ユーザーコントロールを作成する

UserControl1.xaml

<UserControl x:Class="WpfApplication1.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <StackPanel>
            <Button Content="ボタン" Click="Button_Click" />
            <TextBlock Name="textBlock" />
        </StackPanel>
    </Grid>
</UserControl>

UserControl1.cs

using System.Windows;
using System.Windows.Controls;

namespace WpfApplication1 {
    public partial class UserControl1 : UserControl {
        public UserControl1() {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e) {
            this.textBlock.Text = "こんにちは";
        }
    }
}

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication1"
        Title="MainWindow">
    <StackPanel>
        <local:UserControl1 />
        <local:UserControl1 />
    </StackPanel>
</Window>

検証