父窗口向子窗口发送消息

父窗口源代码：

MainWindow.xaml

<Window x:Class="CommunicationByTwoWin.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" ResizeMode="CanMinimize">
    <Grid HorizontalAlignment="Center">
        
        <TextBox  Name="txt_input" HorizontalAlignment="Left" Height="232" Margin="85,44,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="189" 
           AcceptsReturn="True" TextChanged="TextBox_TextChanged"/>
        <Button Content="Send" HorizontalAlignment="Left" Margin="300,137,0,0" VerticalAlignment="Top" Width="75" Height="46" Click="OnButtonClick"/>

    </Grid>
</Window>

MainWindow.xaml.cs

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

        private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
        {

        }

        private  void OnButtonClick(object sender,RoutedEventArgs e)
        {
            string content = txt_input.Text;
            SubWin sub = new SubWin();
            sub.TextContent = content;
            sub.ShowDialog();
          
        }
    }
}

子窗口源代码

SubWin.xaml

<Window x:Class="CommunicationByTwoWin.SubWin"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="SubWin" Height="300" Width="300" ResizeMode="CanMinimize">
    <Grid>
        <TextBox Name="textBox"  Loaded="text_Load">
            
        </TextBox>
    </Grid>
</Window>

SubWin.xaml.cs

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.Shapes;

namespace CommunicationByTwoWin
{
    /// <summary>
    /// SubWin.xaml 的交互逻辑
    /// </summary>
    public partial class SubWin : Window
    {
        public String TextContent { get; set; }
        public SubWin()
        {
            InitializeComponent();
        }
        private  void  text_Load(object sender,RoutedEventArgs e)
        {
            textBox.Text = TextContent;
        }
    }
}

运行效果图