using System.Windows.Forms;
using System.Threading;
// ...
[DllImport( "user32.dll" )]
public static extern IntPtr GetForegroundWindow();

class Win32Window:IWin32Window
{   public IntPtr Handle
    {   get
        {   return handle;  }
    }
    private IntPtr handle;

    public Win32Window(IntPtr wndHandle)
    {   handle = wndHandle;  }
}

private string fullFileName = "";

private void showDialog()
{   IntPtr b1WindowHandle = GetForegroundWindow();
    Win32Window b1Window       = new Win32Window(b1WindowHandle);
    OpenFileDialog ofd = new OpenFileDialog();
    ofd.Title = "Test";
    ofd.InitialDirectory = "";
    ofd.Filter = "Any file|*.*";
    if ( ofd.ShowDialog( b1Window ) == DialogResult.OK ) // Returns immediately
    //if ( ofd.ShowDialog()         == DialogResult.OK )  // Works
    {   fullFileName = ofd.FileName;  }
}

private bool OnMenu(MenuEvent me) // Call this in a menu event handler
{   string msg;
    Thread dlgThread = new Thread(showDialog);
    dlgThread.SetApartmentState(ApartmentState.STA);
    dlgThread.Start();
    dlgThread.Join();
    // Ui.App below is a SAPbouiCOM.Application:
    if (fullFileName == "")
    {   msg = "No file has been selected.";  }
    else
    {   msg = "Selected file is: " + fullFileName;  }
    Ui.App.MessageBox(msg, 1, "", "", "");
    return false; // this is for BubbleEvent;
}