About Shell Extensions

Windows supports two groups of shell extensions. The first group are registered for each type of file:

  • Context menu handlers. They add items to the context menu for a particular file object. The context menu is displayed when the user clicks a file object with mouse button 2.

  • Icon handlers. They typically add instance-specific icons for file objects. They can also be used to add icons for all files belonging to the same class.

  • Data handlers. They provide a type-specific IDataObject interface to be passed to the OLE DoDragDrop function.

  • Drop handlers. They provide type-specific drop behavior to files that can accept drag and drop objects.

  • Property sheet handlers. They add pages to the property sheet dialog box that the shell displays for a file object. The pages are specific to a class of files or a particular file object.

The second group of shell extensions are associated with file operations such as move, copy, rename, and so on:

  • Copy hook handlers. They are called when a folder object is about to be copied, moved, deleted, or renamed. They can either allow or prevent the operation.

  • Drag and drop handlers. They are context menu handlers that the system calls when the user drops an object after dragging it to a new position.

The design of a shell extension is based on the OLE Component Object Model (COM). The shell accesses an object through interfaces. An application implements the interfaces in a shell extension dynamic-link library (DLL), which is essentially an OLE in-process server DLL.

This overview explains how to create shell extensions and describes how the shell interacts with them.

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