using System;

public class Program

{

    public static void Main()

    {

        var a = new A();

        A c = null;

        B b = null;

        Guid r;

        //Console.WriteLine(Guid.TryParse(a?.GetB(b.A), out r));

        Console.WriteLine(Guid.TryParse(c?.GetB(b.A), out r));

    }

    public class A

    {

        public string GetB(A a) => a == null ? Guid.NewGuid().ToString() : "Blah";

    }

    public class B

    { 

        public A A { get; set; }

    }

}

​