This implementation demonstrates loading and executing a WPF application as a plugin with in a host application, using only native .NET capabilities.

The solution involves three projects: AbstractionLayer for defining interfaces, WPFIPluginDemo as the main application, and WpfPlugin3 as the loadable plugin module.

The AbstractionLayer project defines the IPlugin interface:

using System;

namespace AbstractionLayer
{
    public interface IPlugin
    {
        void Initialize();
    }
}

The main application's MainWindow.xaml inculdes two buttons for discovering and displaying plugins:

<Window x:Class="WPFIPluginDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="450" Width="800">
    <StackPanel>
        <Button x:Name="btnSearchPlugins" Content="Discover Plugins" Click="BtnSearchPlugins_OnClick"/>
        <Button x:Name="btnShowPlugin" Content="Display Plugin" Click="BtnShowPlugin_OnClick" Margin="0,10,0,0"/>
    </StackPanel>
</Window>

The corresponding code-behind handles plugin discovery and execution:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Windows;
using System.Windows.Controls;
using AbstractionLayer;
using Path = System.IO.Path;

namespace WPFIPluginDemo
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void BtnSearchPlugins_OnClick(object sender, RoutedEventArgs e)
        {
            LoadPlugins(GetCurrentProgramPath() + "\\Plugins");
        }

        public static string GetCurrentProgramPath()
        {
            var entryAssembly = Assembly.GetEntryAssembly();
            var location = entryAssembly.Location;
            return Path.GetDirectoryName(location);
        }

        public List<IPlugin> PluginList = new List<IPlugin>();
        
        public void LoadPlugins(string directory)
        {
            try
            {
                var files = Directory.GetFiles(directory, "*.exe", SearchOption.TopDirectoryOnly);

                foreach (var file in files)
                {
                    Assembly assembly = Assembly.LoadFrom(file);
                    var types = assembly.GetTypes().Where(t => typeof(IPlugin).IsAssignableFrom(t) && !t.IsInterface);

                    foreach (var type in types)
                    {
                        IPlugin instance = (IPlugin)Activator.CreateInstance(type);
                        instance.Initialize();
                        PluginList.Add(instance);
                    }
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex);
                throw;
            }
        }

        private void BtnShowPlugin_OnClick(object sender, RoutedEventArgs e)
        {
            Type pluginType = PluginList[0].GetType();
            MethodInfo method = pluginType.GetMethod("Show");
            method.Invoke(PluginList[0], null);
        }
    }
}

The plugin module WpfPlugin3 implements the IPlugin interface:

using System;
using AbstractionLayer;

namespace WpfPlugin3
{
    public class WpfPlugin : IPlugin
    {
        public MainWindow WindowInstance = null;
        
        public void Initialize()
        {
            WindowInstance = new MainWindow();
        }

        public void Show()
        {
            if (WindowInstance != null)
            {
                WindowInstance.ShowDialog();
            }
        }
    }
}

The plugin's main window is defined as follows:

<Window x:Class="WpfPlugin3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="450" Width="800" WindowStyle="None">
    <Grid>
        <Button Content="WpfPlugin3" Width="200" Height="200" />
    </Grid>
</Window>

To execute this setup, place the WpfPlugin3.exe file inside the Plugins folder under the debug output directory of WPFIPluginDemo.