OOP Menghitung Penjualan Barang Dengan VB.Net

Public Class Form1
    Public Class Barang
        Public kode As String = 5
        Public nama As String = 10
        Public satuan As Decimal
        Public jumlah As Integer
        Public uang As Integer

        Public Property mKode() As String
            Get
                Return kode
            End Get
            Set(ByVal value As String)
                kode = value
            End Set
        End Property



        Public Property mNama() As String
            Get
                Return nama
            End Get
            Set(ByVal value As String)
                nama = value
            End Set
        End Property

        Public Property mSatuan() As Decimal
            Get
                Return satuan
            End Get
            Set(ByVal value As Decimal)
                satuan = value
            End Set
        End Property

        Public Property mJumlah() As Integer
            Get
                Return jumlah
            End Get
            Set(ByVal value As Integer)
                jumlah = value
            End Set
        End Property


        Public Function harga(ByRef mSatuan As Decimal, ByVal mJumlah As Integer) As Double
            Return satuan * jumlah
        End Function

        Public Function promo(ByVal harga As Double)
            If harga >= 2000000 Then
                Return 0.06 * mSatuan * mJumlah

            ElseIf harga >= 1500000 Then
                Return 0.05 * mSatuan * mJumlah

            ElseIf harga >= 1000000 Then
                Return 0.03 * mSatuan * mJumlah
            ElseIf harga >= 500000 Then
                Return 0.02 * mSatuan * mJumlah
            Else
                Return 0
            End If
        End Function

        Public Function total(ByVal harga As Double, ByVal promo As Double) As Double
            Return harga - promo
        End Function

    End Class


    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim objbarang As New Barang
        With objbarang
            .kode = TextBox1.Text
            .nama = TextBox2.Text
            .satuan = TextBox3.Text
            .jumlah = TextBox4.Text
            .uang = TextBox7.Text
            TextBox8.Text = TextBox7.Text - TextBox6.Text


        End With
    End Sub

    Private Sub TextBox4_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox4.TextChanged
        If TextBox3.Text * TextBox4.Text >= 2000000 Then
            TextBox5.Text = 0.06 * TextBox3.Text * TextBox4.Text
            TextBox6.Text = TextBox3.Text * TextBox4.Text - TextBox5.Text
        ElseIf TextBox3.Text * TextBox4.Text >= 1500000 Then
            TextBox5.Text = 0.05 * TextBox3.Text * TextBox4.Text
            TextBox6.Text = TextBox3.Text * TextBox4.Text
        ElseIf TextBox3.Text * TextBox4.Text >= 1000000 Then
            TextBox5.Text = 0.03 * TextBox3.Text * TextBox4.Text
            TextBox6.Text = TextBox3.Text * TextBox4.Text
        Else
            TextBox5.Text = 0
            TextBox6.Text = TextBox3.Text * TextBox4.Text
        End If
    End Sub
End Class