You typically create a class diagram in a project for the custom types that you’ve created in that project. You can also include types on a class diagram that come from assemblies that your project references, including the .NET Framework itself.
To add elements to a class diagram representing types in the .NET Framework, do the following:
Add a new class diagram to the project. (Right-click on project, select Add, New Item, then select Class Diagram).
Switch from Solution Explorer to Class View and expand Project References until you find a namespace or type that you want to look at.
Left-click and drag the selected namespace onto the class diagram. All types from that namespace will show up on the class diagram (initially zoomed out).
You can zoom in and expand types to see more details.
