For more information about designing icons, see Chapter 13, "Visual Design."
The system uses the registry to determine which icon to display for a specific file. You register an icon for every data file type that your application supports and that you want the user to be able to distinguish easily. Create a DefaultIcon subkey entry under the application identifier subkey you created and define its value as the filename containing the icon. Typically, you use the application's executable filename and the index of the icon within the file. The index value corresponds to the icon resource within the file. A positive number represents the icon's position in the file. A negative number corresponds to the inverse of the resource ID number of the icon. The icon for your application should always be the first icon resource in your executable file. The system always uses the first icon resource to represent executable files. This means the index value for your data files will be a number greater than 0.
HKEY_CLASSES_ROOT
ApplicationIdentifier = Type Name
DefaultIcon = path [,index]
Instead of registering the application's executable file, you can register the name of a dynamic link library file (.DLL), an icon file (.ICO), or bitmap file (.BMP) to supply your data file icons. If an icon does not exist or is not registered, the system supplies an icon derived from the icon of the file type's registered application. If no icon is available for the application, the system supplies a generic icon. These icons do not make your files uniquely identifiable, so design and register icons for both your application and its data file types. Include the following sizes: 16 x 16 pixel (16 color), 32 x 32 pixel (16 color), and 48 x 48 pixel (256 color).
Note
For more information about designing icons, see Chapter 13, "Visual Design."