Vb.net

    Private Sub ChargerComboBoxCategorie()

        Me.cbxCategorie.Items.Add("Michel")
        Me.cbxCategorie.Items.Add("Claude")
        Me.cbxCategorie.Items.Add("Pierre")

    End Sub

    Private Sub btnSaisie_Click(sender As Object, e As EventArgs) Handles btnSaisie.Click

        Dim Valeur As String = String.Empty
        Dim Element As Object = Nothing

        Valeur = InputBox("Question", "Titre", "Choix")

        Element = (From Entx In cbxCategorie.Items Select Entx Where Entx.Equals(Valeur)).FirstOrDefault

        If (Element IsNot Nothing) Then
            MessageBox.Show("Valide")
        Else
            MessageBox.Show("Invalide")
        End If

    End Sub