How to take a screenshot of the content of a ScrollViewer - wpf

Question

I'd like to take a screenshot of whatever is inside a ScrollViewer, not just what is seen inside the ViewPort. But everything I tried so far results in completely black images or in images where only parts of the content are shown and everything else is black. I assumed that it's maybe because I didn't specify any backgrounds, but changing the background of my Window or my TreeView to White didn't change a thing.
Can you tell me what is wrong or how else I can take a screenshot of the full content of a ScrollViewer?
Here's a little sample I put together (press 'P' after the main window is shown).
The main window (just a TreeView in a Window that is too small to show the full content):
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:ScreenshotTest"
mc:Ignorable="d"
Title="MainWindow"
Height="200" Width="300"
KeyDown="Window_KeyDown">
<TreeView x:Name="Tree">
<TreeViewItem Header="Node 1 - Lorem ipsum dolor sit amet, consetetur sadipscing elitr" IsSelected="True" IsExpanded="True">
<TreeViewItem Header="Node 1.1" />
<TreeViewItem Header="Node 1.2" IsExpanded="True">
<TreeViewItem Header="Node 1.2.1"/>
</TreeViewItem>
<TreeViewItem Header="Node 1.3" IsExpanded="True">
<TreeViewItem Header="Node 1.3.1" />
<TreeViewItem Header="Node 1.3.2" />
<TreeViewItem Header="Node 1.3.3" />
</TreeViewItem>
</TreeViewItem>
<TreeViewItem Header="Node 2 - sed diam nonumy eirmod tempor invidunt ut labore et dolore" IsExpanded="True">
<TreeViewItem Header="Node 2.1"/>
<TreeViewItem Header="Node 2.2" IsExpanded="True">
<TreeViewItem Header="Node 2.2.1"/>
</TreeViewItem>
<TreeViewItem Header="Node 2.3" IsExpanded="True">
<TreeViewItem Header="Node 2.3.1" />
<TreeViewItem Header="Node 2.3.2" />
<TreeViewItem Header="Node 2.3.3" />
</TreeViewItem>
</TreeViewItem>
</TreeView>
</Window>
And the code behind to the window:
Class MainWindow
Private Sub Window_KeyDown(sender As Object, e As KeyEventArgs)
If e.Key <> Key.P Then
Exit Sub
End If
Dim sv As ScrollViewer = GetFirstChildOfType(Of ScrollViewer)(Me.Tree)
Me.CaptureScreen(sv.Content, "C:\Temp\Screenshot.png")
End Sub
Private Shared Function GetFirstChildOfType(Of T As DependencyObject)(obj As DependencyObject) As T
Dim result As T
Dim child As DependencyObject
If (obj Is Nothing) Then
Return Nothing
End If
If (VisualTreeHelper.GetChildrenCount(obj) = 0) Then
Return Nothing
End If
For index As Integer = 0 To VisualTreeHelper.GetChildrenCount(obj)
child = VisualTreeHelper.GetChild(obj, index)
result = TryCast(child, T)
If (result Is Nothing) Then
result = GetFirstChildOfType(Of T)(child)
End If
If (result IsNot Nothing) Then
Return result
End If
Next index
Return Nothing
End Function
Private Sub CaptureScreen(source As UIElement, filename As String)
Dim height As Double
Dim renderHeight As Double
Dim width As Double
Dim renderWidth As Double
Dim renderTarget As RenderTargetBitmap
Dim vb As VisualBrush
Dim dv As DrawingVisual
Dim encoder As PngBitmapEncoder
Try
height = source.RenderSize.Height
renderHeight = height
width = source.RenderSize.Width
renderWidth = width
renderTarget = New RenderTargetBitmap(Convert.ToInt32(renderWidth), Convert.ToInt32(renderHeight), 96, 96, PixelFormats.Pbgra32)
vb = New VisualBrush(source)
dv = New DrawingVisual
Using dc As DrawingContext = dv.RenderOpen
dc.DrawRectangle(vb, Nothing, New Rect(New Point(0, 0), New Point(width, height)))
End Using
renderTarget.Render(dv)
encoder = New PngBitmapEncoder
encoder.Frames.Add(BitmapFrame.Create(renderTarget))
Using fs As IO.FileStream = New IO.FileStream(filename, IO.FileMode.Create, IO.FileAccess.Write)
encoder.Save(fs)
End Using
Catch ex As Exception
Stop
End Try
End Sub
End Class
I'm fine with answers in C# as well.

Answer 1

After a lot of trial and error I found out, that I got several things wrong.
First, I used the wrong UIElement. I had to take the first StackPanel instead of the first ScrollViewer.
Second, although my Window has the background set to white, all my controls inside Window doesn't have a background, that's probably why my resulting picture is black.
So, changing just one function of my code to
Private Sub Window_KeyDown(sender As Object, e As KeyEventArgs)
Dim panel As StackPanel
If e.Key <> Key.P Then
Exit Sub
End If
panel = GetFirstChildOfType(Of StackPanel)(Me.Tree)
panel.Background = Brushes.White
panel.UpdateLayout()
Me.CaptureScreen(panel, "C:\Temp\Screenshot.png")
End Sub
did the trick, now I at least get a picture with a white background and all the elements of my TreeView, not just the ones visible on screen.
Now I have some other problems, but that's an issue for another question.