It looks like ContentDialog now.

ModernWpf.MessageBox.Test/App.xaml

@@ -1,9 +1,10 @@
-<Application x:Class="ModernWpfMessageBox.Test.App"
-             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
-             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-             xmlns:local="clr-namespace:ModernWpfMessageBox.Test"
-             xmlns:ui="http://schemas.modernwpf.com/2019"
-             ShutdownMode="OnLastWindowClose">
+<Application
+    x:Class="ModernWpfMessageBox.Test.App"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="clr-namespace:ModernWpfMessageBox.Test"
+    xmlns:ui="http://schemas.modernwpf.com/2019"
+    ShutdownMode="OnLastWindowClose">
     <Application.Resources>
         <ResourceDictionary>
             <ResourceDictionary.MergedDictionaries>

ModernWpf.MessageBox.Test/App.xaml.cs

@@ -1,27 +1,30 @@
-using System.Windows;
-using ModernWpf;
+using ModernWpf;
 using ModernWpf.Controls;
+using System.Windows;
 
-namespace ModernWpfMessageBox.Test {
-    public partial class App : Application {
-        protected override void OnStartup(StartupEventArgs e) {
+namespace ModernWpfMessageBox.Test
+{
+    public partial class App : Application
+    {
+        protected override void OnStartup(StartupEventArgs e)
+        {
             base.OnStartup(e);
 
-            var title = "Some title";
-            var message = "This is a looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong test text!";
+            string title = "Some title";
+            string message = "This is a looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong test text!";
 
-            // MessageBox.Show(message, title, MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
-            // MessageBox.Show("adawdawda", title, MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
+            System.Windows.MessageBox.Show(message, title, MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
+            System.Windows.MessageBox.Show("adawdawda", title, MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
             ShutdownMode = ShutdownMode.OnExplicitShutdown;
-            // ModernWpf.MessageBox.Show("This is a test text!", "Some title", MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
-            // ModernWpf.MessageBox.Show(message, title, MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
-            ModernWpf.MessageBox.Show("redadwada", null, MessageBoxButton.OK, Symbol.Admin);
-            ModernWpf.MessageBox.Show("redadwada", null, MessageBoxButton.OK, SymbolGlyph.Airplane);
-            ModernWpf.MessageBox.Show("redadwada", null, MessageBoxButton.OK, SymbolGlyph.Airplane, MessageBoxResult.OK);
-            ModernWpf.MessageBox.ShowAsync(message, title, MessageBoxButton.YesNoCancel, MessageBoxImage.Question).GetAwaiter().GetResult();
-            ModernWpf.MessageBox.ShowAsync(message, title, MessageBoxButton.YesNoCancel, MessageBoxImage.Hand, MessageBoxResult.Cancel).GetAwaiter().GetResult();
-            ModernWpf.MessageBox.EnableLocalization = false;
-            ModernWpf.MessageBox.ShowAsync("Press Alt and you should see underscores!", null, MessageBoxButton.YesNoCancel, MessageBoxImage.Hand).GetAwaiter().GetResult();
+            ModernWpf.Controls.MessageBox.Show("This is a test text!", "Some title", MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
+            ModernWpf.Controls.MessageBox.Show(message, title, MessageBoxButton.YesNoCancel, MessageBoxImage.Question);
+            ModernWpf.Controls.MessageBox.Show("redadwada", null, MessageBoxButton.OK, ((char)Symbol.Admin).ToString());
+            ModernWpf.Controls.MessageBox.Show("redadwada", null, MessageBoxButton.OK, SymbolGlyph.Airplane);
+            ModernWpf.Controls.MessageBox.Show("redadwada", null, MessageBoxButton.OK, SymbolGlyph.Airplane, MessageBoxResult.OK);
+            ModernWpf.Controls.MessageBox.ShowAsync(message, title, MessageBoxButton.YesNoCancel, MessageBoxImage.Question).GetAwaiter().GetResult();
+            ModernWpf.Controls.MessageBox.ShowAsync(message, title, MessageBoxButton.YesNoCancel, MessageBoxImage.Hand, MessageBoxResult.Cancel).GetAwaiter().GetResult();
+            ModernWpf.Controls.MessageBox.EnableLocalization = false;
+            ModernWpf.Controls.MessageBox.ShowAsync("Press Alt and you should see underscores!", null, MessageBoxButton.YesNoCancel, MessageBoxImage.Hand).GetAwaiter().GetResult();
             Shutdown();
         }
     }

