<Window x:Class="demo1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:demo1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid Margin="0,0,3.6,0">
        <Grid.RowDefinitions>
            <RowDefinition Height="25"></RowDefinition>
            <RowDefinition Height="25"></RowDefinition>
            <RowDefinition Height="25"></RowDefinition>
            <RowDefinition Height="25"></RowDefinition>
        </Grid.RowDefinitions>
        
        <TextBox Grid.Row="0" Grid.Column="0"  Name="pass"/>
        <Button Grid.Row="2" Grid.Column="0" Click="Button_Click" >Click Me</Button>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace demo1
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            this.pass.Text = "Hello World!";
        }
    }
}