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.