Try to create a prism dialog service with custom modern ui window

Dialogs/AssemblyInfo.cs

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+using System.Windows;
+
+[assembly: ThemeInfo(
+    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located
+                                     //(used if a resource is not found in the page,
+                                     // or application resource dictionaries)
+    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located
+                                              //(used if a resource is not found in the page,
+                                              // app, or any theme specific resource dictionaries)
+)]

Dialogs/DialogService.cs

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+//using System;
+//using Prism.Services.Dialogs;
+
+//namespace Dialogs
+//{
+//    public class DialogService : IDialogService
+//    {
+//        /// <inheritdoc />
+//        public void Show(string name, IDialogParameters parameters, Action<IDialogResult> callback)
+//        {
+//            TODO_IMPLEMENT_ME();
+//        }
+
+//        /// <inheritdoc />
+//        public void Show(string name, IDialogParameters parameters, Action<IDialogResult> callback, string windowName)
+//        {
+//            TODO_IMPLEMENT_ME();
+//        }
+
+//        /// <inheritdoc />
+//        public void ShowDialog(string name, IDialogParameters parameters, Action<IDialogResult> callback)
+//        {
+//            TODO_IMPLEMENT_ME();
+//        }
+
+//        /// <inheritdoc />
+//        public void ShowDialog(string name, IDialogParameters parameters, Action<IDialogResult> callback, string windowName)
+//        {
+//            TODO_IMPLEMENT_ME();
+//        }
+//    }
+//}

Dialogs/Dialogs.csproj

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+<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
+
+  <PropertyGroup>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <UseWPF>true</UseWPF>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Prism.Wpf" Version="8.0.0.1740-pre" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Reference Include="ModernWpf.Controls">
+      <HintPath>..\..\..\.nuget\packages\modernwpfui\0.9.1\lib\netcoreapp3.0\ModernWpf.Controls.dll</HintPath>
+    </Reference>
+  </ItemGroup>
+
+</Project>

Dialogs/MessageBoxView.xaml

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+<ui:ContentDialog
+    x:Class="Dialogs.MessageBoxView"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:local="clr-namespace:Dialogs"
+    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?"
+    d:DataContext="{d:DesignInstance local:MessageBoxViewModel}"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    mvvm:ViewModelLocator.AutoWireViewModel="True"
+    CloseButtonText="Cancel"
+    DefaultButton="Primary"
+    PrimaryButtonCommand="{Binding CloseDialogCommand}"
+    PrimaryButtonCommandParameter="true"
+    PrimaryButtonText="Save"
+    SecondaryButtonText="Don't Save"
+    mc:Ignorable="d">
+    <StackPanel HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
+        <!--  Content body  -->
+        <TextBlock Text="Lorem ipsum dolor sit amet, adipisicing elit." TextWrapping="Wrap" />
+        <CheckBox Content="Upload your content to the cloud." />
+    </StackPanel>
+</ui:ContentDialog>

Dialogs/MessageBoxView.xaml.cs

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+using System;
+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
+{
+    /// <summary>
+    /// Interaktionslogik für MessageBoxView.xaml
+    /// </summary>
+    public partial class MessageBoxView
+    {
+        public MessageBoxView()
+        {
+            InitializeComponent();
+         }
+    }
+}

Dialogs/MessageBoxViewModel.cs

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+using System;
+using Prism.Commands;
+using Prism.Services.Dialogs;
+
+namespace Dialogs
+{
+    public class MessageBoxViewModel : IDialogAware
+    {
+        /// <inheritdoc />
+        public bool CanCloseDialog()
+        {
+            return true;
+        }
+
+        /// <inheritdoc />
+        public void OnDialogClosed()
+        {
+            
+        }
+
+        /// <inheritdoc />
+        public void OnDialogOpened(IDialogParameters parameters)
+        {
+        }
+
+        /// <inheritdoc />
+        public string Title { get; }
+
+
+        private DelegateCommand<string> _closeDialogCommand;
+        public DelegateCommand<string> CloseDialogCommand => _closeDialogCommand ??= new DelegateCommand<string>(CloseDialog);
+
+        private void CloseDialog(string obj)
+        {
+            if (obj == "true")
+            {
+                RaiseRequestClose(new DialogResult(ButtonResult.OK));
+            }
+
+        }
+
+        /// <inheritdoc />
+        public event Action<IDialogResult> RequestClose;
+
+        public virtual void RaiseRequestClose(IDialogResult dialogResult)
+        {
+            RequestClose?.Invoke(dialogResult);
+        }
+    }
+}