<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Content="Button" HorizontalAlignment="Left" Margin="106,92,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click_1"/>
        <TextBox x:Name="TextBox1" HorizontalAlignment="Left" Height="23" Margin="196,92,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="252"/>

    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Windows;

namespace WpfApplication1
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            TextBox1.Text = "Hello World！";
        }
    }
}