Chapter 8. Data Handlers

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Data handlers allow you to define custom handling for copy-and-paste operations involving files of a given type. Normally, when you select a file and press Ctrl-C or select Copy from the Explorer menu, the shell copies the name of the file using the CF_HDROP format. This is evident in the clsDropFiles class that we used to implement the IShellExtInit::Initialize and IDropTarget::Drop methods in Chapter 7. But what if we want to change this behavior for our file type? Take a bitmap file for instance. What if, instead of copying the file, we want to copy the actual image contained within the file? Then you would be able to highlight the file in Explorer and make it available to any program that knows how to handle CF_BITMAP information (Microsoft Paint, Adobe Photoshop, etc.). Or what if we want to copy information from the file, say in CF_TEXT format, to the clipboard? We might want to copy its dimensions, for example, or the color depth of the file. This would allow us to select a file in the shell, press Ctrl-C, and copy pertinent information directly from the file into other programs like Microsoft Word or Excel.

Data handlers are required to implement two interfaces. These interfaces are IPersistFile and IDataObject . The IPersistFile implementation serves the same purpose it did for our icon handler and drop handler. We will just implement Load to get the name of the file in question.

We have used IDataObject before, but we have never implemented it. Admittedly, when IDataObject was last discussed (see Chapter 4), all you got was a really glossed overview of the interface. In this chapter, we will talk more about this interface and get a little better idea of how it works. But by no means will the discussion be complete. To write a data handler, we need to implement only three of nine available methods. But after we implement these methods, you should be able to explore the rest of IDataObject with better understanding.

Our data handler will be fairly simple, although the implementation is somewhat involved. When a .rad file is copied in the shell, the data handler will determine the type of animal represented by the file and build the string "The ( animal name ) is on the clipboard." This string is then made available to any program that allows for text transfers ( CF_TEXT format) over the clipboard via the Paste command. Figure 8.1, for example, shows a string generated by the data handler for .rad files that has been pasted into Notepad.

Figure 8.1. Accessing the .rad file data handler from Notepad

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