My WPF Application instance implements a custom interface, but second DLL doesn't see the interface

C#
public partial class App : System.Windows.Application, Platform.HttpClientOwner.IHttpClientOwner
{
    private static readonly System.Net.Http.HttpClient client = new();

    public System.Net.Http.HttpClient HttpClient => client;
}

C#
public partial class App : System.Windows.Application, Platform.HttpClientOwner.IHttpClientOwner
{
    private static readonly System.Net.Http.HttpClient client = new();

    public System.Net.Http.HttpClient HttpClient => client;
}

C#
static Class()
{
    if(System.Windows.Application.Current is not Platform.HttpClientOwner.IHttpClientOwner)
                    throw new System.InvalidProgramException("NetworkMgr can only be used by applications that implement " +
                        "BestChat.Platform.HttpClientOwner.IHttpClientOwner.");
.
.
.
}

C#
static Class()
{
    if(System.Windows.Application.Current is not Platform.HttpClientOwner.IHttpClientOwner)
                    throw new System.InvalidProgramException("NetworkMgr can only be used by applications that implement " +
                        "BestChat.Platform.HttpClientOwner.IHttpClientOwner.");
.
.
.
}

public partial class EntryPoint
    : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        var x = Application.Current == this;
    }
}

public partial class EntryPoint
    : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        var x = Application.Current == this;
    }
}

public class MyService
{
    public MyService(HttpClient httpClient)
        => _httpClient;

    public void DoSomeStuff();

    private readonly HttpClient _httpClient;
}

public class MyService
{
    public MyService(HttpClient httpClient)
        => _httpClient;

    public void DoSomeStuff();

    private readonly HttpClient _httpClient;
}

public class MyWindow
    : Window
{
    public MyWindow(MyService service)
        => _service = service;

    private void OnButtonClick(object sender, ClickEventHandler e)
        => _service.DoSomeStuff();
    
    private readonly MyService _service;
}

public class MyWindow
    : Window
{
    public MyWindow(MyService service)
        => _service = service;

    private void OnButtonClick(object sender, ClickEventHandler e)
        => _service.DoSomeStuff();
    
    private readonly MyService _service;
}

protected override OnStartup(StartupEventArgs e)
{
    _serviceProvider = new ServiceCollection()
        .AddHttpClientFactory()
        .AddSingleton<MyService>()
        .AddTransient<Window>()
        .BuildServiceProvider();

    _window = _serviceProvider.GetRequiredService<Window>();

    _window.Show();
}

protected override OnStartup(StartupEventArgs e)
{
    _serviceProvider = new ServiceCollection()
        .AddHttpClientFactory()
        .AddSingleton<MyService>()
        .AddTransient<Window>()
        .BuildServiceProvider();

    _window = _serviceProvider.GetRequiredService<Window>();

    _window.Show();
}

namespace BestChat.GUI
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : System.Windows.Application, Platform.HttpClientOwner.IHttpClientOwner
    {
        private static System.Net.Http.HttpClient client = new();

        public System.Net.Http.HttpClient HttpClient => client;

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

            client = new System.Net.Http.HttpClient();
        }
    }
}

namespace BestChat.GUI
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : System.Windows.Application, Platform.HttpClientOwner.IHttpClientOwner
    {
        private static System.Net.Http.HttpClient client = new();

        public System.Net.Http.HttpClient HttpClient => client;

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

            client = new System.Net.Http.HttpClient();
        }
    }
}

<Application
    x:Class="Best_Chat.GUI.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Best_Chat.GUI"
    xmlns:ctrls="clr-namespace:BestChat.GUI.Ctrls;assembly=BestChat.GUI.Ctrls"
    StartupUri="MainWnd.xaml">

<Application
    x:Class="Best_Chat.GUI.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Best_Chat.GUI"
    xmlns:ctrls="clr-namespace:BestChat.GUI.Ctrls;assembly=BestChat.GUI.Ctrls"
    StartupUri="MainWnd.xaml">