Why is Interfaces behaviour different when using value types and reference types

Question

I did the following example in c#

    interface IChangeable
    {
        void Change(params Int32[] array);
    }

    struct SomeValueType : IChangeable
    {
        private Int32 m_X;

        public SomeValueType(int X)
        {
            m_X = X;
        }

        public void Change(params Int32[] array)
        {
            m_X = array[0];
        }

        public override string ToString()
        {
            return String.Format("Crt value of m_X: {0}", m_X);
        }
    }

And in Main:

    static void Main(String[] args)
    {
        SomeValueType someValueType = new SomeValueType(5);
        Console.WriteLine(someValueType);                                   // No boxing occured. It showed: 5

        Object someRefType = someValueType;                                 // Boxed
        Console.WriteLine(someRefType);                                     // Also Shows 5 (from heap)

        someValueType.Change(2);                                            // Change Value of x not o's
        Console.WriteLine(someValueType + " " + someRefType);               // 2 5

        ((SomeValueType)someRefType).Change(3);                             // Copies to a temp stack so no change ocuured in o
        Console.WriteLine(someRefType);                                     // 5

        IChangeable itfStackChange = (IChangeable)someValueType;
        itfStackChange.Change(7);
        Console.WriteLine(someValueType);                                   // Shows 2 and not 7 ... why?

        IChangeable itfChange = (IChangeable)someRefType;
        itfChange.Change(1);                                                // Unboxes the value of o, making the value of x 1 boxes o again?
        Console.WriteLine(someRefType);                                     // Shows 1
    }

Now I am wondering what happens when I do:

        IChangeable itfStackChange = (IChangeable)someValueType;      //value was 2 and its still 2
        itfStackChange.Change(7);
        Console.WriteLine(someValueType);  

But if I change the definition of struct to class like in:

class SomeValueType : IChangeable

It writes 7 and not 2.

Answer 1

Value types semantics are such that the value gets copied on assignment. This means that when you change after assignment, the variables point to different objects.

For reference types the reference gets copied, meaning that when you change after assignment, both variables point to the same object.

See Value Types and Reference Types on MSDN.