I have a dummy class where I am testing arrays. I've noticed that when I want to dynamically allocate size of array at runtime, fields that indicate this size have to be static. I know I should probably use collections for this kind of code, but I'm more interested why do these fields have to be static? Is there any particular reason behind this?

class Foo
{
    private static int x;
    private static int y;

    private int[,] bar = new int[ x, y ];

    public Foo( int a, int b )
    {
        x = a;
        y = b;
    }
}