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QueryContinueDragEventHandler Delegate

Represents the method that will handle the QueryContinueDrag event of a Control.

Namespace: System.Windows.Forms
Assembly: System.Windows.Forms (in system.windows.forms.dll)

public delegate void QueryContinueDragEventHandler (
	Object sender,
	QueryContinueDragEventArgs e
)
/** @delegate */
public delegate void QueryContinueDragEventHandler (
	Object sender, 
	QueryContinueDragEventArgs e
)
JScript supports the use of delegates, but not the declaration of new ones.

Parameters

sender

The source of an event.

e

A QueryContinueDragEventArgs that contains the event data.

When you create a QueryContinueDragEventHandler delegate, you identify the method that will handle the event. To associate the event with your event handler, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate. For more information about handling events with delegates, see Events and Delegates.

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 UseCustomCursorsCheckCheckBox 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 DropLocationLabelLabel.

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.

This code excerpt demonstrates using the QueryContinueDragEventHandler delegate with the QueryContinueDrag event. See the DoDragDrop method for the complete code example.

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;
        }
    }
}

private void listDragSource_QueryContinueDrag(Object sender, 
    System.Windows.Forms.QueryContinueDragEventArgs e)
{
    // Cancel the drag if the mouse moves off the form.
    ListBox lb = (ListBox)sender;

    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.get_MousePosition().get_X() - screenOffset.get_X() 
            < f.get_DesktopBounds().get_Left() 
            || Control.get_MousePosition().get_X() 
            - screenOffset.get_X() > f.get_DesktopBounds().get_Right() 
            || Control.get_MousePosition().get_Y() - screenOffset.get_Y() 
            < f.get_DesktopBounds().get_Top() 
            || Control.get_MousePosition().get_Y() - screenOffset.get_Y() 
            > f.get_DesktopBounds().get_Bottom()) {
            e.set_Action(DragAction.Cancel);
        }
    }
} //listDragSource_QueryContinueDrag

Windows 98, Windows 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.

.NET Framework

Supported in: 2.0, 1.1, 1.0

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