How to use C# class library with webbrowser and rgiesecke.dllexport

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If I try to create a C# class library that shows a form with a webbrowser in it, the execution of the library shows the error: System.Threading.ThreadStateException: ActiveX control, and it does not allow to show the form and the application ends.

I have a console application made in C++, which loads a C# class library, which, when started, should show a form with a webbrowser in it, but when executing the code to show the form the application shows the previous error and finish.

I believe that the unmanaged package using RGiesecke.DllExport is incompatible with the normal threads of an application

What I have tried:

I try:

C#

[PermissionSet(SecurityAction.Demand, Name = "FullTrust")]
public class MainModule
{
    [DllExport(ExportName = "ObjectWebJS", CallingConvention =
    CallingConvention.StdCall)]
    static public int ObjectWebJS(int a, int b)
    {
        LoadForm();
        return 0;
    }
    [STAThread]
    public static void LoadForm()
    {
        ObjectForm form1 = new ObjectForm();
        form1.ShowDialog();
    }
}

Posted 22-Nov-18 10:45am

fisadmaster

Updated 23-Nov-18 2:17am

v3

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Comments

KarstenK 23-Nov-18 2:08am

The reason is that to show an UI the executing thread needs that rights. An app has this rights - but not a dll.

fisadmaster 23-Nov-18 4:45am

Thanks for the comment but you are wrong because when I do not use the RGiesecke package the C# class library works as it should and I can load it from a console application in C++ and show the form with the webbrowser.

What I understand is that the RGiesecke package causes the C# class library to run on a multi thread and that is incompatible with the webbrowser component.

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Richard Deeming · Accepted Answer · 2018-11-23T02:18:00

STAThreadAttribute Class (System) | Microsoft Docs[^]:
Apply this attribute to the entry point method (the Main() method in C# and Visual Basic). It has no effect on other methods.

Your calling code is in an MTA thread. Applying the [STAThread] attribute to the method it calls will have no effect.

You need to test the current thread's apartment state, and spin up a new STA thread if necessary.

C#

public static void LoadForm()
{
    if (Thread.CurrentThread.ApartmentState == ApartmentState.STA)
    {
        LoadFormCore();
    }
    else
    {
        ThreadStart run = LoadFormCore;
        Thread t = new Thread(run);
        t.SetApartmentState(ApartmentState.STA);
        t.Start();
        t.Join();
    }
}

private static void LoadFormCore()
{
    ObjectForm form1 = new ObjectForm();
    form1.ShowDialog();
}