public static class Extensions
    {
        public static RangeCheck IsInRange<T>(this T pValue, T pBorder1, T pBorder2) where T: IComparable
        {
            return (RangeCheck)(Math.Sign(pBorder1.CompareTo(pBorder2)) == 0 ? 
                Math.Abs(Math.Sign(pValue.CompareTo(pBorder1))) + 1 : 
                Math.Abs(Math.Sign(pValue.CompareTo(pBorder1)) + Math.Sign(pValue.CompareTo(pBorder2))));
        }
    }



    public enum RangeCheck
    {
        Inside = 0,
        Border = 1,
        Outside = 2
    }


TestCode:

        private void Test()
        {

            int border1 = 3;
            int border2 = 7;

            System.Diagnostics.Debug.WriteLine("RealBorder (3-7)");

            for (int value = 1; value < 10; value++)
            {
                System.Diagnostics.Debug.WriteLine("{0}={1}", value, value.IsInRange(border1, border2));
            }


            border1 = 5;
            border2 = 5;

            System.Diagnostics.Debug.WriteLine("EqualBorder (5)");

            for (int value = 1; value < 10; value++)
            {
                System.Diagnostics.Debug.WriteLine("{0}={1}", value, value.IsInRange(border1, border2));
            }
        }
    }