static string Number = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

/// <summary>
/// 压缩ID标识
/// </summary>
/// <param name="n"></param>
/// <returns></returns>
public string Short(long n) {
    string result = string.Empty;
    int l = Number.Length;
    while (n / l >= 1) {
        result = Number[(int)(n % l)] + result;
        n /= l;
    }
    result = Number[(int)n] + result;
    return result;
}

/// <summary>
/// 还原ID标识
/// </summary>
/// <param name="s"></param>
/// <returns></returns>
public long UnShort(string s) {
    long result = 0;
    if (s.HasValue()) {
        s = s.Trim();
        int l = s.Length;
        int m = Number.Length;
        for (int x = 0; x < l; x++) {
            result += Number.IndexOf(s[l - 1 - x]) * (long)Math.Pow(m, x);
        }
    }
    return result;
}