ModernWpf.MessageBox.Test/ModernWpf.MessageBox.Test.csproj

@@ -2,9 +2,8 @@
 
   <PropertyGroup>
     <OutputType>WinExe</OutputType>
-    <TargetFrameworks>net45;net462;netcoreapp3.0;net5.0-windows10.0.18362</TargetFrameworks>
+    <TargetFramework>net6.0-windows10.0.18362.0</TargetFramework>
     <UseWPF>True</UseWPF>
-    <StartupObject></StartupObject>
   </PropertyGroup>
 
   <ItemGroup>

ModernWpf.MessageBox/Extensions/DWMAPI.cs

@@ -0,0 +1,217 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace MS.Win32
+{
+    /// <summary>
+    /// Used by Desktop Window Manager (DWM)
+    /// </summary>
+    internal static class DWMAPI
+    {
+        /// <summary>
+        /// DWMWINDOWATTRIBUTE enumeration. (dwmapi.h)
+        /// <para><see href="https://github.com/electron/electron/issues/29937"/></para>
+        /// </summary>
+        [Flags]
+        public enum DWMWINDOWATTRIBUTE : uint
+        {
+            /// <summary>
+            /// Enables or forcibly disables DWM transitions. The pvAttribute parameter points to a value of type BOOL. TRUE to disable transitions, or FALSE to enable transitions.
+            /// </summary>
+            DWMWA_TRANSITIONS_FORCEDISABLED = 3,
+
+            /// <summary>
+            /// Enables content rendered in the non-client area to be visible on the frame drawn by DWM.
+            /// </summary>
+            DWMWA_ALLOW_NCPAINT = 4,
+
+            /// <summary>
+            /// Retrieves the bounds of the caption button area in the window-relative space.
+            /// </summary>
+            DWMWA_CAPTION_BUTTON_BOUNDS = 5,
+
+            /// <summary>
+            /// Forces the window to display an iconic thumbnail or peek representation (a static bitmap), even if a live or snapshot representation of the window is available.
+            /// </summary>
+            DWMWA_FORCE_ICONIC_REPRESENTATION = 7,
+
+            /// <summary>
+            /// Cloaks the window such that it is not visible to the user.
+            /// </summary>
+            DWMWA_CLOAK = 13,
+
+            /// <summary>
+            /// If the window is cloaked, provides one of the following values explaining why.
+            /// </summary>
+            DWMWA_CLOAKED = 14,
+
+            /// <summary>
+            /// Freeze the window's thumbnail image with its current visuals. Do no further live updates on the thumbnail image to match the window's contents.
+            /// </summary>
+            DWMWA_FREEZE_REPRESENTATION = 15,
+
+            /// <summary>
+            /// Allows a window to either use the accent color, or dark, according to the user Color Mode preferences.
+            /// </summary>
+            DWMWA_USE_IMMERSIVE_DARK_MODE_OLD = 19,
+
+            /// <summary>
+            /// Allows a window to either use the accent color, or dark, according to the user Color Mode preferences.
+            /// </summary>
+            DWMWA_USE_IMMERSIVE_DARK_MODE = 20,
+
+            /// <summary>
+            /// Controls the policy that rounds top-level window corners.
+            /// <para>Windows 11 and above.</para>
+            /// </summary>
+            DWMWA_WINDOW_CORNER_PREFERENCE = 33,
+
+            /// <summary>
+            /// The color of the thin border around a top-level window.
+            /// </summary>
+            DWMWA_BORDER_COLOR = 34,
+
+            /// <summary>
+            /// The color of the caption.
+            /// <para>Windows 11 and above.</para>
+            /// </summary>
+            DWMWA_CAPTION_COLOR = 35,
+
+            /// <summary>
+            /// The color of the caption text.
+            /// <para>Windows 11 and above.</para>
+            /// </summary>
+            DWMWA_TEXT_COLOR = 36,
+
+            /// <summary>
+            /// Width of the visible border around a thick frame window.
