Random number/letter generator not working correctly [duplicate]

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• Vb.net Random Number generator generating same number many times 1 answer

Hello all I am trying to figure out why my code below only produces the same random letters/number for all 4/5/8 varibles:

Public Function GenerateRandomString(ByRef iLength As Integer, Optional numberOnly As Boolean = False) As String
    If numberOnly Then
        Dim RandomClass As New Random()
        Dim RememberSet As New HashSet(Of Integer)
        Dim RandomNumber As Integer

        While RememberSet.Count < 3
            RandomNumber = RandomClass.Next(1, 3)

            If RememberSet.Add(RandomNumber) Then
                Return RandomNumber
            End If
        End While
    Else
        Dim rdm As New Random()
        Dim sResult As String = ""
        Dim allowChrs() As Char = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLOMNOPQRSTUVWXYZ0123456789".ToCharArray()

        For i As Integer = 0 To iLength - 1
            sResult += allowChrs(rdm.Next(0, allowChrs.Length))
        Next

        Return sResult
    End If
End Function

And this is the code that I use to test it out:

Private Sub testing()
   Dim fourValue1 As String = Nothing
   Dim fourValue2 As String = Nothing
   Dim fourValue3 As String = Nothing
   Dim twelveValue As String = Nothing
   Dim eightValue As String = Nothing

   fourValue1 = GenerateRandomString(4)
   fourValue2 = GenerateRandomString(4)
   fourValue3 = GenerateRandomString(4)
   twelveValue = GenerateRandomString(12)
   eightValue = GenerateRandomString(8)

   debug.print(fourValue1)
   debug.print(fourValue2)
   debug.print(fourValue3)
   debug.print(twelveValue)
   debug.print(eightValue)
End Sub

And this is what I get for the output for the above code:

Z4Gb
Z4Gb
Z4Gb
Z4Gb5Sq28VOr
Z4Gb5Sq2

So are you see Z4Gb is in each of the random generated variables and then Z4Gb5Sq2 is within the 8 and 12 variables.

So what am I doing incorrect?