Possible to enforce nullable reference types?

Question

If a class library was written with nullable reference types enabled, the developer may not always check for null, as the compiler will not give warnings/errors.

Take the following code in a library with nullable reference types enabled:

public class Class1
{
    public void MyMethod(string s)
    {
        // don't check for null as compiler is happy
        s.ToLower();
    }
    public void MyMethod2(string? s)
    {
        // check for null as compiler is warning otherwise
        if (s != null)
        {
            this.MyMethod(s);
        }
    }
}

If a consumer called Class1.MyMethod without enabling nullable reference types, they would not get a warning when making the following call:

var c = new Class1();
c.MyMethod(null);

Is it possible to somehow force—or at least warn—consuming code to enable nullable reference types?

Answer 1

It is not possible for good reason. If it could imagine you add a new library to your project and it forces you to update hundreds of thousands of lines of code.

As the nullability has no runtime effect, you as a library author should always test your arguments in public interface to validate invalid inputs. The library internals can safely rely on nullability if you validate inputs on public interfaces.