Operating a runtime webbrowser control from a background thread vb.net

Question

I'm trying to use a bunch of webbrowsers in some background threads. This works no problem when I use webbrowser controls that i have placed on the form in design view but now when they are created at runtime.

I declare the webbrowsers array globally:

Dim webbroswers(-1) As WebBrowser

The following code is on the main thread:

ReDim Preserve webbroswers(somenum)

  For i = 0 To sumnum

            webbroswers(currentbrowsermax + i) = New WebBrowser

  Next

Then this code is run on the background thread:

If webbroswers(num).InvokeRequired Then
    webbroswers(num).Invoke(Sub() webbroswers(num).Navigate(someurl))
Else
    webbroswers(num).Invoke(Sub() webbroswers(num).Navigate(someurl))

The program crashes at this point with the following error:

Unable to get the window handle for the 'WebBrowser' control. Windowless ActiveX controls are not supported.

Any help on this would be great. Also if anyone knows how to suppress script errors then I think this might help. I've tried: WebBrowser(num).ScriptErrorsSuppressed = True but this doesn't work (it doesn't work elsewhere in my code when running on the main thread either) Thanks!

I am not sure about what you are intending to do in the condition which crashes. Also not sure about why you want to use BWG: it is meant for "non-GUI actions" and a browser is a GUI element (and thus has to be run in the main thread). Please, describe the exact conditions you want to account for. — varocarbas
Basically I have some code that does stuff with the webbroswer. I want to make this multithreaded so it can do it with multiple webbrowsers (pointed to slightly different addresses) — FraserOfSmeg
Example where you can use the BGW: the browser1 connects to url1 and starts downloading stuff; meanwhile you want to allow users to use the browser2 to go anywhere they want. That is, the BWG is not meant to deal with browser1 and browser2 (navigating to different urls), but to allow a GUI-element to be shown while expensive, non-GUI actions are performed (in this case, browser1 downloading things). If all (or most) of what you want to do requires the GUI (for example: having two browsers opened just used to navigate), the BWG wouldn't be required. — varocarbas
Actually, even in the aforementioned example, the BGW wouldn't make too much sense: if you want to ignore the GUI elements of one webbrowser, just make it invisible. The true utility of the BGW is when you cannot hide the GUI; for example: using webbrowser1 to navigate to other urls, while it is downloading from url1. — varocarbas
What are you trying to do? The WebBrowser needs an event-loop in an STA thread to work. If you want to retrieve content without display then the WebClient is your best choice. — Pragmateek

Hans Passant Hans Passant · Accepted Answer · 2013-11-11T17:29:03

The Control.InvokeRequired and Invoke members use the Handle property to figure out what thread owns the control. Problem is, the Handle is null for the web browsers that you created. A control only has a valid handle when you made it visible on a form. Which you didn't do. It will then try to create the handle but that's a fail whale, an ActiveX control like WebBrowser needs a valid Parent. Without Me.Control.Add(), as was done in your original version, it won't have one.

The workaround is simple, you just need another control with a valid Handle property. Any will do, it only cares about the thread that owns the handle, not the value of the handle.

You have one: your form. Use Me.InvokeRequired and Me.Invoke() instead. Or Application.OpenForms(0) if you can't easily get a reference to the form object, best avoided.

Operating a runtime webbrowser control from a background thread vb.net

1 Answers