However, I noticed that the Plugin class is using the [Export] attribute from the System.Composition namespace, while the App class is using the [Import] attribute from the System.ComponentModel.Composition namespace. These are two different versions of MEF and they're not compatible with each other.

However, I noticed that the Plugin class is using the [Export] attribute from the System.Composition namespace, while the App class is using the [Import] attribute from the System.ComponentModel.Composition namespace. These are two different versions of MEF and they're not compatible with each other.

namespace Wpf.Shared
{
    public interface IPlugin
    {
        void Process();
        string Name { get; }
        string Description { get; }
    }
}

namespace Wpf.Shared
{
    public interface IPlugin
    {
        void Process();
        string Name { get; }
        string Description { get; }
    }
}

namespace Wpf.Private;

[Export(typeof(IPlugin))]
public class Plugin : IPlugin
{
    public string Name => "Plugin";
    public string Description => "Plugin";

    public void Process()
    {
        Console.WriteLine("Plugin");
    }
}

namespace Wpf.Private;

[Export(typeof(IPlugin))]
public class Plugin : IPlugin
{
    public string Name => "Plugin";
    public string Description => "Plugin";

    public void Process()
    {
        Console.WriteLine("Plugin");
    }
}

public partial class App : Application
{
    [Import(typeof(IPlugin))]
    public IPlugin plugin;

    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        string plugins = System.IO.Path.Combine(
            Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
            "WPFAPP", "plugins");

        if (!Directory.Exists(plugins))
        {
            Directory.CreateDirectory(plugins);
        }

        var catalog = new DirectoryCatalog(plugins);

        try
        {
            var container = new CompositionContainer(catalog);
            container.ComposeParts(this);
        }
        catch (Exception ex)
        {
            Console.WriteLine($"ex: {ex}");
        }
    }
}

public partial class App : Application
{
    [Import(typeof(IPlugin))]
    public IPlugin plugin;

    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        string plugins = System.IO.Path.Combine(
            Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
            "WPFAPP", "plugins");

        if (!Directory.Exists(plugins))
        {
            Directory.CreateDirectory(plugins);
        }

        var catalog = new DirectoryCatalog(plugins);

        try
        {
            var container = new CompositionContainer(catalog);
            container.ComposeParts(this);
        }
        catch (Exception ex)
        {
            Console.WriteLine($"ex: {ex}");
        }
    }
}