System.Windows.Media
is a namespace - most of the types within it are in the assembly PresentationFramework.dll
or PresentationCore.dll
.
You should look up whichever type you're interested in (via MSDN), and check which assembly it's in. You've got to distinguish between namespaces and assemblies - they're different concepts. You add a reference to an assembly as part of the project configuration, but specify the namespace in your source code (usually via a using
directive).