I am developing a UserControl that consists of a block with a heading and a list of items (as ItemsControl). The usercontrol is added dynamically to a canvas. I
Is your user control defined to change in height to reflect the size of the contents? By default your user control will be a fixed size and not change in height just because the items control has more entries.
I think you need to add your user control inside a grid before then measuring the grid. This is how I measure controls and it seems to work well, even when measuring the control directly does not work as in your case...
MyControl control1 = new MyControl();
... your setup code for control1...
Dim containerGrid As New Grid
containerGrid.Children.Add(control1)
containerGrid.Measure(New Size(Double.MaxValue, Double.MaxValue))
containerGrid.Arrange(New Rect(0, 0, Double.MaxValue, Double.MaxValue))
...now check the grid.ActualWidth and grid.ActualHeight