Question

For example,

  class Age
  {
    public int Year
    {
      get;
      set;
    }
    public Age(int year)
    {
      Year = year;
    }
  }

  class Person
  {
    public Age MyAge
    {
      get;
      set;
    }
    public Person(Age age)
    {
      age.Year = ( age.Year * 2 );
      MyAge = age;      
    }
  }

[Client]

Age a = new Age(10);
Person p = new Person( a );

When a new Age class is constructed the Year property is: 10. However, the Person class changes the Year to 20, even though there is no ref keyword…

Can someone explain why Year is not still 10?

Answer 1

A reference to the object is passed by value. With that reference, the object instance itself can be modified.

Consider the following example to understand what that statement means:

public class A
{
    private MyClass my = new MyClass();

    public void Do()
    {
        TryToChangeInstance(my);
        DoChangeInstance(my);
        DoChangeProperty(my);
    }

    private void TryToChangeInstance(MyClass my)
    {
        // The copy of the reference is replaced with a new reference.
        // The instance assigned to my goes out of scope when the method exits.
        // The original instance is unaffected.
        my = new MyClass(); 
    }

    private void DoChangeInstance(ref MyClass my)
    {
        // A reference to the reference was passed in
        // The original instance is replaced by the new instance.
        my = new MyClass(); 
    }

    private void DoChangeProperty(MyClass my)
    {
        my.SomeProperty = 42; // This change survives the method exit.
    }
}