Generar claves de forma automática en windows form

Public Class Form1
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Label1.Text = generarClave(4, 65, 90) & generarClave(4, 48, 57)
    End Sub
    Function generarClave(n As Integer, li As Integer, ls As Integer) As String
        Dim cadena As String = ""
        Dim car As String = ""
        Dim r As New Random
        For i = 1 To n
            car = Chr(r.Next(li, ls))
            While InStr(cadena, car)
                car = Chr(r.Next(li, ls))
            End While
            cadena = cadena & car
        Next
        Return cadena
    End Function
End Class