Another C# question about references/collections/value types

Question

I have the following code:

public class Test
{
    public static void Main()
    {
     List<Person> list = new List<Person>();
     Person person = new Person() { Name="Chris" };
     list.Add(person);

     person = new Person(){ Name="Wilson the cat" };
     list.Add(person);

     Console.WriteLine(list[0].Name);
     Console.WriteLine(list[1].Name);
     Console.ReadLine();
    }
}

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

My question is where does the first person instance go? Does the CLR magically create a new instance of it somewhere? Is there anyway of referencing it outside of the list - e.g. where does it go after the method has completed? What method is used for storing objects in a collection (that was 4 questions).

Answer 1

1) It goes on the heap (or large object heap if > 84K)

2) Yes, a new instance is created. It can be referenced by accessing it through the list.

3) The collection probably uses a list, but you shouldn't need to know the internal details unless you require specific speed or space properties.

Answer 2

    List<Person> list = new List<Person>();

    Person person = new Person() { Name="Chris" };
    // your person object lives on the heap. The person variable is just
    // a pointer of that object on the heap. Note that the pointer lives
    // on the stack, and the object it points to lives on the heap.

    list.Add(person);
    // when you add your person to to the list, all it does it pass a
    // copy of the *pointer* to the list's method. List has access to this
    // object through the pointer.

    person = new Person(){ Name="Wilson the cat" };
    // new'ing up a instance of person creates another person object on
    // the heap. Note that this doesn't overwrite our original person created above,
    // because our original person sits in an entirely differently memory 
    // location.

    // We did, however overwrite our pointer variable by assigning it a new
    // location to point at. This doesn't affect the object we put into our
    // list since the list received a copy of our original pointer :)

    list.Add(person);

    Console.WriteLine(list[0].Name);
    // list[0] has a pointer to the first object we created


    Console.WriteLine(list[1].Name);
    // list[1] has a pointer to the second object we created.

    Console.ReadLine();

    // when this methods goes out of scope (i.e. when the stack frame is
    // popped), the pointers will be dropped from memory, and the objects
    // on the heap will no longer have any live references to them, so
    // they'll be eaten by the garbage collector.