Dragging a Controls around a Form

A frequent question I see on the forums is how can you drag controls around in a form with the mouse.  Drag and drop is a very big and complex subject but this is one subset that can be implemented very quickly in VB. For this sample I've created a simple form with Visual Studio and added a TextBox named TextBox1.  This sample will work for virtually any control though with a little modification.

To implement this fetaure we need to handle 3 events on the TextBox

• MouseDown
• MouseUp
• MouseMove

We need to handle the Up and Down events so we can record whether or not the user clicked on the TextBox.  As opposed to the user clicking elsewhere on the form (or anywhere else on the screen for that matter).  In addition to knowing the user clicked on the TextBox we need to also record where the Mouse was when the user Clicked.

This brings up the issue of screen versus client coordinates.  Events like MouseMove have a Location property which contains a Point relative to the client.  In Client Coordinates 0,0 referes to the top left corner of the control.  In Screen Coordinates 0,0 refers to the top left corner of your actual monitor.  To do a move we to find the difference in the MousePoints.  It's easier to do this using Screen Coordinates. 

Whenever we detect a mouse move event, we need to calculate how much the mouse moved in terms of x and y.  Then modify the location of the control by the same amount and the control will be dragable.

 Public Class Form1

    Private m_mouseHeldOnTextBox As Boolean
    Private m_mouseHeldPoint As Point

    Private Sub OnTextBoxMouseMove(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseMove
        If m_mouseHeldOnTextBox Then
            Dim currentPoint As Point = TextBox1.PointToScreen(e.Location)
            Dim xDelta As Integer = currentPoint.X - m_mouseHeldPoint.X
            Dim yDelta As Integer = currentPoint.Y - m_mouseHeldPoint.Y
            TextBox1.Location = New Point(TextBox1.Location.X + xDelta, TextBox1.Location.Y + yDelta)
            m_mouseHeldPoint = currentPoint
        End If
    End Sub

    Private Sub OnTextBoxMouseDown(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseDown
        If (e.Button = Windows.Forms.MouseButtons.Left) Then
            m_mouseHeldOnTextBox = True
            m_mouseHeldPoint = TextBox1.PointToScreen(e.Location)
        End If
    End Sub

    Private Sub OnTextBoxMouseUp(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseUp
        m_mouseHeldOnTextBox = False
    End Sub
End Class