WPF Textbox et Bouton

        List<TextBox> textBoxes;//la liste devient "globale" à la classe pour ne pas refaire n fois la même requête, et servir aux 2 interactions
        int indexTextBox;//idem pour l'index

        public MainWindow()
        {
            InitializeComponent();

            textBoxes = leGrid.Children.OfType<TextBox>().OrderBy(t => Convert.ToInt32(t.Tag)).ToList();
        }

        private void TextBox_KeyDown(object sender, System.Windows.Input.KeyEventArgs e)
        {
            if(e.Key == System.Windows.Input.Key.Enter)
            {
                SetIndex(sender);

                if (++indexTextBox == textBoxes.Count)//si een ajoutant 1, on atteint un index trop grand, on remet à zéro
                    indexTextBox = 0;

                textBoxes[indexTextBox].Focus();
            }
        }

        private void SetIndex(object sender)
        {
            TextBox textBox = sender as TextBox; //on caste sender en textbox
            if (textBox == null)//si en fait sender n'était pas une textbox on sort
                return;

            indexTextBox = textBoxes.IndexOf(textBox);//index du textbox en cours dans la liste
        }

        private void TextBox_GotFocus(object sender, RoutedEventArgs e)
        {
            SetIndex(sender);//au cas ou le focus soit parvenu autrement (souris ou Tab par exemple)
        }

        private void ButTest_Click(object sender, RoutedEventArgs e)
        {
            textBoxes[indexTextBox].Text = "Coucou";
        }

    <Grid x:Name="leGrid">
        <TextBox HorizontalAlignment="Left" Height="23" Margin="445,69,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120" KeyDown="TextBox_KeyDown" Tag="1" GotFocus="TextBox_GotFocus"/>
        <TextBox HorizontalAlignment="Left" Height="23" Margin="445,125,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120" KeyDown="TextBox_KeyDown" Tag="3" GotFocus="TextBox_GotFocus"/>
        <TextBox HorizontalAlignment="Left" Height="23" Margin="445,153,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120" KeyDown="TextBox_KeyDown" Tag="4" GotFocus="TextBox_GotFocus"/>
        <TextBox HorizontalAlignment="Left" Height="23" Margin="445,97,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120" KeyDown="TextBox_KeyDown" Tag="2" GotFocus="TextBox_GotFocus"/>
        <Button Content="Test" HorizontalAlignment="Left" Margin="471,208,0,0" VerticalAlignment="Top" Width="75" Click="ButTest_Click"/>