可参考以下代码：

/// <summary>
        /// 取中间文本 + static string GetMiddleStr(string oldStr,string preStr,string nextStr)
        /// </summary>
        /// <param name="oldStr">原文</param>
        /// <param name="preStr">前文</param>
        /// <param name="nextStr">后文</param>
        /// <returns></returns>
        public static string GetMiddleStr(string oldStr, string preStr, string nextStr)
        {
            string tempStr = oldStr.Substring(oldStr.IndexOf(preStr) + preStr.Length);
            tempStr = tempStr.Substring(0, tempStr.IndexOf(nextStr));
            return tempStr;
        }