Why does this value in the struct not change at all?

Question

That's very simple question I believe. Could anybody explain why this code outputs 1000, not 1050

public class Program
    {
        public static void Main()
        {
            Bus b = new Bus(1000);
            ((Car)b).IncreaseVolume(50);
            Console.WriteLine(b.GetVolume());
        }
    }

    public interface Car
    {
        int GetVolume();
        void IncreaseVolume(int amount);
    }

    public struct Bus : Car
    {
        private int volume;

        public Bus(int volume)
        {
            this.volume = volume;
        }

        public int GetVolume()
        {
            return volume;
        }

        public void IncreaseVolume(int amount)
        {
            volume += amount;
        }
    }
}

Answer 1

Casting a value type (struct) to an interface boxes the value. So you're invoking the method on the boxed copy of the value, not on the value itself.

Answer 2

Value types (struct) are passed around by value, but interfaces are considered to be reference types (not value types). Let's see:

Bus b = new Bus(1000);

Now b contains the value of a Bus with its volume set to 1000.

Car c = (Car)b;

Now the value in b is copied and made into a reference type (boxed) of Car. Now c contains a pointer to the boxed copy.

c.IncreaseVolume(50);

On the reference type, you call IncreaseVolume, which is a member of the Car interface. It receives the reference to the boxed value. It takes a managed pointer to the value in the box (to make it a value type again).

void Car.IncreaseVolume(int amount)
{
    ((Bus)this).IncreaseVolume(amount);
}

Now your method will act on the value in the box:

public void IncreaseVolume(int amount)
{
    volume += amount;
}

Now the method returns. Notice how no operation ever acted on the value in b, only on copies of it. So the next statement will print 1000:

Console.WriteLine(b.GetVolume());

That's it.