.net截取字符串

public static string CutLen(string str, int Length)
    {
        int num = 0;
        int num2 = 0;
        foreach (char ch in str)
        {
            if (ch > '/x007f')
            {
                num += 2;
            }
            else
            {
                num++;
            }
            if (num > Length)
            {
                str = str.Substring(0, num2 - 2) + "...";
                return str;
            }
            num2++;
        }
        return str;
    }