private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            Button btn = sender as Button;
            textKey ="";



        }

Thành

private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            Button btn = sender as Button;
            TextKey ="";



        }

Bạn quên ghi hoa

@k9

COde day a:

    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private string textKey;
        public string TextKey
        {
            get
            {
                return textKey;
            }

            set
            {
                textKey = value;
                OnPropertyChanged("TextKey");
            }
        }

        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Button but = sender as Button;

            TextKey += but.Content.ToString();


        }



        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            Button btn = sender as Button;
            textKey ="";



        }

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