截取指定长度的字符串，区分汉字和字符

using System.Text;
using System.Text.RegularExpressions;     

 

           int current = 30;
            int location_http = lblContext.Text.IndexOf("http");
            int location_img = lblContext.Text.IndexOf("img");
            if (location_http < current && location_http > 0)
            {
                current = location_http;
            }
            if (location_img < current && location_img > 0)
            {
                current = location_img;
            }
 

            lblContext.Text = GetFirstString(lblContext.Text, current);
     

   public static string GetFirstString(string stringToSub, int length)
        {
            Regex regex = new Regex("[\u4e00-\u9fa5]+", RegexOptions.Compiled);
            char[] stringChar = stringToSub.ToCharArray();
            StringBuilder sb = new StringBuilder();
            int nLength = 0;
            bool isCut = false;
            for (int i = 0; i < stringChar.Length; i++)
            {
                if (regex.IsMatch((stringChar[i]).ToString()))
                {
                    sb.Append(stringChar[i]);
                    nLength += 2;
                }
                else
                {
                    sb.Append(stringChar[i]);
                    nLength = nLength + 1;
                }

                if (nLength >= length)
                {
                    isCut = true;
                    break;
                }
            }
            if (isCut)
                return sb.ToString() + "..";
            else
                return sb.ToString();
        }