Bewerken

Delen via

Drag-and-drop mouse behavior overview (Windows Forms .NET)

  • Artikel

Windows Forms includes a set of methods, events, and classes that implement drag-and-drop behavior. This topic provides an overview of the drag-and-drop support in Windows Forms.

Drag-and-drop events

There are two categories of events in a drag and drop operation: events that occur on the current target of the drag-and-drop operation, and events that occur on the source of the drag and drop operation. To perform drag-and-drop operations, you must handle these events. By working with the information available in the event arguments of these events, you can easily facilitate drag-and-drop operations.

Events on the current drop target

The following table shows the events that occur on the current target of a drag-and-drop operation.

Mouse Event Description
DragEnter This event occurs when an object is dragged into the control's bounds. The handler for this event receives an argument of type DragEventArgs.
DragOver This event occurs when an object is dragged while the mouse pointer is within the control's bounds. The handler for this event receives an argument of type DragEventArgs.
DragDrop This event occurs when a drag-and-drop operation is completed. The handler for this event receives an argument of type DragEventArgs.
DragLeave This event occurs when an object is dragged out of the control's bounds. The handler for this event receives an argument of type EventArgs.

The DragEventArgs class provides the location of the mouse pointer, the current state of the mouse buttons and modifier keys of the keyboard, the data being dragged, and DragDropEffects values that specify the operations allowed by the source of the drag event and the target drop effect for the operation.

Events on the drop source

The following table shows the events that occur on the source of the drag-and-drop operation.

Mouse Event Description
GiveFeedback This event occurs during a drag operation. It provides an opportunity to give a visual cue to the user that the drag-and-drop operation is occurring, such as changing the mouse pointer. The handler for this event receives an argument of type GiveFeedbackEventArgs.
QueryContinueDrag This event is raised during a drag-and-drop operation and enables the drag source to determine whether the drag-and-drop operation should be canceled. The handler for this event receives an argument of type QueryContinueDragEventArgs.

The QueryContinueDragEventArgs class provides the current state of the mouse buttons and modifier keys of the keyboard, a value specifying whether the ESC key was pressed, and a DragAction value that can be set to specify whether the drag-and-drop operation should continue.

Performing drag-and-drop

Drag-and-drop operations always involve two components, the drag source and the drop target. To start a drag-and-drop operation, designate a control as the source and handle the MouseDown event. In the event handler, call the DoDragDrop method providing the data associated with the drop and the a DragDropEffects value.

Set the target control's AllowDrop property set to true to allow that control to accept a drag-and-drop operation. The target handles two events, first an event in response to the drag being over the control, such as DragOver. And a second event which is the drop action itself, DragDrop.

The following example demonstrates a drag from a Label control to a TextBox. When the drag is completed, the TextBox responds by assigning the label's text to itself.

// Initiate the drag
private void label1_MouseDown(object sender, MouseEventArgs e) =>
    DoDragDrop(((Label)sender).Text, DragDropEffects.All);

// Set the effect filter and allow the drop on this control
private void textBox1_DragOver(object sender, DragEventArgs e) =>
    e.Effect = DragDropEffects.All;

// React to the drop on this control
private void textBox1_DragDrop(object sender, DragEventArgs e) =>
    textBox1.Text = (string)e.Data.GetData(typeof(string));

' Initiate the drag
Private Sub Label1_MouseDown(sender As Object, e As MouseEventArgs)
    DoDragDrop(DirectCast(sender, Label).Text, DragDropEffects.All)
End Sub

' Set the effect filter and allow the drop on this control
Private Sub TextBox1_DragOver(sender As Object, e As DragEventArgs)
    e.Effect = DragDropEffects.All
End Sub

' React to the drop on this control
Private Sub TextBox1_DragDrop(sender As Object, e As DragEventArgs)
    TextBox1.Text = e.Data.GetData(GetType(String))
End Sub

For more information about the drag effects, see Data and AllowedEffect.

See also