code for random password generation - UPDATED

' In button click event check inputdata and call GeneratePassword function
Private Sub btnGenerate_Click(sender As Object, e As EventArgs) Handles btnGenerate.Click
        ' First we must check that the number in
        ' txtLength.Text property is a real integer number or not
        If Not IsNumeric(txtLength.Text) Then
            MessageBox.Show("Password length must be a whole number")
            txtLength.Focus()
            Exit Sub
        End If

        ' It was an integer so we can keep on
        ' now we check that the pass length is not below 6
        ' because we want the password to be minimum 6 char long
        If CInt(txtLength.Text) < 6 Then
            MessageBox.Show("Password length must be 6 or higher")
            txtLength.Focus()
            Exit Sub
        End If

        ' Check Number of special chars too
        If Not IsNumeric(txtNumSpecialChars.Text) Then
            MessageBox.Show("Num Specialchars must be a whole number")
            txtNumSpecialChars.Focus()
            Exit Sub
        End If

        Dim PassLength As Integer = CInt(txtLength.Text)
        Dim NumSpecialChars As Integer = CInt(txtNumSpecialChars.Text)

        txtPassword.Text = GeneratePassword(PassLength, NumSpecialChars)

    End Sub

' Function to return a random generated password
Public Function GeneratePassword(passwordLength As Integer, Optional numSpecialChars As Integer = 0) As String
        Dim RandomClass As New Random()
        Dim UsableChars() 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", "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"}
        Dim SpecialChars() As String = {"=", "+", "-", "_", "(", ")", "*", "&", "^", "%", "$", "#", "@", "!", "~", "`", "{", "}", "[", "]", ":", "'", "?", "/", ">", ".", ",", "<"}
        Dim FinalPassword As String = ""
        Dim index As Integer = 0
        Dim i As Integer

        If numSpecialChars = 0 Then
            For i = 1 To passwordLength
                index = RandomClass.Next(0, UsableChars.Length() - 1)
                FinalPassword += UsableChars(index)
            Next
        Else
            ' Subtract numSpecialChars from passwordlength
            ' since the password should use special chars also
            passwordLength -= numSpecialChars

            ' run through UsableChars finding random chars for password length
            For i = 1 To passwordLength
                index = RandomClass.Next(0, UsableChars.Length() - 1)
                FinalPassword += UsableChars(index)
            Next

            ' run trough SpecialChars finding random specialchars for numSpecialChars
            For i = 1 To numSpecialChars
                index = RandomClass.Next(0, SpecialChars.Length - 1)
                FinalPassword += SpecialChars(index)
            Next

            ' Next we should shuffle the password so we mix up chars and special chars
            FinalPassword = ShuffleString(FinalPassword)
        End If

        ' Return the shuffled generated password
        Return FinalPassword

    End Function

    Private Function ShuffleString(ByVal strInput As String) As String

        Dim strOutput As String = ""
        Dim rand As New System.Random
        Dim intPlace As Integer

        While strInput.Length > 0

            intPlace = rand.Next(0, strInput.Length)
            strOutput += strInput.Substring(intPlace, 1)
            strInput = strInput.Remove(intPlace, 1)

        End While

        Return strOutput
    End Function