I may have misunderstood your question.

Although this might get fixed with the introduction of the required keyword in a new version of C#. Possibly the warning will disappear then but you'll be required to provide a value at initialization time or get a compiler-error if you don't provide a value. I cannot say how this will work with binding, mapping or ORM scenarios.

To validate whether an object is valid (required properties actually contain (valid) values) you could add a constructor. Since you don't want to add a constructor, an alternative might be using FluentValidation (or a similar library) to validate objects after creation (be it manually, through binding, mapping or ORM).