WPF - Hidden Window Shows In Screenshot - wpf

Question

My simple goal is to take a screenshot.
My application has a main window that has multiple purposes and one of them is taking a screenshot.
What I do is hide the window by using MainWindow.Hide(); and then showing a second window that handles the screenshot taking process.
this.Hide();
SnapshotManager snapshotMgr = new SnapshotManager();
snapshotMgr.Closed += SnapshotMgrClosed;
snapshotMgr.Show();
The second window is basically this:
<Grid>
<Image Name="MainImage" Stretch="None" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
</Image>
<Border BorderBrush="Transparent" BorderThickness="0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<Border.Background>
<SolidColorBrush Color="Black" Opacity="0.4"/>
</Border.Background>
</Border>
</Grid>
with the properties:
AllowsTransparency="True"
Background="Transparent"
WindowStyle="None"
When the user clicks a button to take the screenshot I hide the main window and show this one.
Then I save the screen to the image in my second window like so:
Bitmap snapshot = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);
Graphics graphics = Graphics.FromImage(snapshot);
graphics.CopyFromScreen(new System.Drawing.Point(0,0), new System.Drawing.Point(0,0), Screen.PrimaryScreen.Bounds.Size, CopyPixelOperation.SourceCopy);
//-----------------------------------
// Save the screen to MainImage
//-----------------------------------
MemoryStream ms = new MemoryStream();
snapshot.Save(ms, ImageFormat.Bmp);
ms.Position = 0;
BitmapImage bi = new BitmapImage();
bi.BeginInit();
bi.StreamSource = ms;
bi.EndInit();
MainImage.Source = bi;
The problem is that for some reason my main window shows up in the screenshot.
The MainWindow.Hide() probably doesn't work like I think it does.
How can I take a screenshot with the main window?
Thank you!
Edit:
The following code works but uses Sleep(200) which I really would like to avoid.
private void OnTakeSnapshot(object sender, RoutedEventArgs e)
{
this.LayoutUpdated += RunSnapshotMgr;
this.Hide();
}
private void RunSnapshotMgr(object sender, EventArgs e)
{
if (IsVisible == true) return;
Thread.Sleep(200);
SnapshotManager snapshotMgr = new SnapshotManager();
snapshotMgr.Closed += SnapshotMgrClosed;
snapshotMgr.Show();
this.LayoutUpdated -= RunSnapshotMgr;
}