Single-threaded apartment - cannot instantiate ActiveX control

Question

I need to get information about applied CSS styles in HTML page. I used AxWebBrowser and iterate IHTMLDOMNode. I'm able to get all the data I need and move the code into my application. The problem is that this part is running inside of the background worker and I got exception when trying to instantiate the control.

AxWebBrowser browser = new AxWebBrowser();

ActiveX control '8856f961-340a-11d0-a96b-00c04fd705a2' cannot be instantiated
because the current thread is not in a single-threaded apartment.

Is there any way how to solve this or other option than AxWebBrowser?

score 95 · Accepted Answer · edited Mar 30 '11 at 20:58

95

The problem you're running into is that most background thread / worker APIs will create the thread in a Multithreaded Apartment state. The error message indicates that the control requires the thread be a Single Threaded Apartment.

You can work around this by creating a thread yourself and specifying the STA apartment state on the thread.

var t = new Thread(MyThreadStartMethod);
t.SetApartmentState(ApartmentState.STA);
t.Start();

edited Mar 30 '11 at 20:58

Factor Mystic

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answered Sep 13 '09 at 18:23

JaredPar

733,204
149
1,241
1,454

1

Thanks, great this is working. One more questions/problem. The class I'm using is just class and the AxWebBrowser looks like it needs to be added into this.Controls(). Is there way how to fake the Controls? Or will I need to have separated Form for that? – martin.malek Sep 13 '09 at 18:36
@martin.malek There's no great way to fake that. The best bet is to create a new form. – JaredPar Sep 13 '09 at 18:46
1

Hi, the code should be t.SetApartmentState(ApartmentState.STA); – Santiago Corredoira Nov 28 '10 at 12:55
Confirmed Dawkins comment, I also had to change it to AppartmentState.STA. – Boog Mar 08 '11 at 18:37
I've met this error when I want to open a windowsfrom from my XNA-Game. I've opened the form with JaredPar's code and it works. Can I carry this code inside the windows form's code ? – icaptan Sep 07 '11 at 22:19
2

Beware that this code is not enough to properly implement an STA thread, it must also pump a message loop. Particularly WebBrowser will malfunction, it will not fire its DocumentCompleted event. Check [this post](http://stackoverflow.com/a/21684059/17034) for an alternative. – Hans Passant Apr 18 '14 at 10:29
How do you do this in the Compact Framework. (i.e. compact framework has no `[STAThread]` ability.) – bvdb Jul 28 '16 at 10:02
I am using Task, is there any solution? – camino Dec 25 '21 at 11:50

score 66 · Answer 2 · answered May 21 '11 at 17:37

Go ahead and add [STAThread] to the main entry of your application, this indicates the COM threading model is single-threaded apartment (STA)

example:

static class Program
{
    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new WebBrowser());
    }
}

doesnt work in framework 4.5 - gives invalid arguments on last line — pat capozzi, Nov 27 '13 at 17:16

Ahmad · Answer 3 · 2014-12-12T18:55:46.077

4

If you used [STAThread] to the main entry of your application and still get the error you may need to make a Thread-Safe call to the control... something like below. In my case with the same problem the following solution worked!

Private void YourFunc(..)
{
    if (this.InvokeRequired)
    {
        Invoke(new MethodInvoker(delegate()
        {
           // Call your method YourFunc(..);
        }));
    }
    else
    {
        ///
    }

edited Dec 12 '14 at 18:55

answered Dec 12 '14 at 18:16

Ahmad

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What should be invoked in here? The browser? In which state? In my case the browser is sitting on a dialog. – C4d Nov 08 '17 at 07:47

score 0 · Answer 4 · answered Oct 13 '21 at 11:04

0

My problem with this was that I had my Main method marked as async. So fixed it just by making it static void and adding .wait() to all awaitable methods inside.

answered Oct 13 '21 at 11:04

Grengas

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Single-threaded apartment - cannot instantiate ActiveX control

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