WebBrowser control: "Specified cast is not valid."

Question

I used a WebBrowser control to navigate to the login page for a WordPress blog. The page loads fine but whenever I try to access the WebBrowser from a thread. I get a specific cast is not valid exception. Also when debugging, everything freezes up for around 5 seconds. When debugging and I try to access the control. I get timed out errors on all of the member variables.

//in constructor of main form
Thread.CurrentThread.ApartmentState = ApartmentState.STA;
this.CheckForIllegalCrossThreadCalls = false;

mainThreadHandle = new Thread(mainThread);
mainThreadHandle.Start();

private void mainThread()
{
    wbMain.Navigate("http://example.com/");

    //navigating is set to false in the document complete event.
    navigating = true;

    while (navigating == true)
        Thread.Sleep(5000);

    try
    {
        //Where I get the issues
        MessageBox.Show(wbMain.DocumentText);
    }
    catch (Exception e)
    {

    }

    Thread.Sleep(1000);
}

Can you post the code where the `Specified cast is not valid` exception? — Bala R, May 01 '11 at 14:01
You have a bug in your code. You need to look at your code and find the bug. — SLaks, May 01 '11 at 14:02
The code freeze for 5 seconds because you send it to sleep for 5000 miliseconds. — Shadow The GPT Wizard, May 01 '11 at 14:24
@Shadow Wizard I said freeze, not sleep. I know the difference. It freezes for around 5 seconds whenever I try to access the control itself. My entire IDE freezes. — Drake, May 01 '11 at 14:32
Were you able to resolve it? I have encountered a similar issue - http://stackoverflow.com/questions/22820382/how-to-update-documenttext-with-another-documenttext — LCJ, Apr 02 '14 at 18:52

score 9 · Accepted Answer · edited May 23 '17 at 12:03

WebBrowser is a COM component under the hood. An apartment threaded one, COM takes care of calling its methods in a thread-safe way. Your Navigate() call works for that reason, it is actually executed on the UI thread. What doesn't work is the DocumentText property, it is implemented in the .NET wrapper and they somewhat fumbled the code. It bombs when the COM interop support in the CLR notices that a thread in the MTA tries to access a property of a component that lives on an STA.

Your call to SetApartmentState() isn't correct. It is made on the wrong thread, the UI thread already is STA. Also the reason it doesn't bomb, you cannot change the apartment state of a thread after it is started. You must call it on the Thread object you created. It still doesn't solve your problem, two STA threads are not compatible.

Two basic ways to solve your problem. The first one is that you create the WebBrowser object itself on a separate STA thread. The code in this answer shows you how to do that.

The browser you create that way cannot also be visible on your form. Which is the second way, marshal the call yourself with Control.Invoke(). Doing this is however pretty pointless, all of your code executes on the UI thread anyway, you get no concurrency. There is no free lunch here. Running it on a thread only gives you headaches. If you need time to process the document text then run that code on a separate thread.

Thanks for the info.. I have a related question as an extention to this http://stackoverflow.com/questions/22820382/how-to-update-documenttext-with-another-documenttext — LCJ, Apr 02 '14 at 19:19

score 1 · Answer 2 · answered May 01 '11 at 14:18

1

Why not use event model when work with WebBrowser control?

Navigating, Navigated, ProgressChanged, DocumentCompleted

answered May 01 '11 at 14:18

abatishchev

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I am for getting the state of the browser, but for the task I'm doing it requires a separate thread to not hang the app. – Drake May 01 '11 at 14:26

WebBrowser control: "Specified cast is not valid."

2 Answers2

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