How memory is allocated to reference types in C#?

Question

Hi I have some doubts in the memory allocation of the reference types.Please clarify my questions that are commented in between the code below.

 class Program
    {
        static void Main(string[] args)
        {
            testclass objtestclass1 = new testclass();
            testclass objtestclass2 = new testclass();
            testclass objtestclass3 = new testclass();
            // Is seperate memory created for all the three objects that are created above ? 
            objtestclass1.setnumber(1);
            objtestclass2.setnumber(2);
            Console.Write(objtestclass1.number);
            Console.Write(objtestclass2.number);
            objtestclass3 = objtestclass1;
            //When we assign one object to another object is the existing memory of       the         objtestclass3 be cleared by GC
            Console.Write(objtestclass3.number);
            objtestclass3.setnumber(3);
            Console.Write(objtestclass3.number);
            Console.Write(objtestclass1.number);
            Console.Read();   
            }

            public class testclass
            {
                public int number = 0;
                public void setnumber(int a)
                {
                    number = a;
                }

            }

Thanks.

Answer 1

The instance of testclass is on the heap. Each instance will consist of:

• A sync block
• A type reference
• The field number

On a 32-bit Windows .NET, this will take 12 bytes.

The local variables within the Main method (objtestclass1 etc) will be on the stack - but they're references, not objects. Each reference will be 4 bytes (again on a 32-bit CLR).

The difference between references and objects is important. For example, after this line:

objtestclass3 = objtestclass1;

You're making the values of the two variables the same - but those values are both references. In other words, both variables refer to the same object, so if you make a change via one variable you'll be able to see it via the other variable. You can think of the reference as a bit like a URL - if we both have the same URL, and one of us edits the page it refers to, we'll both see that edit.

For more information on this, see my article on reference types and another one on memory.

Answer 2

Is seperate memory created for all the three objects that are created above ?

Yes.

When we assign one object to another object is the existing memory of
the objtestclass3 be cleared by GC

Not exactly. First, you aren't really assigning one object to another. You are reassigning a variable that contains a reference to memory on the heap. Second, once memory on the heap no longer has any variables referring to it, the GC will detect that fact and reclaim the memory, but it is never really "cleared."