Try to let it work

App/App.xaml.cs

@@ -3,6 +3,7 @@ using Dialogs;
 using ModernWpf;
 using ModernWpfPlayground.Types;
 using Prism.Ioc;
+using Prism.Services.Dialogs;
 
 namespace ModernWpfPlayground
 {
@@ -21,6 +22,7 @@ namespace ModernWpfPlayground
 
         protected override void RegisterTypes(IContainerRegistry containerRegistry)
         {
+            containerRegistry.RegisterSingleton<IDialogService, Dialogs.DialogService>();
             containerRegistry.Register<MainWindow>();
             containerRegistry.Register<MainWindowViewModel>();
             containerRegistry.RegisterDialog<MessageBoxView, MessageBoxViewModel>();

App/MainWindowViewModel.cs

@@ -8,6 +8,7 @@ using Microsoft.Win32;
 using ModernWpfPlayground.MvvmStuff;
 using ModernWpfPlayground.Types;
 using Prism.Commands;
+using Prism.Logging;
 using Prism.Services.Dialogs;
 using YamlDotNet.Serialization;
 using static ModernWpf.ThemeManager;
@@ -18,15 +19,17 @@ namespace ModernWpfPlayground
     public class MainWindowViewModel : BaseViewModel
     {
         private readonly IDialogService _service;
+        private readonly ILoggerFacade _logger;
         private const string AppName = "TaBEA 3.0.0";
         private string? _path;
         private string _title = AppName;
         private readonly ISerializer _serializer;
         private readonly IDeserializer _deserializer;
 
-        public MainWindowViewModel(IDialogService service)
+        public MainWindowViewModel(IDialogService service, ILoggerFacade logger)
         {
             _service = service;
+            _logger = logger;
             ShowDialogCommand = new DelegateCommand(ShowDialog);
             CloseCommand = new DelegateCommand(() => Application.Current.Shutdown());
             OpenViewModelCommand = new DelegateCommand(LoadViewModel);
@@ -135,12 +138,12 @@ namespace ModernWpfPlayground
 
         private void ShowDialogService()
         {
-            void executeCallBack(IDialogResult result)
+            static void ExecuteCallBack(IDialogResult result)
             {
-
+                MessageBox.Show(Application.Current.MainWindow, "DialogResult = " + result.Result);
             }
 
-            _service.Show(nameof(MessageBoxView), default, executeCallBack);
+            _service.ShowDialog(nameof(MessageBoxView), default, ExecuteCallBack);
 
         }
 

Dialogs/DialogService.cs

@@ -1,32 +1,103 @@
-//using System;
+using System;
-//using Prism.Services.Dialogs;
+using System.Linq;
+using System.Threading.Tasks;
+using System.Windows;
+using ModernWpf.Controls;
+using Prism.Common;
+using Prism.Ioc;
+using Prism.Services.Dialogs;
 
-//namespace Dialogs
+namespace Dialogs
-//{
+{
-//    public class DialogService : IDialogService
+    /// <summary>
-//    {
+    /// Implements <see cref="IDialogService"/> to show modal and non-modal dialogs.
-//        /// <inheritdoc />
+    /// </summary>
-//        public void Show(string name, IDialogParameters parameters, Action<IDialogResult> callback)
+    /// <remarks>
-//        {
+    /// The dialog's ViewModel must implement IDialogAware.
-//            TODO_IMPLEMENT_ME();
+    /// </remarks>
-//        }
+    public class DialogService : IDialogService
+    {
+        private readonly IContainerExtension _containerExtension;
 
-//        /// <inheritdoc />
+        /// <summary>
-//        public void Show(string name, IDialogParameters parameters, Action<IDialogResult> callback, string windowName)
+        /// Initializes a new instance of the <see cref="DialogService"/> class.
-//        {
+        /// </summary>
-//            TODO_IMPLEMENT_ME();
+        /// <param name="containerExtension"></param>
-//        }
+        public DialogService(IContainerExtension containerExtension)
+        {
+            _containerExtension = containerExtension;
+        }
 
-//        /// <inheritdoc />
+        /// <summary>
-//        public void ShowDialog(string name, IDialogParameters parameters, Action<IDialogResult> callback)
+        /// Shows a non-modal dialog.
-//        {
+        /// </summary>
-//            TODO_IMPLEMENT_ME();
+        /// <param name="name">The name of the dialog to show.</param>
-//        }
+        /// <param name="parameters">The parameters to pass to the dialog.</param>
+        /// <param name="callback">The action to perform when the dialog is closed.</param>
+        public void Show(string name, IDialogParameters parameters, Action<IDialogResult> callback)
+        {
+            throw new NotImplementedException();
+        }
 
