Chapter 13: Drag and Drop, and the Clipboard

Overview

The clipboard is an object where programs can save and restore data. A program can save data in multiple formats and retrieve it later, or another program might retrieve the data. Windows, rather than Visual Basic, provides the clipboard, so it is available to every application running on the system, and any program can save or fetch data from the clipboard.

The clipboard can store remarkably complex data types. For example, an application can store a representation of a complete object in the clipboard for use by other applications that know how to use that kind of object.

Drag-and-drop support enables the user to drag information from one control to another. The controls may be in the same application or in different applications. For example, your program could let the user drag items from one list to another, or it could let the user drag files from Windows Explorer into a file list inside your program.

A drag occurs in three main steps. First, a drag source control starts the drag, usually when the user presses the mouse down on the control. The control starts the drag, indicating the data that it wants to drag and the type of drag operations it wants to perform (such as Copy, Link, or Move).

When the user drags over a control, that control is a possible drop target. The control examines the kind of data being dragged and the type of drag operation requested (such as Copy, Link, or Move). The drop target then decides whether it will allow the drop and what type of feedback it should give to the user. For example, if the user drags a picture over a label control, the label might refuse the drop and display a no drop icon (a circle with a line through it). If the user drags the picture over a PictureBox that the program is using to display images, it might display a drop link icon (a box with a curved arrow in it).

Finally, when the user releases the mouse, the current drop target receives the data and does whatever is appropriate. For example, if the drop target is a TextBox control and the data is a string, the TextBox control might display the string. If the same TextBox control receives a file name, it might read the file and display its contents.

The following sections describe drag-and-drop events in more detail and give several examples of common drag-and-drop tasks. The section “Using the Clipboard” near the end of the chapter explains how to use the clipboard. Using it is very similar to using drag and drop, although it doesn’t require as much user feedback, so it is considerably simpler.