Gets which drag-and-drop operations are allowed by the originator (or source) of the drag event.
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
When a control initiates a drag-and-drop operation by calling the Control.DoDragDrop method, it specifies the permissible effects of the operation. For example, when you drag a file from a source, if the file is read-only (or from a read-only storage medium such as a CD), the source will indicate that the file can be copied, but not transferred, to the target.
Before attempting to perform an operation on the dragged data, you should examine this property to ensure that the operation is allowed.
The following example demonstrates a drag-and-drop operation between two ListBox controls. The example calls the DoDragDrop method when the drag action starts. The drag action starts if the mouse has moved more than SystemInformation.DragSize from the mouse location during the MouseDown event. The IndexFromPoint method is used to determine the index of the item to drag during the MouseDown event.
The example also demonstrates using custom cursors for the drag-and-drop operation. The example assumes that two cursor files, 3dwarro.cur and 3dwno.cur, exist in the application directory, for the custom drag and no-drop cursors, respectively. The custom cursors will be used if the UseCustomCursorsCheck CheckBox is checked. The custom cursors are set in the GiveFeedback event handler.
The keyboard state is evaluated in the DragOver event handler for the right ListBox, to determine what the drag operation will be based upon state of the SHIFT, CTRL, ALT, or CTRL+ALT keys. The location in the ListBox where the drop would occur is also determined during the DragOver event. If the data to drop is not a String, then the DragEventArgs.Effect is set to DragDropEffects.None. Finally, the status of the drop is displayed in the DropLocationLabel Label.
The data to drop for the right ListBox is determined in the DragDrop event handler and the String value is added at the appropriate place in the ListBox. If the drag operation moves outside the bounds of the form, then the drag-and-drop operation is canceled in the QueryContinueDrag event handler.
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
Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.