-//        /// <inheritdoc />
+        /// <summary>
-//        public void ShowDialog(string name, IDialogParameters parameters, Action<IDialogResult> callback, string windowName)
+        /// Shows a non-modal dialog.
-//        {
+        /// </summary>
-//            TODO_IMPLEMENT_ME();
+        /// <param name="name">The name of the dialog to show.</param>
-//        }
+        /// <param name="parameters">The parameters to pass to the dialog.</param>
-//    }
+        /// <param name="callback">The action to perform when the dialog is closed.</param>
-//}
+        /// <param name="windowName">The name of the hosting window registered with the IContainerRegistry.</param>
+        public void Show(string name, IDialogParameters parameters, Action<IDialogResult> callback, string windowName)
+        {
+            throw new NotImplementedException();
+        }
+
+        /// <summary>
+        /// Shows a modal dialog.
+        /// </summary>
+        /// <param name="name">The name of the dialog to show.</param>
+        /// <param name="parameters">The parameters to pass to the dialog.</param>
+        /// <param name="callback">The action to perform when the dialog is closed.</param>
+        public void ShowDialog(string name, IDialogParameters parameters, Action<IDialogResult> callback)
+        {
+            ShowDialogInternal(name, parameters, callback, null);
+        }
+
+        /// <summary>
+        /// Shows a modal dialog.
+        /// </summary>
+        /// <param name="name">The name of the dialog to show.</param>
+        /// <param name="parameters">The parameters to pass to the dialog.</param>
+        /// <param name="callback">The action to perform when the dialog is closed.</param>
+        /// <param name="windowName">The name of the hosting window registered with the IContainerRegistry.</param>
+        public void ShowDialog(string name, IDialogParameters parameters, Action<IDialogResult> callback,
+            string windowName)
+        {
+            ShowDialogInternal(name, parameters, callback, windowName);
+        }
+
+        void ShowDialogInternal(string name, IDialogParameters parameters, Action<IDialogResult> callback,
+            string windowName)
+        {
+            var content = _containerExtension.Resolve<object>(name);
+            if (!(content is ContentDialog contentDialog))
+                throw new NullReferenceException("A dialog's content must be a content dialog");
+
+            if (!(contentDialog.DataContext is IDialogAware viewModel))
+                throw new NullReferenceException("A dialog's ViewModel must implement the IDialogAware interface");
+
+            MvvmHelpers.ViewAndViewModelAction<IDialogAware>(viewModel, d => d.OnDialogOpened(parameters));
+            Action<IDialogResult> closeHandler = default;
+
+            closeHandler += e =>
+            {
+                viewModel.RequestClose -= closeHandler;
+                callback?.Invoke(e);
+            };
+            viewModel.RequestClose += closeHandler;
+
+            if (contentDialog.Owner == null)
+                contentDialog.Owner = Application.Current?.Windows.OfType<Window>().FirstOrDefault(x => x.IsActive);
+            contentDialog.ShowAsync().Await();
+        }
+    }
+}

Dialogs/MessageBoxView.xaml

@@ -7,7 +7,7 @@
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
     xmlns:mvvm="http://prismlibrary.com/"
     xmlns:ui="http://schemas.modernwpf.com/2019"
-    Title="Save your work?"
+    Title="{Binding Title}"
     d:DataContext="{d:DesignInstance local:MessageBoxViewModel}"
     d:DesignHeight="450" 
     d:DesignWidth="800"

Dialogs/MessageBoxView.xaml.cs

@@ -1,15 +1,5 @@
-using System;
+using System.Linq;
-using System.Collections.Generic;
-using System.Text;
 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 Dialogs
 {
@@ -21,6 +11,7 @@ namespace Dialogs
         public MessageBoxView()
         {
             InitializeComponent();
+            Owner = Application.Current.Windows.OfType<Window>().FirstOrDefault(x => x.IsActive);
         }
     }
 }

Dialogs/MessageBoxViewModel.cs

@@ -24,7 +24,7 @@ namespace Dialogs
         }
 
         /// <inheritdoc />
-        public string Title { get; }
+        public string Title { get; } = "Hallo Holger";
 
 
         private DelegateCommand<string> _closeDialogCommand;