为什么使用值类型和引用类型时接口的行为不同?

我用c#做了下面的例子

    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);
        }
    }

    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
    }

现在我想知道当我这样做时会发生什么:

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

但是如果我把struct的定义改成class，如下所示

class SomeValueType : IChangeable

它写的是7而不是2