#697 – Dragging Data Out of Your Application

When you use a user interface element as a drag-and-drop source, you initiate the drag-and-drop operation by calling the DragDrop.DoDragDrop method.  The data that you specify when calling this method can then be dragged onto other elements in your application that act as drop targets.   You can also drag data out to another application that knows how to act as a drag-and-drop target.

In the example below, we left-click and drag on the Image, then dropping data into Microsoft Word.  The actual data dropped is textual.

        private void Image_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            string textToDrag = "On November 23, 1936, the first issue of the pictorial magazine Life is published, featuring a cover photo of the Fort Peck Dam by Margaret Bourke-White.";

            DragDrop.DoDragDrop((DependencyObject)e.Source, textToDrag, DragDropEffects.Copy);
        }

#696 – A Drop Target Can Receive Data from Other Applications

When you designate an element in your user interface as a drop target for a drag-and-drop operation, it can receive data that is dragged from other elements in your application, provided that those elements initiate the drag-and-drop operation.  But a drop target element in your application can also serve as a target for a drag source in another application.

You can set the AllowDrop property to true for an element and then add code for the element’s Drop event, where you call IDataObject.GetData to get the dragged data.

In the example below, we’ve made a ListBox the drop target.  We can then drag some text onto it from Microsoft Word.

        private void lbDrop(object sender, DragEventArgs e)
        {
            string draggedText = (string)e.Data.GetData(DataFormats.StringFormat);

            ListBox lbox = e.Source as ListBox;
            lbox.Items.Add(draggedText);
        }

#695 – Implementing Drag-and-Drop Behavior

Below is code that implements some simple drag-and-drop behavior, dragging text from a TextBox to a ListBox.

In the XAML, we specify handlers for the MouseDown event on the TextBox and the Drop event on the ListBox.  We also set the AllowDrop property on the ListBox to true.

        <StackPanel Orientation="Vertical" Grid.Column="0" Margin="10">
            <Label Content="Enter some text and then drag to list"/>
            <TextBox Text="" MouseDown="txtMouseDown"/>
        </StackPanel>
        <ListBox Grid.Column="1" Margin="10" AllowDrop="True" Drop="lbDrop"/>

In the MouseDown event handler for the TextBox, we initiate drag-and-drop using the DragDrop.DoDragDrop method.

        private void txtMouseDown(object sender, MouseButtonEventArgs e)
        {
            TextBox txtElement = e.Source as TextBox;

            DragDrop.DoDragDrop(txtElement, txtElement.SelectedText, DragDropEffects.Copy);
        }

In the Drop event handler for the ListBox, we get the data being dragged and add an item to the ListBox.

        private void lbDrop(object sender, DragEventArgs e)
        {
            string draggedText = (string)e.Data.GetData(DataFormats.StringFormat);

            ListBox lbox = e.Source as ListBox;
            lbox.Items.Add(draggedText);
        }

#694 – An Example of Custom Drag-and-Drop Behavior

You can define your own drag-and-drop behavior in an application, defining the content to be dragged and the controls that the user drags the content from and to.  The basic process for drag-and-drop is:

• User presses and holds a mouse button (typically left mouse button) on a control that contains the data to be dragged (the source element)
• User moves mouse pointer over another control, where the data is to be dropped (the target element)
• User releases mouse button
• Data is “dropped” onto target control (control is typically updated to reflect new content)

In the example below, the user can select some text in the TextBox and drag it to the ListBox, adding a new item to the list for each chunk of text dragged.