Well, after a little headache... this solution looks good for me.
XAML
<combobox x:name="nameSelectionTypeStat" height="24" xmlns:x="#unknown">
VerticalAlignment="Center" MinWidth="75">
<comboboxitem content="Stats. au trimestre"></comboboxitem>
<comboboxitem content="Stats. à l'année"></comboboxitem>
</combobox>
<Button x:Name="nameValiderTypeStat" Content="Valider le type" Height="24"
VerticalAlignment="Center" Click="nameValiderTypeStat_Click_1" />
CodeBehind :
private void nameValiderTypeStat_Click_1(object sender, RoutedEventArgs e)
{
if (nameSelectionTypeStat.SelectedIndex == 0)
{
MessageBox.Show("Choice 1");
}
else if (nameSelectionTypeStat.SelectedIndex == 1)
{
MessageBox.Show("Choice 2");
}
}