using System.Text;

public static class ObjectExtension
{
    public static string ToEllipsisString(this string message, int length)
    {
        var index = 0;
        var sb = new StringBuilder();
        foreach (var item in message)
        {
            if (IsChineseCharacter(item))
            {
                index += 2;
            }
            else
            {
                index += 1;
            }

            sb.Append(item);
            if (length <= index)
            {
                return sb.Append("...").ToString();
            }
        }

        return sb.ToString();
    }

    private static bool IsChineseCharacter(char c)
    {
        return (19968 <= (int)c) && ((int)c <= 171941);
    }
}