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Article:
  Implementing Drag and Drop in Windows Forms
Subject:   Items disappear when dragging, even if not dropped anywhere
Date:   2004-10-14 12:38:44
From:   sdx
Response to: Items disappear when dragging, even if not dropped anywhere

Imports System
Imports System.Drawing
Imports System.Windows.Forms


Public NotInheritable Class Form1
Inherits System.Windows.Forms.Form


Friend WithEvents ListDragSource As System.Windows.Forms.ListBox
Friend WithEvents ListDragTarget As System.Windows.Forms.ListBox
Friend WithEvents UseCustomCursorsCheck As System.Windows.Forms.CheckBox
Friend WithEvents DropLocationLabel As System.Windows.Forms.Label


Private indexOfItemUnderMouseToDrag As Integer
Private indexOfItemUnderMouseToDrop As Integer


Private dragBoxFromMouseDown As Rectangle
Private screenOffset As Point


Private MyNoDropCursor As Cursor
Private MyNormalCursor As Cursor


<System.STAThread()> _
Public Shared Sub Main()
System.Windows.Forms.Application.Run(New Form1)
End Sub 'Main


Public Sub New()
MyBase.New()


Me.ListDragSource = New System.Windows.Forms.ListBox
Me.ListDragTarget = New System.Windows.Forms.ListBox
Me.UseCustomCursorsCheck = New System.Windows.Forms.CheckBox
Me.DropLocationLabel = New System.Windows.Forms.Label


Me.SuspendLayout()


' ListDragSource
Me.ListDragSource.Items.AddRange(New Object() {"one", "two", "three", "four", _
"five", "six", "seven", "eight", _
"nine", "ten"})
Me.ListDragSource.Location = New System.Drawing.Point(10, 17)
Me.ListDragSource.Size = New System.Drawing.Size(120, 225)


' ListDragTarget
Me.ListDragTarget.AllowDrop = True
Me.ListDragTarget.Location = New System.Drawing.Point(154, 17)
Me.ListDragTarget.Size = New System.Drawing.Size(120, 225)


' UseCustomCursorsCheck
Me.UseCustomCursorsCheck.Location = New System.Drawing.Point(10, 243)
Me.UseCustomCursorsCheck.Size = New System.Drawing.Size(137, 24)
Me.UseCustomCursorsCheck.Text = "Use Custom Cursors"


' DropLocationLabel
Me.DropLocationLabel.Location = New System.Drawing.Point(154, 245)
Me.DropLocationLabel.Size = New System.Drawing.Size(137, 24)
Me.DropLocationLabel.Text = "None"


' Form1
Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)
Me.ClientSize = New System.Drawing.Size(292, 270)
Me.Controls.AddRange(New System.Windows.Forms.Control() {Me.ListDragSource, _
Me.ListDragTarget, Me.UseCustomCursorsCheck, _
Me.DropLocationLabel})


Me.Text = "Drag and Drop Example"
Me.ResumeLayout(False)
End Sub


Private Sub ListDragSource_MouseDown(ByVal sender As Object, ByVal e As MouseEventArgs) Handles ListDragSource.MouseDown


' Get the index of the item the mouse is below.
indexOfItemUnderMouseToDrag = ListDragSource.IndexFromPoint(e.X, e.Y)


If (indexOfItemUnderMouseToDrag <> ListBox.NoMatches) Then


' Remember the point where the mouse down occurred. The DragSize indicates
' the size that the mouse can move before a drag event should be started.
Dim dragSize As Size = SystemInformation.DragSize


' Create a rectangle using the DragSize, with the mouse position being
' at the center of the rectangle.
dragBoxFromMouseDown = New Rectangle(New Point(e.X - (dragSize.Width / 2), _
e.Y - (dragSize.Height / 2)), dragSize)
Else
' Reset the rectangle if the mouse is not over an item in the ListBox.
dragBoxFromMouseDown = Rectangle.Empty
End If


End Sub


Private Sub ListDragSource_MouseUp(ByVal sender As Object, ByVal e As MouseEventArgs) Handles ListDragSource.MouseUp


' Reset the drag rectangle when the mouse button is raised.
dragBoxFromMouseDown = Rectangle.Empty
End Sub


Private Sub ListDragSource_MouseMove(ByVal sender As Object, ByVal e As MouseEventArgs) Handles ListDragSource.MouseMove


If ((e.Button And MouseButtons.Left) = MouseButtons.Left) Then


' If the mouse moves outside the rectangle, start the drag.
If (Rectangle.op_Inequality(dragBoxFromMouseDown, Rectangle.Empty) And _
Not dragBoxFromMouseDown.Contains(e.X, e.Y)) Then


' Creates custom cursors for the drag-and-drop operation.
Try
MyNormalCursor = New Cursor("3dwarro.cur")
MyNoDropCursor = New Cursor("3dwno.cur")


Catch
' An error occurred while attempting to load the cursors so use
' standard cursors.
UseCustomCursorsCheck.Checked = False
Finally
' The screenOffset is used to account for any desktop bands
' that may be at the top or left side of the screen when
' determining when to cancel the drag drop operation.
screenOffset = SystemInformation.WorkingArea.Location


' Proceed with the drag and drop, passing in the list item.
Dim dropEffect As DragDropEffects = ListDragSource.DoDragDrop(ListDragSource.Items(indexOfItemUnderMouseToDrag), _
DragDropEffects.All Or DragDropEffects.Link)


' If the drag operation was a move then remove the item.
If (dropEffect = DragDropEffects.Move) Then
ListDragSource.Items.RemoveAt(indexOfItemUnderMouseToDrag)


