使用Stopwatch类可以来确定应用程序的执行时间，在C#中进行字符串截取时，可通过正则的方法截取，也可以通过字符串循环的方截取。通过Stopwatch类来测试哪种方法执行的时间更短。

正则截取字符串：

        /// <summary>
        /// 截取字符串，获取数值或变量
        /// </summary>
        /// <param name="str">字符串</param>
        /// <param name="start">开始字符串</param>
        /// <param name="end">结束字符串</param>
        /// <returns></returns>
        public static string CutOut(string str, string start, string end)
        {
            Regex rg = new Regex("(?<=(" + start + "))[.\\s\\S]*?(?=(" + end + "))", RegexOptions.Multiline | RegexOptions.Singleline);
            return rg.Match(str).Value;
        }

字符串循环截取字符串：

        /// <summary>
        /// 截取字符串，获取数值或变量
        /// </summary>
        /// <param name="sourse">字符串</param>
        /// <param name="startstr">开始字符串</param>
        /// <param name="endstr">结束字符串</param>
        /// <returns></returns>
        public static string MidStrEx(string sourse, string startstr, string endstr)
        {
            string result = string.Empty;
            int startindex, endindex;
            try
            {
                startindex = sourse.IndexOf(startstr);
                if (startindex == -1)
                    return result;
                string tmpstr = sourse.Substring(startindex + startstr.Length);
                endindex = tmpstr.IndexOf(endstr);
                if (endindex == -1)
                    return result;
                result = tmpstr.Remove(endindex);
            }
            catch (Exception ex)
            {
                throw new ApplicationException("数据异常" + ex.Message);
            }
            return result;
        }

通过Stopwatch类来测试两种方法的执行时间：

        /// <summary>
        /// 测试函数执行时间,直接访问字符串指定的方法
        /// </summary>
        /// <param name="type">函数名</param>
        /// <param name="testStr">待处理字符串</param>
        /// <param name="startStr">开始字符串</param>
        /// <param name="endStr">结束字符串</param>
        /// <returns></returns>
        public static TimeSpan runTime(string type,string testStr,string startStr,string endStr)
        {
            Type t = typeof(AjaxController);//括号中的为所要使用的函数所在的类的类名。
            MethodInfo mt = t.GetMethod(type);
            TimeSpan time = new TimeSpan(0);
            if (mt != null)
            {
                Stopwatch stopWatch = new Stopwatch();
                stopWatch.Start();
                //要测试的代码块
                string str = (string)mt.Invoke(null, new object[] { testStr,startStr,endStr });
                stopWatch.Stop();
                time = stopWatch.Elapsed;
            }
            return time;
        }

主方法调用：

using System;
using System.Diagnostics;
using System.Threading;
class Program
{
    static void Main(string[] args)
    {
            string testStr = "<span class=\"end\"></span><img id=\"1611620870664\" src=\"AQAAAQABAAD/2wBDAAMCAgMCAgMDAw\" title =\"打开文件说明文档\" onclick=\"openFileEdit(1611620870664)\" height=\"40px\" weight=\"30px\" style=\"vertical-align:middle;cursor:pointer\"><span class=\"hide\">;title=\"打开文件说明文档\" onclick=\"openFileEdit(1611620870664)\" height=\"40px\" weight=\"30px\" style=\"vertical-align:middle;cursor:pointer\"><span class=\"hide\">;title=\"打开文件说明文档\" onclick=\"openFileEdit(1611620870664)\" height=\"40px\" weight=\"30px\" style=\"vertical-align:middle;cursor:pointer\"><span class=\"hide\">;title=\"打开文件说明文档\" onclick=\"openFileEdit(1611620870664)\" height=\"40px\" weight=\"30px\" style=\"vertical-align:middle;cursor:pointer\"><span class=\"hide\">;";
            string testStrTow = "<span class=\"end\"></span><img id=\"1611620870664\" src=\"4d4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCAgMCAgMDAwMEAwMEBQgFBQQEBQoHBdata:image/jpg;base64,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\" title =\"打开文件说明文档\" onclick=\"openFileEdit(1611620870664)\" height=\"40px\" weight=\"30px\" style=\"vertical-align:middle;cursor:pointer\"><span class=\"hide\">;title=\"打开文件说明文档\" onclick=\"openFileEdit(1611620870664)\" height=\"40px\" weight=\"30px\" style=\"vertical-align:middle;cursor:pointer\"><span class=\"hide\">;title=\"打开文件说明文档\" onclick=\"openFileEdit(1611620870664)\" height=\"40px\" weight=\"30px\" style=\"vertical-align:middle;cursor:pointer\"><span class=\"hide\">;title=\"打开文件说明文档\" onclick=\"openFileEdit(1611620870664)\" height=\"40px\" weight=\"30px\" style=\"vertical-align:middle;cursor:pointer\"><span class=\"hide\">;";
            var timeC = runTime("CutOut", testStr, "src=\"", "\"");  //截取的字符串较短
            var timeM = runTime("MidStrEX", testStr, "src=\"", "\"");  //截取的字符串较短
            var timeC1 = runTime("CutOut", testStrTow, "src=\"", "\"");  //截取的字符串较长
            var timeM1 = runTime("MidStrEX", testStrTow, "src=\"", "\"");  //截取的字符串较长
    }
}

结果

timeC ：{00:00:00.0001441}
timeM：{00:00:00.0001529}
timeC1：{00:00:00.0000950}
timeM1 ：{00:00:00.0000243}

结论

通过几轮测试，可以得知，当需要截取的数据较短时，使用正则的方法所执行的时间更短，当需要截取的数据较长时，使用字符串循环的方法所执行的时间更短。

注：Stopwatch类中还有其他属性值