Please see my comment to the question.
As you can see, if really every time you search for practice problem "they either belong to c/c++ or python and others", it gives you at least one simple recipe for getting "C# problems": take the formulation of such problem, scratch out the words "C", "C++" or "Python" and write "C#" instead. From this moment, you got something to work at. :-)
As to the really new problems… to me, they are extremely hard to invent. Extremely. But there is a great number of well-known problems which you can transform into some computer problems by using some fantasy and logical thinking. I can suggest one very rich source of problems: many
books by
Martin Gardner, especially
Mathematical Games columns.
—SA