Question

Here, Person is a class, hence a reference type, it should be changed when I pass it as parameter in another function and assign it there.

Why program.person is null here after call to ChangePerson ?

namespace IndirectlyPropertySet
{
    class Program
    {
        public Person _person;

        public Person person
        {
            get { return _person; }
            set
            {
                if (_person != value)
                {
                    _person = value;
                }
            }
        }

        static void Main(string[] args)
        {
            Program program = new Program();

            ChangePerson(program.person); 
            // Why program.person is null after executing this ?

            program.person = new Person() { Name = "b", Age = 2 };
        }

        static void ChangePerson(Person p)
        {
            Person pe = new Person() { Name = "a", Age = 1 };
            p = pe;
        }
    }

    public class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
}

Answer 1

As an addition to @JonSkeet’s answer, if you rewrote your code to do this instead it would work.

static void ChangePerson(ref Person p)
{
    Person pe = new Person() { Name = "a", Age = 1 };
    p = pe;
}

void Main()
{
    Person p = program.person;
    ChangePerson(ref p);
    // program.person will now be set to "a" with age 1 at this point
    program.person = p;   

    program.person = new Person() { Name = "b", Age = 2 };
}

The key point is to pass the reference type by reference i.e. adding the ref keyword to your parameter.