>>>Sort of. What I can't quite get my head around is the creation of my custom base classes (which are supposed to be generic) within a specific solution, and them then being available to other soltions?<
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>Well, when you need to add the custom base classes to another solution, you simply add a reference to it, just like you would with any other class library. And add it to your toolbox as well, so you can drag the controls to your IDE. The namespace for your custom base classes should be something like MyCompany.WinUI.MyBaseClasses.

Thanks, although my question related to how to create my base classes in the first place, in that VS will not let you do File / New / Class without a Solution open.

However, looking back over this thread, I note that you suggested to somebody else to compile into a DLL and add this to the Toolbox. So should I be creating and compiling a separate "Solution" for my base classes / common utilities, and referencing this in the app Solutions where I use it?

Kevin