You should be able to get the Bounds of your selectionRectangle and see if they exceed the Width and/or Height of your canvas before committing the drag operation.
selectionRectangle.MouseMove += new MouseEventHandler(Rectangle_MouseMove_1);
private bool drag = false;
private Point startPt;
private int wid;
private int hei;
private Point lastLoc;
private double CanvasLeft, CanvasTop;
private void Rectangle_MouseMove_1(object sender, MouseEventArgs e)
{
try
{
if (drag)
{
var newX = (startPt.X + (e.GetPosition(BackPanel).X - startPt.X));
var newY = (startPt.Y + (e.GetPosition(BackPanel).Y - startPt.Y));
Point offset = new Point((startPt.X - lastLoc.X), (startPt.Y - lastLoc.Y));
CanvasTop = newY - offset.Y;
CanvasLeft = newX - offset.X;
// check if the drag will pull the rectangle outside of it's host canvas before performing
// TODO: protect against lower limits too...
if ((CanvasTop + selectionRectangle.Height > BackPanel.Height) || (CanvasLeft + selectionRectangle.Width > BackPanel.Width) || CanvasTop < 0 || CanvasLeft < 0)
{
return;
}
selectionRectangle.SetValue(Canvas.TopProperty, CanvasTop);
selectionRectangle.SetValue(Canvas.LeftProperty, CanvasLeft);
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
}