Operation en visual basic .net

Public Class Form1
    Dim operation As String


    Private Function addition(ByVal chiffre1 As Integer, ByVal chiffre2 As Integer)
        Dim Resultat As Integer
        If operation = "addition" Then
            Resultat = chiffre1 + chiffre2
        ElseIf operation = "soustraction" Then
            Resultat = chiffre1 - chiffre2
        ElseIf operation = "multiplication" Then
            Resultat = chiffre1 * chiffre2
        ElseIf operation = "division" Then
            Resultat = chiffre1 / chiffre2
        End If
        Return Resultat
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Label1.Text = "le resultat est : " & addition(TextBox1.Text, TextBox2.Text)
        MsgBox(ComboBox1.SelectedItem)
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        If ComboBox1.SelectedItem = "+" Then
            operation = "addition"
        ElseIf ComboBox1.SelectedItem = "-" Then
            operation = "soustraction"
        ElseIf ComboBox1.SelectedItem = "x" Then
            operation = "multiplication"
        ElseIf ComboBox1.SelectedItem = "/" Then
            operation = "division"
        End If
    End Sub
End Class