What is so special about Generic.xaml?

I've been trying to figure out how to organize my ResourceDictionary files for reuse and sharing with other members of my team.

I keep coming across "Generic.xaml", but if I look on MSDN for Generic.xaml or just do a Google search, I only seem to get blog posts and forum questions that happen to mention it--I can't seem to hit upon anything really authoritative and clear.

What is the difference between Generic.xaml and MyRandomlyNamedResourceDictionary.xaml? It seems like either way, I have to reference ResourceDictionaries stored in libraries with the Source attribute. E.g.,:

<Application.Resources>
    <ResourceDictionary
        Source="/CommonLibraryWpfThemes;component/Themes/Generic.xaml"
</Application.Resources>

So what advantage does Generic.xaml provide exactly? Does it have any purpose if I'm not trying to give my application multiple "looks" (i.e., if I have only one theme)?

Every Control in WPF has a default Style that provides, among other things, the Control's default ControlTemplate. WPF looks for the default style in a special resource dictionary in the Themes folder in the same assembly as the control. The key for the default style is provided by the Control.DefaultStyleKey dependency property, the default value of which is overridden in each sub-class of Control.

The name of the resource dictionary depends on the current Windows theme e.g. on Vista using the Aero theme, the dictionary is called Aero.NormalColor.xaml, on XP using the default theme it is Luna.NormalColor.xaml. If the style is not found in the theme dictionary, it looks in Generic.xaml i.e for controls whose look doesn't depend on the theme.

This only applies to any custom controls you have defined i.e. classes derived from Control, directly or indirectly. You can change the default style for a standard control by deriving from it and calling DefaultStyleKeyProperty.OverrideMetadata in the static constructor, but you then have to supply the full style including ControlTemplate.

Note that you can tell WPF to look in an external assembly for your default style by using the ThemeInfo attribute. The external assembly must be named <YourAssembly>.<ThemeName>.dll e.g. PresententationFramework.Aero.dll.

For a generic.xaml file (case insensitive) to be something special, two conditions must be met:

• It must be in the Themes sub-root folder in the project
• The assembly must be marked with the ThemeInfoAttribute (usually in AssemblyInfo.cs)

Then it serves as the default lookup location for any default styles you wish to apply to your Controls. Note also that for a style to be the default it must declare both its TargetType and x:Key as the Type of Control which is to be styled.

If you wish to add entire themes and theme switching to your application, that is accomplished with some coding, this technique merely defines the default resource dictionary.