moving multiple controls - wpf

Question

I need to move a multiple textboxes with a mouse cursor.
I decided that I do it that way.
If a textbox is clicked (and Control button is pressed) textbox is added to list of selected items. Then when button is still pressed and when mouse moves I do an operation of moving controls. However my code doesn't work well. Textboxes are moving but very very fast. Here is my code
List<TextBox> items;
private void txtBox_PreviewMouseDown(object sender, RoutedEventArgs e)
{
isClicked = true;
startPoint = Mouse.GetPosition( (sender as TextBox).Parent);
items = CurrentSelection;
}
private void txtBox_PreviewMouseMove(object sender, RoutedEventArgs e)
{
Point mousePos = Mouse.GetPosition(parentCanvas);
if (isClicked)
{
foreach (TextBox item in items)
{
double left = Canvas.GetLeft(item);
double top = Canvas.GetTop(item);
Canvas.SetLeft(item, left + (startPoint.X - mousePos.X));
Canvas.SetTop(item, top + (startPoint.Y - mousePos.Y));
}
}
}
Basically I iterate through all selected items and change their position on canvas. However I probably calculate a new position in a wrong way.

Answer 1

The problem is, that you always calculate the delta to the initial start point. You must actualize startPoint after every call to txtBox_PreviewMouseMove. Somethin like...
private void txtBox_PreviewMouseMove(object sender, RoutedEventArgs e) {
Point mousePos = Mouse.GetPosition(parentCanvas);
if (isClicked){
foreach (TextBox item in items) {
double left = Canvas.GetLeft(item);
double top = Canvas.GetTop(item);
Canvas.SetLeft(item, left + (startPoint.X - mousePos.X));
Canvas.SetTop(item, top + (startPoint.Y - mousePos.Y));
}
startPoint=mousePoint;
}
}
...should do the job. Another thing I have seen, is that the direction is probably inverted. This can be easily corrected. Change the calculcation to...
Canvas.SetLeft(item, left + (mousePos.X-startPoint.X ));
Canvas.SetTop(item, top + (mousePos.Y-startPoint.Y));
... and this problem should also be gone.