Incorrect pixel coordinates - wpf

Question

I am using the following code to get pixel coordinates of the mouse position.
private void canvas_MouseMove(object sender, MouseEventArgs e)
{
startPoint = e.GetPosition(this);
}
However, if the user uses the scroll bar to move down a bit then this code does not work. I get the wrong coordinates and the object does not get drawn under the mouse position. Any ideas what going on here?

Answer 1

What you're doing should (more or less) work. You might have some problem with exactly which MouseMove event you are listening to, though.
Here is a sample app that demonstrates the desired behavior.
The XAML:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid Margin="12">
<ScrollViewer Name="scrollViewer"
HorizontalScrollBarVisibility="Auto"
VerticalScrollBarVisibility="Auto"
MouseMove="ScrollViewer_MouseMove">
<Canvas Name="canvas" Width="800" Height="600" />
</ScrollViewer>
</Grid>
</Window>
The code-behind:
using System.Windows;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Shapes;
namespace WpfApplication1
{
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private void ScrollViewer_MouseMove(object sender, MouseEventArgs e)
{
Point mousePosition = e.GetPosition(canvas);
var rectangle = new Rectangle
{
RenderTransform = new TranslateTransform(
mousePosition.X,
mousePosition.Y),
Stroke = Brushes.Black,
};
canvas.Children.Add(rectangle);
}
}
}
The key here is I bound the MouseMove event to the ScrollViewer, and checked the position of the Canvas.