I can't Data Bind to a local variable in WPF/XAML

Question

I want a textbox to display the value of a variable when I click it (an iteration of 1 to 100), I do not know what I am doing Wrong:

When I run the project nothing i

Answer 1

implement INotifyPropertyChanged:

public partial class MainWindow : Window, INotifyPropertyChanged
    {
        public MainWindow()
        {
            this.InitializeComponent();
        }

        private string _txt;
        public string txt
        {
            get
            {
                return _txt;
            }
            set
            {
                if (_txt != value)
                {
                    _txt = value;
                    OnPropertyChanged("txt");
                }
            }
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            txt = "changed text";
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

XAML:

yes

and don't forget about adding the DataContext property of your window: