i want to get familiar on using abstract classes and interfaces in C#,
i've read about how they work; trying to understand how And when to use them.
what simple programs or project ideas that i could practice on the use of abstraction and interfaces?

This is usually an interview question. Be sure to use a search engine to find prior dissertations about abstractions and interface in your choice of language but from my view there is no hard demand for such. But and this is borne out by the longer answers which I won't expand on here, using these can reduce work later in the product's life cycle.

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