程序代码：

        /// <summary>
        /// 从字符串的指定位置截取指定长度的子字符串
        /// </summary>
        /// <param name="str">原字符串</param>
        /// <param name="startIndex">子字符串的起始位置</param>
        /// <param name="length">子字符串的长度</param>
        /// <param name="showEllipsis">是否显示省略号</param>
        /// <returns>子字符串</returns>
        public static string CutString(string str, int startIndex, int length, bool showEllipsis)
        {
            if (startIndex >= 0)
            {
                if (length < 0)
                {
                    length = length*-1;
                    if (startIndex - length < 0)
                    {
                        length = startIndex;
                        startIndex = 0;
                    }
                    else
                        startIndex = startIndex - length;
                }

                if (startIndex > str.Length)
                    return "";
            }
            else
            {
                if (length < 0)
                    return "";
                else
                {
                    if (length + startIndex > 0)
                    {
                        length = length + startIndex;
                        startIndex = 0;
                    }
                    else
                        return "";
                }
            }

            if (str.Length - startIndex < length)
            {
                length = str.Length - startIndex;
            }
            string elli = string.Empty;
            if (length < str.Length && showEllipsis)
            {
                elli = "…";
            }

            return str.Substring(startIndex, length) + elli;
        }