C#

using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Text;

public class MyGen
{
    private const string LL = "keygen";

    public virtual string getSerial(string userId, string licenseNum)
    {

        DateTime cal = new DateTime();
        cal.Add(1, 3);           // This is Java Code
        cal.Add(6, -1);          // This is Java Code

        NumberFormat nf = new java.text.DecimalFormat("000");                     // This is Java Code
        licenseNum = nf.format(Convert.ToInt32(licenseNum));                         // This is Java Code
        string verTime = new StringBuilder("-").Append(new java.text.SimpleDateFormat("yyMMdd").format(cal.getTime())).Append("0").ToString();                              // This is Java Code
        string type = "YE3MP-";
        string need = new StringBuilder(userId.Substring(0, 1)).Append(type).Append("300").Append(licenseNum).Append(verTime).ToString();
        string dx = new StringBuilder(need).Append(LL).Append(userId).ToString();
        int suf = this.decode(dx);
        string code = new StringBuilder(need).Append(Convert.ToString(suf)).ToString();
        return this.change(code);
    }

    private int decode(string s)
    {
        int i;
        char[] ac;
        int j;
        int k;
        i = 0;
        ac = s.ToCharArray();
        j = 0;
        k = ac.Length;
        while (j < k)
        {
            i = (31 * i) + ac[j];
            j++;
        }
        return Math.Abs(i);
    }

    private string change(string s)
    {
        sbyte[] abyte0;
        char[] ac;
        int i;
        int k;
        int j;
        abyte0 = s;                               // This is Java Code
        ac = new char[s.Length];
        i = 0;
        k = abyte0.Length;
        while (i < k)
        {
            j = abyte0[i];
            if ((j >= 48) && (j <= 57))
            {
                j = (((j - 48) + 5) % 10) + 48;
            }
            else if ((j >= 65) && (j <= 90))
            {
                j = (((j - 65) + 13) % 26) + 65;
            }
            else if ((j >= 97) && (j <= 122))
            {
                j = (((j - 97) + 13) % 26) + 97;
            }
            ac[i] = (char)j;
            i++;
        }
        return Convert.ToString(ac);
    }

    public MyGen()
        : base()
    {
    }

    static void Main(string[] args)
    {
        try
        {
            Console.WriteLine("please input register name:");
            string userId = null;
            userId = System.Console.ReadLine();
            MyGen MyGen = new MyGen();
            string res = MyGen.getSerial(userId, "5");
            Console.WriteLine("Serial:" + res);
            System.Console.ReadLine();
        }
        catch (IOException ex)
        {
        }
    }
}