Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim TabIva As New IvaManager
        TabIva.Codice = Me.TextBox2.Text
        TabIva.Descrizione = Me.TextBox1.Text
        TabIva.Aliquota = CDec(Me.TextBox3.Text)
        Dim prova As New Clsiva
        prova.SaveIva(TabIva.Codice, TabIva.Descrizione, TabIva.Aliquota)
    End Sub

oppure così:

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim prova As New Clsiva
        prova.SaveIva(propertyIva.Codice, propertyIva.Descrizione, propertyIva.Aliquota)
    End Sub

    Private Function propertyIva() As IvaManager
        Dim TabIva As New IvaManager
        TabIva.Codice = Me.TextBox2.Text
        TabIva.Descrizione = Me.TextBox1.Text
        TabIva.Aliquota = CDec(Me.TextBox3.Text)
        Return TabIva
    End Function