30 Day Trial Example Update

Hi this is a simple bit of code to allow you to add 30 day trial to your programs.
Hope you may find it usfull.
Comments and suggestions welcome.

Hi this is a new update the code now been converted to a class with a few fixes to the code.

class code

vbnet Code:

Option Explicit On
Imports System.IO
 
Public Class cTrial
 
    Private m_DaysUsed As Integer = 0
    Private m_MaxDays As Integer = 30
    Private m_Ended As Boolean = False
    Private m_Filename As String = vbNullString
 
    Public ReadOnly Property DaysUsed As Integer
        Get
            'Return number of days used.
            Return m_DaysUsed
        End Get
    End Property
 
    Public ReadOnly Property DaysLeft As Integer
        Get
            'Return number of days left.
            Return (MaxDays - m_DaysUsed)
        End Get
    End Property
 
    Public Property Filename As String
        Get
            Return m_Filename
        End Get
 
        Set(ByVal value As String)
            'Set filename.
            m_Filename = value
            'Check if trial file is found.
            If Not File.Exists(Filename) Then
                'Create trial file.
                If Not UpdateFile(Now.Date.AddDays(MaxDays), 0) Then
                    Throw New FileNotFoundException
                    Exit Property
                End If
            End If
 
            'Check the date.
            CheckDate(Now.Date)
        End Set
    End Property
 
    Public ReadOnly Property HasEnded As Boolean
        Get
            Return m_Ended
        End Get
    End Property
 
    Public Property MaxDays As Integer
        Get
            Return m_MaxDays
        End Get
        Set(ByVal value As Integer)
            m_MaxDays = value
        End Set
    End Property
 
    Private Sub CheckDate(ByVal Date1 As DateTime)
        Dim sr As StreamReader = Nothing
        Dim Date2 As DateTime = Nothing
        Dim Flag As Integer = 0
        Dim Tmp As String = vbNullString
 
        Try
            sr = New StreamReader(Filename)
            'Read in file contents.
            Tmp = XorString(sr.ReadToEnd())
            'Close file.
            sr.Close()
 
            'Read trial flag.
            Flag = Integer.Parse(Tmp(0))
            'Read date.
            Date2 = Tmp.Substring(1)
 
            'Check if trial finished.
            If (Flag <> 0) Then
                m_DaysUsed = MaxDays
                m_Ended = True
            Else
                'Return number of days used.
                m_DaysUsed = MaxDays - DateDiff("d", Date1, Date2) + 1
                'Check if in range.
                m_Ended = (DaysUsed <= 0) Or (DaysUsed > MaxDays)
            End If
        Catch ex As Exception
            m_DaysUsed = MaxDays
            m_Ended = True
        End Try
 
        If HasEnded Then
            m_DaysUsed = MaxDays
            'Add date and trial end flag.
            UpdateFile(Date2, 1)
        End If
 
    End Sub
 
    Private Function UpdateFile(ByVal EndDate As DateTime, ByVal Flag As Integer) As Boolean
        Dim sw As StreamWriter = Nothing
        'File format is as follows flag,date
        'flag 0 = still in trial
        'flag 1 = trial has ended.
 
        Try
            sw = New StreamWriter(Filename)
            'Write date and trial flag.
            sw.Write(XorString(Flag & EndDate.Date))
            'Close file.
            sw.Close()
        Catch ex As Exception
            Return False
        End Try
 
        Return True
    End Function
 
    Private Function XorString(ByVal source As String) As String
        Dim sb As New System.Text.StringBuilder()
 
        Rnd(-1)
        'Set Random seed.
        Randomize(1830)
 
        For Each c As Char In source
            'Encrypt bye
            Dim b As Byte = Asc(c) Xor Int(Rnd() * 256) Mod 255
            'Append char to stringbuilder.
            sb.Append(Chr(b))
        Next c
        'Return string
        Return sb.ToString()
 
    End Function
End Class

Example code put into a new form.

vbnet Code:

Option Explicit On
Imports System.IO
 
Public Class frmmain
 
    Private MyTrial As New cTrial()
 
    Private Sub ShowMsg(ByVal msg As String)
        MessageBox.Show(msg, "Trial-Demo",
                        MessageBoxButtons.OK, MessageBoxIcon.Information)
    End Sub
 
    Private Sub frmmain_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
 
        With MyTrial
            'Set the number of trial days.
            .MaxDays = 30
            'Tril file, hide this file somewere were no one can find it.
            .Filename = Path.Combine(Application.StartupPath, "trial.sys")
 
            'Check if trial has ended.
            If .HasEnded Then
                ShowMsg("Your " & .MaxDays & " day trial has eneded.")
            Else
                ShowMsg("You are on day " & .DaysUsed & " of " & .MaxDays & " days.")
            End If
        End With
 
    End Sub
 
End Class

Looks like, without me testing, all someone has to do to keep using it is set back the clock on their computer. I had a trial version of Toad that I used for years that way :D

Quote:

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Looks like, without me testing, all someone has to do to keep using it is set back the clock on their computer. I had a trial version of Toad that I used for years that way

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Yep it's basic tho the date is encrypted so they need to decrypt first plus you can hide the file somewere hard to find.
I am going to think of something that if the date is set back, just need to think of something.

How to add this? I understand that I have to add new class but how about that " example code put into form" ?:/

Hey thanks for sharing such a wonderfull code, i learned alot from it i just have a question how does decryption works? ( i understood encryption using XOR gate)

thanks for your time.