public class Test
{
    private int x,y,z; //number of these properties is large

    [Pure]
    public void Foo()
    {
        //only x must be allowed to change
    }
}

Contract.Ensures(Contract.OldValue<int>(y) == y);
Contract.Ensures(Contract.OldValue<int>(z) == z);
...//and for other large number of properties