Dictionaries are merged by adding a ResourceDictionary to the generic collection referenced by MergedDictionaries. A merged ResourceDictionary does not have resource elements defined within it in markup. Instead, the merged dictionary is a ResourceDictionary with no markup child elements defined (or with no elements added through code), but with a URI specified for Source. The Source designation enables the merged dictionary to come from an external source, such as a separate resource assembly that ships with the application, "loose XAML", or a specific localization or global resources assembly.
Specifying resource elements within a ResourceDictionary that has Source specified is not supported.
Lookup behavior within the MergedDictionaries collection will search the last-added ResourceDictionary first, and the search stops as soon as a requested key is found. Duplicate keys across the collection of merged dictionaries are not illegal. For more information on merged dictionary behavior, see Merged Resource Dictionaries.