You need to learn to distinguish between assembly names and namespaces. They are not in correspondence 1-1.
If you want access, for example. System.Windows.Controls.Button, you need to reference the assembly PresentationFramework, which seems to contain most of the classes documented on MSDN, like being in the System.Windows.Controls namespace .
Multiple assemblies can contribute classes / controls to a single namespace. And there is no requirement that all (or even most) classes from the assembly be in the same namespace.
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