(htmlDocument);
 myHandler.Handler += new DHTMLEvent(this.BrowserEventHandler);
 htmlDocument.onmouseover = myHandler;

And because I didn’t want the IE to eat other events (a famous but solved question), I should create a DHTMLEventHandler class like this:

public delegate void DHTMLEvent(IHTMLEventObj obj);

[ComVisible(true)]
public class DHTMLEventHandler
{
    public DHTMLEvent Handler;

    private IHTMLDocument2 Document;

    public DHTMLEventHandler(IHTMLDocument2 doc)
    {
        Document = doc;
    }

    [DispId(0)]
    public void Call()
    {
        Handler(Document.parentWindow.@event);
    }
}

And my BrowserEventHandler method is like this:

public void BrowserEventHandler(IHTMLEventObj e)
    {
    }

Still empty (but I want to get the element’s innerHtml under cursor), but still this method is empty and I get :

A first chance exception of type ‘System.InvalidCastException’ occurred in FindText.exe

Whenever I move the mouse.

Questions:

1. Why I’m getting this exception?
2. Is it possible to get e.srcElement.innerhtml in a variable and use in my c# application?

Let me know if there’s something unclear and ambiguous. Thanks in advance for any help and ideas. I greatly appreciate any comments and helps.

Update:

To answer my second question, it seems possible to get the e.srcElement.innerHTML of the IHTMLEventObj e but I really don’t know why the BrowserEventHandler method is never called. Any ideas or hints?

update2:

Ok, I found the problem. First I changed some settings of my project to get the line where the exception occurs first (since it’s a first chance exception) like this:

In visual studio: Menu >> Debug >> Exceptions >> CLR exceptions >> system and check the throw option of SystemInvalidCastException.

I found the problem is with this line of DHTMLEventHandler class:

Handler(Document.parentWindow.@event); 

It throws Invalid Cast Exception which I think is related to Threads according to my recent searches.

I guess I should set ApartmentState to ApartmentState.STA. But I don’t know how. any ideas?

Answer 1

Ok,I found the solution! 🙂

I needed to change DHTMLEventHandler class like this:

public delegate void DHTMLEvent(IHTMLEventObj obj);

[ComVisible(true)]
[ClassInterface(ClassInterfaceType.AutoDispatch)]
public class DHTMLEventHandler
{
    private Thread currentThread;
    public DHTMLEvent Handler;

    private IHTMLDocument2 Document;

    public DHTMLEventHandler(IHTMLDocument2 doc)
    {
        Document = doc;
    }

    [DispId(0)]
     [STAThread]
    public void Call()
    {
        currentThread = Thread.CurrentThread;
       Thread parentWin = new Thread(new ThreadStart(pWindowHandler));
        parentWin.SetApartmentState(ApartmentState.STA);
        parentWin.Start();
        currentThread.Suspend();
       // Handler(Document.parentWindow.@event);

    }
    public void pWindowHandler()
    {
        Handler(Document.parentWindow.@event);
        currentThread.Resume();
    }
}

But if anybody else tries to use my solution, I don’t know whether It has any side effects in other situations. At least it works fine for me! 🙂