We have in Toptal has several stages (wrote about it here https://toster.ru/answer?answer_id=492571).
There Algorithmics - you can indulge in https://codility.com/programmers/lessons/
There are other issues - for C# there to watch https://www.toptal.com/resources
Also give the test project. For web developers is usually to distribute a small SPA application (without page reloads) to edit any of the entities list (total CRUD) based on RESTful services with login and registration of users. A fairly typical task.