多边形面积计算

/// <summary>
        /// 多边形面积计算(不论凸凹)
        /// </summary>
        /// <param name="count">坐标点数</param>
        /// <param name="X">坐标的横坐标集合</param>
        /// <param name="Y">坐标的纵坐标集合</param>
        /// <returns></returns>
        private double AoArea(int count,double [] X,double [] Y)
        {
            double area;
            if (count < 3)
                return 0;
            area= Y[0] * (X[count - 1] - X[1]);
            for (int i = 1; i <count; i++)
                area+= Y[i] * (X[(i - 1)] - X[(i + 1)%count]);
            return area/2;
        }