Question

Well I have made this random string generate but I have recently noticed a fatal flaw. When I generate a random string, close the program and generate another random string it is the same as the first generation. Here is the code:

   Public Function RandomString(ByVal length As Integer) As String
    Dim strb As New System.Text.StringBuilder
    Dim chars() As String = {"A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"} 

    Dim UpperBound As Integer = UBound(chars)

    For x As Integer = 1 To length
        strb.Append(chars(Int(Rnd() * UpperBound)))
    Next

    Return strb.ToString

End Function

Private Sub PictureBox1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles PictureBox1.Click
    Try
        System.Diagnostics.Process.Start("Link Removed!")
    Catch
    End Try
End Sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim repeatCnt As Integer

    'Check for valid numeric entry
    If Integer.TryParse(TextBox2.Text, repeatCnt) Then
        For repeatIdx As Integer = 1 To repeatCnt
            Dim rndstring As String
            'Generate random string...
            rndstring = RandomString(24)
            '...and append to text box with a line break
            RichTextBox1.Text &= rndstring & vbCrLf
        Next
    Else
        MessageBox.Show("Please enter a valid integer number in the text box")
    End If
End Sub

Answer 1

Your problem is that you’re running Rnd() instead of using the Random class.

In order to keep Rnd() from giving the same sequence of “random” numbers every time, Random.Next() should be called (in order to use the machine clock for the initial value / seed).

So in your case:

Dim random As New System.Random()
Public Function RandomString(ByVal length As Integer) As String
   Dim strb As New System.Text.StringBuilder
   Dim chars() As String = {"A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"} 

   Dim UpperBound As Integer = UBound(chars)

   For x As Integer = 1 To length
      strb.Append(chars(Int(random.Next(UpperBound)))
   Next

   Return strb.ToString

End Function