Type of Interface

an abstract class can derive from an interface but not vice versa. For an example, to calculate a rectangle square you can use:

 interface IMy
        {
            double RectangleCalculation(int a, int b);
            int A
            {
                get;
                set;
            }
            int B
            {
                get;
                set;
            }
        }

        abstract class MyClass : IMy
        {
            public virtual double RectangleCalculation(int a, int b)
            {
                return a * b;
            }
          
           public int a;
           public int A
            {
                get { return a; }
                set { a = value; }
            }

           public int b;
           public int B
            {
                get { return b; }
                set { b = value; }
            }
        }

Hope above help.