Custom Null Checking Patterns - How to add without using Attributes?

Hello!

I see that the common solution for Custom Null-Checking Methods is this

But I also see here that custom null checking patterns can also be created:

My question is:

How do I implement a custom null checking method pattern without using attribute contract annotations?

Example Method:

I would like to accomplish this without using [CanBeNull] and [ContractAnnotation("value:null => false;value:notnull => true")] annotiations. 

Is it possible?