MainWindow.xaml

<Window x:Class="Test.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:Test"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <StackPanel>
        <Button Click="Button_Click">Click</Button>
        <TextBlock x:Name="txblData"></TextBlock>
    </StackPanel>
</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 Test
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Window2 w=new Window2();
            w.Show();
            //Sử dụng event SendDataWindow2 đã tạo bên Window2.xaml.cs
            w.SendDataWindow2 += W_SendDataWindow2;
        }

        private void W_SendDataWindow2(object sender, Window2EventArgs e)
        {
            //Hiển thị Data lên TextBlock
            txblData.Text = e.Data;
        }
    }
}

Window2.xaml

<Window x:Class="Test.Window2"
        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:Test"
        mc:Ignorable="d"
        Title="Window2" Height="450" Width="800">
    <StackPanel>
        <Button Content="Click" Click="Button_Click"></Button>
        <TextBox x:Name="txtData"></TextBox>
    </StackPanel>
</Window>

Window2.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 Test
{
    /// <summary>
    /// Interaction logic for Window2.xaml
    /// </summary>
    public partial class Window2 : Window
    {
        public Window2()
        {
            InitializeComponent();
        }
        //Tạo event
        private event EventHandler<Window2EventArgs> sendDataWindow2;
        public event EventHandler<Window2EventArgs> SendDataWindow2
        {
            add { sendDataWindow2 += value; }
            remove { sendDataWindow2 -= value; }
        }
        private void Button_Click(object sender, RoutedEventArgs e)
        {
            string data=txtData.Text;
            //Sử dụng Event sendDataWindow2 để truyền data qua MainWindow
            if (sendDataWindow2 != null)
            {
                sendDataWindow2(this,new Window2EventArgs(data));
            }
        }
    }
    //Tạo 1 class event cho riêng mình kể thừa lại EventArgs(Event chuẩn .NET)
    public class Window2EventArgs:EventArgs
    {
        public string Data { get; set; }
        public Window2EventArgs(string data)
        {
            Data = data;
        }
    }
}