' Select the previous item in the list as long as the list has an item.
If (indexOfItemUnderMouseToDrag > 0) Then
ListDragSource.SelectedIndex = indexOfItemUnderMouseToDrag - 1


ElseIf (ListDragSource.Items.Count > 0) Then
' Selects the first item.
ListDragSource.SelectedIndex = 0
End If
End If


' Dispose the cursors since they are no longer needed.
If (Not MyNormalCursor Is Nothing) Then _
MyNormalCursor.Dispose()


If (Not MyNoDropCursor Is Nothing) Then _
MyNoDropCursor.Dispose()
End Try


End If
End If
End Sub
Private Sub ListDragSource_GiveFeedback(ByVal sender As Object, ByVal e As GiveFeedbackEventArgs) Handles ListDragSource.GiveFeedback


' Use custom cursors if the check box is checked.
If (UseCustomCursorsCheck.Checked) Then


' Set the custom cursor based upon the effect.
e.UseDefaultCursors = False
If ((e.Effect And DragDropEffects.Move) = DragDropEffects.Move) Then
Cursor.Current = MyNormalCursor
Else
Cursor.Current = MyNoDropCursor
End If
End If


End Sub


Private Sub ListDragTarget_DragOver(ByVal sender As Object, ByVal e As DragEventArgs) Handles ListDragTarget.DragOver
' Determine whether string data exists in the drop data. If not, then
' the drop effect reflects that the drop cannot occur.
If Not (e.Data.GetDataPresent(GetType(System.String))) Then


e.Effect = DragDropEffects.None
DropLocationLabel.Text = "None - no string data."
Return
End If


' Set the effect based upon the KeyState.
If ((e.KeyState And (8 + 32)) = (8 + 32) And _
(e.AllowedEffect And DragDropEffects.Link) = DragDropEffects.Link) Then
' KeyState 8 + 32 = CTL + ALT


' Link drag and drop effect.
e.Effect = DragDropEffects.Link


ElseIf ((e.KeyState And 32) = 32 And _
(e.AllowedEffect And DragDropEffects.Link) = DragDropEffects.Link) Then


' ALT KeyState for link.
e.Effect = DragDropEffects.Link


ElseIf ((e.KeyState And 4) = 4 And _
(e.AllowedEffect And DragDropEffects.Move) = DragDropEffects.Move) Then


' SHIFT KeyState for move.
e.Effect = DragDropEffects.Move


ElseIf ((e.KeyState And 8) = 8 And _
(e.AllowedEffect And DragDropEffects.Copy) = DragDropEffects.Copy) Then


' CTL KeyState for copy.
e.Effect = DragDropEffects.Copy


ElseIf ((e.AllowedEffect And DragDropEffects.Move) = DragDropEffects.Move) Then


' By default, the drop action should be move, if allowed.
e.Effect = DragDropEffects.Move


Else
e.Effect = DragDropEffects.None
End If


' Gets the index of the item the mouse is below.


' The mouse locations are relative to the screen, so they must be
' converted to client coordinates.


indexOfItemUnderMouseToDrop = _
ListDragTarget.IndexFromPoint(ListDragTarget.PointToClient(New Point(e.X, e.Y)))


' Updates the label text.
If (indexOfItemUnderMouseToDrop <> ListBox.NoMatches) Then


DropLocationLabel.Text = "Drops before item #" & (indexOfItemUnderMouseToDrop + 1)
Else
DropLocationLabel.Text = "Drops at the end."
End If


End Sub


Private Sub ListDragTarget_DragDrop(ByVal sender As Object, ByVal e As DragEventArgs) Handles ListDragTarget.DragDrop
' Ensures that the list item index is contained in the data.


If (e.Data.GetDataPresent(GetType(System.String))) Then


Dim item As Object = CType(e.Data.GetData(GetType(System.String)), System.Object)


' Perform drag and drop, depending upon the effect.
If (e.Effect = DragDropEffects.Copy Or _
e.Effect = DragDropEffects.Move) Then


' Insert the item.
If (indexOfItemUnderMouseToDrop <> ListBox.NoMatches) Then
ListDragTarget.Items.Insert(indexOfItemUnderMouseToDrop, item)
Else
ListDragTarget.Items.Add(item)


End If
End If
' Reset the label text.
DropLocationLabel.Text = "None"
End If
End Sub
Private Sub ListDragSource_QueryContinueDrag(ByVal sender As Object, ByVal e As QueryContinueDragEventArgs) Handles ListDragSource.QueryContinueDrag
' Cancel the drag if the mouse moves off the form.
Dim lb As ListBox = CType(sender, System.Windows.Forms.ListBox)


If Not (lb Is Nothing) Then


Dim f As Form = lb.FindForm()


' Cancel the drag if the mouse moves off the form. The screenOffset
' takes into account any desktop bands that may be at the top or left
' side of the screen.
If (((Control.MousePosition.X - screenOffset.X) < f.DesktopBounds.Left) Or _
((Control.MousePosition.X - screenOffset.X) > f.DesktopBounds.Right) Or _
((Control.MousePosition.Y - screenOffset.Y) < f.DesktopBounds.Top) Or _
((Control.MousePosition.Y - screenOffset.Y) > f.DesktopBounds.Bottom)) Then


e.Action = DragAction.Cancel
End If
End If
End Sub
Private Sub ListDragTarget_DragEnter(ByVal sender As Object, ByVal e As DragEventArgs) Handles ListDragTarget.DragEnter


' Reset the label text.
DropLocationLabel.Text = "None"
End Sub
Private Sub ListDragTarget_DragLeave(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListDragTarget.DragLeave


' Reset the label text.
DropLocationLabel.Text = "None"
End Sub
End Class