WindowsFormsHost steals focus on activating application even activating by application's other wpf form

Question

To Reproduce my case (.net 4.0)

1. Create a WPF Application (MainWindow.xaml)
2. Add a Winform user control that contains a textbox (UserConrol1.cs - Winform)
3. Put UserControl1 into MainWindow.xaml with windowsformshost
4. Add another WPF Window that contains a textbox(wpf) to project (Window1.xaml)
5. Create and Show Window1 after MainWindow InitializeComponent

Your project is ready,

1. Run Project and set textbox focused in MainWindow.xaml (that in WindowsFormsHost)
2. Deactivate your application by opening a window (Windows file explorer ,notepad, winamp etc.)
3. Try to write in textbox that in Window1 window by clicking textbox with mouse

And you will see that you can't set focus on textbox in Window1 because MainWindow Texbox( in winformshost will steal your focus on you application got activating)

Any idea?

MainWindow.xaml

<Window x:Class="WinFormsHostFocusProblem.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WinFormsHostFocusProblem"
        xmlns:my="clr-namespace:System.Windows.Forms.Integration;assembly=WindowsFormsIntegration"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
      <my:WindowsFormsHost  Focusable="False"  >
         <local:UserControl1>

         </local:UserControl1>
      </my:WindowsFormsHost>

   </Grid>
</Window>

MainWindow.xaml.cs

namespace WinFormsHostFocusProblem
{
   public partial class MainWindow : Window
   {
      public MainWindow()
      {
         InitializeComponent();
         Window1 window1 = new Window1();
         window1.Show();
      }
   }
}

Window1.xaml

<Window x:Class="WinFormsHostFocusProblem.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WinFormsHostFocusProblem"
        xmlns:my="clr-namespace:System.Windows.Forms.Integration;assembly=WindowsFormsIntegration"
        SizeToContent="WidthAndHeight" 
        ResizeMode="NoResize"

        Topmost="True"
        Title="Window1" Height="300" Width="300" Background="Red">
    <Grid>
      <TextBox Height="25">asd</TextBox>
   </Grid>
</Window>

Window1.xaml.cs

namespace WinFormsHostFocusProblem
{
   public partial class Window1 : Window
   {
      public Window1()
      {
         InitializeComponent();
      }
   }
}

Answer 1

We had a similar problem in one of our applications and found that upgrading to .net 4.5 seems to have fixed a good portion of our application's WPF/WinForms focus issues including a similar one to this.

In addition, the _focusedChild field no longer exists in the .net 4.5 version of WindowsFormsHost

Answer 2

I used my MSDN support contract to get an answer to this problem. The engineer was able to repro from yunusayd's sample and confirmed it is almost certainly a bug in WindowsFormsHost.

Thanks to yunus for the minimal repro sample and Keith at Microsoft for tackling the issue and providing a workaround in less than one day.

Workaround code follows. It works by using .NET reflection to change a private variable used in WindowsFormsHost and disable the trigger for the bug. According to the engineer I worked with, this relies on WPF internals, but he spoke with product team members and it should be safe to use. There's no guarantee of lack of side effects, of course, but so far I haven't found any problems in my testing with multiple WindowsFormsHosts in multiple WPF windows (maybe nesting would be trickier). I modified the original workaround to work generically with multiple windows. You can just as easily hardcode references to specific windows and named WindowsFormsHost controls in the Application_Deactivated event and skip the whole "LastActive" scheme and extension methods.

// App.xaml.cs: you must hook up to Application.Deactivated
void Application_Deactivated(object sender, EventArgs e)
{
    foreach (Window w in windows)
    {
        foreach (var host in UI.DependencyObjectExtension.AllLogicalChildren(w).
                     Where(c => c is WindowsFormsHost))
        {
            FIELD_FOCUSED_CHILD.SetValue(host, null);
        }
    }
}


public readonly static FieldInfo FIELD_FOCUSED_CHILD = typeof(System.Windows.Forms.Integration.WindowsFormsHost).
    GetField("_focusedChild", BindingFlags.NonPublic | BindingFlags.Instance);

public static class DependencyObjectExtension
{
    /// <summary>
    /// Returns a collection of o's logical children, recursively.
    /// </summary>
    /// <param name="o"></param>
    /// <returns></returns>
    public static IEnumerable<DependencyObject> AllLogicalChildren(this DependencyObject o)
    {
        foreach (var child in LogicalTreeHelper.GetChildren(o))
        {
            if (child is DependencyObject)
            {
                yield return (DependencyObject)child;

                if (child is DependencyObject)
                {
                    foreach (var innerChild in AllLogicalChildren((DependencyObject)child))
                    {
                        yield return innerChild;
                    }
                }
            }
        }
    }
}