c# generic self-referencing declarations

It means that T must inherit from Person<T>.

This is a typical way to create type-specific methods or properties or parameters in the base class, specific to the actual descendant.

For instance:

public abstract class Base<T> where T : Base<T>, new()
{
    public static T Create()
    {
        var instance = new T();
        instance.Configure(42);
        return instance;
    }

    protected abstract void Configure(int value);
}

public class Actual : Base<Actual>
{
    protected override void Configure(int value) { ... }
}

...

 Actual a = Actual.Create(); // Create is defined in Base, but returns Actual