Using partial mocks for non public methods testing

namespace MockByExample.Calculators
{
    public static class IntExtensions
    {
        /// <summary>
        /// Determines if a value is between two other given values
        /// </summary>
        /// <param name="value">Current value</param>
        /// <param name="from"></param>
        /// <param name="to"></param>
        /// <returns></returns>
        public static bool IsBetween(this int value, int @from, int @to)
        {
            if (@from > @to)
            {
                var tmp = @from;
                @from = @to;
                @to = tmp;
            }
            return value >= @from && value <= @to;
        }
    }
}