Question

I am wondering why when we have p = new Person("TOM", 999); by calling fred.PrintInfo();
It doesn’t change the p to TOM and 999 but by using p.age = 99; we can change the fred’s age well the both constructor and properties are public then what is here that I am missing? I don’t want to do anything with this code I just want the reason.

using System;

class Person
{
    public string fullName;
    public int age;

    public Person(string n, int a)
    {
        fullName = n;
        age = a;
    }

    public void PrintInfo()
    {
        Console.WriteLine("{0} is {1} years old", fullName, age);
    }
}

class MainClass
{
    public static void SendAPersonByValue(Person p)
    {
        p.age = 99;

        p = new Person("TOM", 999);
    }

    public static void Main()
    {
        Person fred = new Person("Fred", 12);
        fred.PrintInfo();
        SendAPersonByValue(fred);
        fred.PrintInfo();
    }
}

Answer 1

fred points to some particular location in memory:

           +------------+
fred ----> | Fred    12 |
           +------------+

Upon calling SendAPersonByValue, p points to that same location:

           +------------+
fred ----> | Fred    12 |
           +------------+
              ^
  p  ---------+

p.age = 99; now changes the value in memory:

           +------------+
fred ----> | Fred    99 |
           +------------+
              ^
  p  ---------+

whereas new Person("TOM", 999); creates a new Person in memory, and p = ... makes p point to it:

           +------------+
fred ----> | Fred    99 |
           +------------+

           +------------+
  p  ----> | TOM    999 |
           +------------+

And this is exactly why fred still contains Fred, 99.

---

Now, if you were to pass fred as a ref parameter, p would become an alias for fred:

             +------------+
fred/p ----> | Fred    12 |
             +------------+

After p.age = 99:

             +------------+
fred/p ----> | Fred    99 |
             +------------+

After p = new Person("TOM", 999);:

             +------------+
             | Fred    99 |    (will be garbage collected eventually)
             +------------+

             +------------+
fred/p ----> | TOM    999 |
             +------------+