C# 中文字符串截取

/// <summary>
    /// 字符截取,中文一个字符两个字节处理
    /// </summary>
    /// <param name="str"></param>
    /// <param name="length"></param>
    /// <returns></returns>
    public static string SubString(string str, int length)
    {
        Regex regex = new Regex("^[\u4e00-\u9fa5]$");

        StringBuilder sb = new StringBuilder();
        int nLength = 0;

        for (int i = 0; i < str.Length; i++)
        {
            if (regex.IsMatch((str[i]).ToString()))
            {
                sb.Append(str[i]);
                nLength += 2;
            }
            else
            {
                sb.Append(str[i]);
                nLength = nLength + 1;
            }
            if (nLength >= length)
                break;
        }
        return sb.ToString();
    }