Couldnt add the namespace using system.windows.controls in a lib

If it's a separate assembly - and each project creates a single assembly - then you probably need to add a Reference to it in each project that you want to use it. This shoudl help: Add references in the Reference Manager - Visual Studio (Windows) | Microsoft Learn[^]

What are you doing in the library that needs access to UI controls? This sounds a bit suspicious in that unless you're writing your own control library, or an extension to controls, you never need to do this. Adding references to UI libraries and interacting with UI controls directly ties your library to only being used by that kind of application. So, for example, if your library does some data processing and interaction with WPF controls, you can only ever use the library in a WPF app. You cannot use it in a web app of any kind if the need arises, forcing you to rewrite the library.

Having said all that, you seem to be wanting to add references to WindowsBase.dll, PresentationFramework.dll, and PresentationCore.dll. After you right-click the References item in Solution Explorer, pick Add Reference. In the dialog that shows up, under the Assemblies item in the top-left corner of the window, you'll find Framework. Click that and you can put a checkmark next to the .DLL's I listed. When done, click OK and the references will be set.