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Control.QueryContinueDrag Event

Occurs during a drag-and-drop operation and allows the drag source to determine whether the drag-and-drop operation should be canceled.

[Visual Basic]
Public Event QueryContinueDrag As QueryContinueDragEventHandler
[C#]
public event QueryContinueDragEventHandler QueryContinueDrag;
[C++]
public: __event QueryContinueDragEventHandler* QueryContinueDrag;

[JScript] In JScript, you can handle the events defined by a class, but you cannot define your own.

Event Data

The event handler receives an argument of type QueryContinueDragEventArgs containing data related to this event. The following QueryContinueDragEventArgs properties provide information specific to this event.

Property Description
Action Gets or sets the status of a drag-and-drop operation.
EscapePressed Gets whether the user pressed the ESC key.
KeyState Gets the current state of the SHIFT, CTRL, and ALT keys.

Remarks

The QueryContinueDrag event is raised when there is a change in the keyboard or mouse button state during a drag-and-drop operation. The QueryContinueDrag event allows the drag source to determine whether the drag-and-drop operation should be canceled.

The following describes how and when events related to drag-and-drop operations are raised.

The DoDragDrop method determines the control under the current cursor location. It then checks to see if the control is a valid drop target.

If the control is a valid drop target, the GiveFeedback event is raised with the drag-and-drop effect specified. For a list of drag-and-drop effects, see the DragDropEffects enumeration.

Changes in the mouse cursor position, keyboard state, and mouse button state are tracked.

  • If the user moves out of a window, the DragLeave event is raised.
  • If the mouse enters another control, the DragEnter for that control is raised.
  • If the mouse moves but stays within the same control, the DragOver event is raised.

If there is a change in the keyboard or mouse button state, the QueryContinueDrag event is raised and determines whether to continue the drag, to drop the data, or to cancel the operation based on the value of the Action property of the event's QueryContinueDragEventArgs.

Note   The DragOver and GiveFeedback events are paired so that as the mouse moves across the drop target, the user is given the most up-to-date feedback on the mouse's position.

If the value is DragAction.Drop, the drop effect value is returned to the source, so the source application can perform the appropriate operation on the source data; for example, cut the data if the operation was a move.

If the value is DragAction.Cancel, the DragLeave event is raised.

By default, the QueryContinueDrag event sets Action to DragAction.Cancel if the ESC key was pressed and sets Action to DragAction.Drop if the left, middle, or right mouse button is pressed.

For more information about handling events, see Consuming Events.

Example

[Visual Basic, C#, C++] 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.

[Visual Basic, C#, C++] 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.

[Visual Basic, C#, C++] 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.

[Visual Basic, C#, C++] 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.

[Visual Basic, C#, C++] This code excerpt demonstrates using the QueryContinueDrag event. See the DoDragDrop method for the complete code example.

[Visual Basic] 
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

[C#] 
private void ListDragSource_QueryContinueDrag(object sender, System.Windows.Forms.QueryContinueDragEventArgs e) {
    // Cancel the drag if the mouse moves off the form.
    ListBox lb = sender as ListBox;

    if (lb != null) {

        Form f = 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) ||
            ((Control.MousePosition.X - screenOffset.X) > f.DesktopBounds.Right) ||
            ((Control.MousePosition.Y - screenOffset.Y) < f.DesktopBounds.Top) ||
            ((Control.MousePosition.Y - screenOffset.Y) > f.DesktopBounds.Bottom)) {

            e.Action = DragAction.Cancel;
        }
    }
}

[C++] 
private:
   void ListDragSource_QueryContinueDrag(Object* sender, 
      System::Windows::Forms::QueryContinueDragEventArgs* e) {
         // Cancel the drag if the mouse moves off the form.
         ListBox* lb = dynamic_cast<ListBox*>(sender);

         if (lb != 0) {

            Form* f = 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) ||
               ((Control::MousePosition.X - screenOffset.X) > f->DesktopBounds.Right) ||
               ((Control::MousePosition.Y - screenOffset.Y) < f->DesktopBounds.Top) ||
               ((Control::MousePosition.Y - screenOffset.Y) > f->DesktopBounds.Bottom))
            {

               e->Action = DragAction::Cancel;
            }
         }
      }

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family

See Also

Control Class | Control Members | System.Windows.Forms Namespace | OnQueryContinueDrag

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