WPF TextBox 获得焦点后，文本框中的文字全选中

textbox.GotFocus 事件处理 Textbox.SelectAll() 是不行的, 这样处理会发生的情况是:
1) textbox1 当前没有焦点, 内容为 someText.
2) 鼠标点击 textbox1, 若单击点位于 someText 之内, 则 someText 被瞬间全选后所有的选择都被取消, 若单击点位于 someText 之外, 则不会发生任何事情, 没有任何选中的内容.

如下是解决办法

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            LIKE_textBox.PreviewMouseDown += new MouseButtonEventHandler(LIKE_textBox_PreviewMouseDown);//注意，这个事件的注册必须在LIKE_textBox获得焦点之前
            LIKE_textBox.GotFocus += new RoutedEventHandler(LIKE_textBox_GotFocus);
            LIKE_textBox.LostFocus += new RoutedEventHandler(LIKE_textBox_LostFocus);
        }

        void LIKE_textBox_LostFocus(object sender, RoutedEventArgs e)
        {
            LIKE_textBox.PreviewMouseDown += new MouseButtonEventHandler(LIKE_textBox_PreviewMouseDown);
        }

        void LIKE_textBox_PreviewMouseDown(object sender, MouseButtonEventArgs e)
        {
            LIKE_textBox.Focus();
            e.Handled = true;
        }

        void LIKE_textBox_GotFocus(object sender, RoutedEventArgs e)
        {
            LIKE_textBox.SelectAll();
            LIKE_textBox.PreviewMouseDown -= new MouseButtonEventHandler(LIKE_textBox_PreviewMouseDown);
        }
    }