IE BHO loaded two times for each tab

Question

I am creating IE BHO using this example: http://code.msdn.microsoft.com/CSBrowserHelperObject-c5f3d2f6

The problem I am having is that for each tab BHO is loaded two times. I have inserted following core in BHO constructor:

        Trace.WriteLine("BHO.ctor");
        Trace.WriteLine("Thread: " + Thread.CurrentThread.ManagedThreadId + " of process: " + Process.GetCurrentProcess().Id);

In destructor:

        Trace.WriteLine("BHO.destructor");
        Trace.WriteLine("Thread: " + Thread.CurrentThread.ManagedThreadId + " of process: " + Process.GetCurrentProcess().Id);

Then in Setsite:

        Trace.WriteLine("Bho.SetSite: " + site);
        Trace.WriteLine("Thread: " + Thread.CurrentThread.ManagedThreadId + " of process: " + Process.GetCurrentProcess().Id);

When I start IE in debug output I see:

[3376] BHO.ctor 
[3376] Thread: 1 of process: 3376 
[3376] Bho.SetSite: System.__ComObject 
[3376] Thread: 1 of process: 3376 
[3376] BHO.ctor 
[3376] Thread: 3 of process: 3376 
[3376] Bho.SetSite: System.__ComObject 
[3376] Thread: 3 of process: 3376

and only when I close it:

[3376] BHO.destructor 
[3376] Thread: 2 of process: 3376 
[3376] BHO.destructor 
[3376] Thread: 2 of process: 3376 

So, BHO is loaded two times and two instances of BHO exist at the same time. Why can this happen?

I discovered this issue when my BHO subscribed to events from third party assembly and I have experienced some strange behaviour (because there was more than 1 subscriber for the event). First I thought that it's a third-party library who causes this but now I removed all extra code from BHO so it basically does nothing but still is loaded two times.

Also if in SetSite I add OnDocumentComplete handler and then in this handler I write URL to debug console using trace I see that for first thread URL will be actual URL of my page (eg www.google.com) and for second thread it will be "about:blank". Also if I open one more tab and do something there and then switch to the first tab I see that one more BHO instance is created (URL: about:blank)

Answer 1

This may be IE's integrity level management.

I'm trying to solve this myself.

I'm currently considering:

http://blogs.msdn.com/b/ieinternals/archive/2011/08/03/internet-explorer-automation-protected-mode-lcie-default-integrity-level-medium.aspx

The second one seems to disappear after a second or so.

Answer 2

What you saw is called virtual-tabs, mentioned only briefly in the docs . We became aware of it only recently after contacting MS support - but these are the exact symptoms (many 'ghost' navigation cycles to about:blank).

If you want to detect them (to ignore, most probably), the suggested semi-official way is -

1. Obtain an IWebBrowser2 interface,
2. Extract an HWND from it,
3. Check this HWND visibility with IsWindowVisible. If it is not visible - it is a virtual tab.