Generate a random alpha-numeric code with any length in VB.Net

Simple function to Generate a random alpha-numeric code with any length in VB.Net

'Generate a random alpha-numeric code with any length.
    Shared Function GetRandomCode(ByVal length As Int16) As String
        Dim num_characters As Integer
        Dim i As Integer
        Dim txt As String = String.Empty
        Dim ch As Integer

        num_characters = length
        Randomize()

        For i = 1 To num_characters
            ch = Int((26 + 26 + 10) * Rnd())
            If ch < 26 Then
                txt = txt & Chr(ch + Asc("A"))
            ElseIf ch < 2 * 26 Then
                ch = ch - 26
                txt = txt & Chr(ch + Asc("a"))
            Else
                ch = ch - 26 - 26
                txt = txt & Chr(ch + Asc("0"))
            End If
        Next i

        Return txt
    End Function