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How to receive data dragged from other applications (part 1 of 6)

Contents and introduction



In article #11 we looked at how to catch files dragged and dropped on our application from Explorer. That's fine as far as it goes, but what about catching objects dragged and dropped from other applications. For example, how do we catch a text selection dragged from a text editor or word processor, or what about an HTML selection dragged from your web browser?

In order to receive such objects we have to make our application into a "drop target" that is recognised by Windows. We do this using COM (or OLE) drag and drop. As is usual with COM, we have to create an object that implements an interface – IDropTarget in this case – and then tell Windows about it. Windows then calls the methods of our implementation of IDropTarget to notify us of drag drop events.

If we accept a dropped object Windows makes the dropped object's data available via an object that supports the IDataObject interface. This object can provide the data in one or more data formats. Several different delivery mechanisms may be supported and objects can be targetted at different output devices. Source applications may also advise how the data should be rendered. Our application needs to examine the available data formats and decide if it can accept any of them. If so we must also be able to extract the data from the data object.

In this article we will:

  • Show how to register an application's window as a drop target.
  • Interact with Windows to give visual feedback about whether we will accept a drop.
  • Show how to examine all data formats supported by an object.
  • Give examples of how to extract data from certain kinds of data object.

The first thing we need to do is to get to know the IDropTarget and IDataObject interfaces. We do this in the next section.