Generate Random Alphanumeric Number

I used the following function to generate random alphanumeric code in one of my project

Public Shared Function GenerateRandomString(ByRef len As Integer, ByRef upper As Boolean) As String
Dim rand As New Random()
Dim allowableChars() As Char = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLOMNOPQRSTUVWXYZ0123456789".ToCharArray()
Dim final As String = String.Empty
For i As Integer = 0 To len - 1
final += allowableChars(rand.Next(allowableChars.Length - 1))
Next
Return IIf(upper, final.ToUpper(), final)
End Function