+            /// <para>Windows 11 and above.</para>
+            /// </summary>
+            DWMWA_VISIBLE_FRAME_BORDER_THICKNESS = 37,
+
+            /// <summary>
+            /// Allows to enter a value from 0 to 4 deciding on the imposed backdrop effect.
+            /// </summary>
+            DWMWA_SYSTEMBACKDROP_TYPE = 38,
+
+            /// <summary>
+            /// Indicates whether the window should use the Mica effect.
+            /// <para>Windows 11 and above.</para>
+            /// </summary>
+            DWMWA_MICA_EFFECT = 1029
+        }
+
+        /// <summary>
+        /// Collection of backdrop types.
+        /// </summary>
+        [Flags]
+        public enum DWMSBT : uint
+        {
+            /// <summary>
+            /// Automatically selects backdrop effect.
+            /// </summary>
+            DWMSBT_AUTO = 0,
+
+            /// <summary>
+            /// Turns off the backdrop effect.
+            /// </summary>
+            DWMSBT_DISABLE = 1,
+
+            /// <summary>
+            /// Sets Mica effect with generated wallpaper tint.
+            /// </summary>
+            DWMSBT_MAINWINDOW = 2,
+
+            /// <summary>
+            /// Sets acrlic effect.
+            /// </summary>
+            DWMSBT_TRANSIENTWINDOW = 3,
+
+            /// <summary>
+            /// Sets blurred wallpaper effect, like Mica without tint.
+            /// </summary>
+            DWMSBT_TABBEDWINDOW = 4
+        }
+
+        /// <summary>
+        /// Abstraction of pointer to an object containing the attribute value to set. The type of the value set depends on the value of the dwAttribute parameter.
+        /// The DWMWINDOWATTRIBUTE enumeration topic indicates, in the row for each flag, what type of value you should pass a pointer to in the pvAttribute parameter.
+        /// </summary>
+        public enum PvAttribute
+        {
+            /// <summary>
+            /// Object containing the <see langowrd="false"/> attribute value to set in dwmapi.h. 
+            /// </summary>
+            Disable = 0x00,
+
+            /// <summary>
+            /// Object containing the <see langowrd="true"/> attribute value to set in dwmapi.h. 
+            /// </summary>
+            Enable = 0x01
+        }
+
+        /// <summary>
+        /// Represents the current DWM color accent settings.
+        /// </summary>
+        public struct DWMCOLORIZATIONPARAMS
+        {
+            /// <summary>
+            /// ColorizationColor
+            /// </summary>
+            public uint clrColor;
+
+            /// <summary>
+            /// ColorizationAfterglow.
+            /// </summary>
+            public uint clrAfterGlow;
+
+            /// <summary>
+            /// ColorizationColorBalance.
+            /// </summary>
+            public uint nIntensity;
+
+            /// <summary>
+            /// ColorizationAfterglowBalance.
+            /// </summary>
+            public uint clrAfterGlowBalance;
+
+            /// <summary>
+            /// ColorizationBlurBalance.
+            /// </summary>
+            public uint clrBlurBalance;
+
+            /// <summary>
+            /// ColorizationGlassReflectionIntensity.
+            /// </summary>
+            public uint clrGlassReflectionIntensity;
+
+            /// <summary>
+            /// ColorizationOpaqueBlend.
+            /// </summary>
+            public bool fOpaque;
+        }
+
+        /// <summary>
+        /// Sets the value of Desktop Window Manager (DWM) non-client rendering attributes for a window.
+        /// </summary>
+        /// <param name="hWnd">The handle to the window for which the attribute value is to be set.</param>
+        /// <param name="dwAttribute">A flag describing which value to set, specified as a value of the DWMWINDOWATTRIBUTE enumeration.</param>
+        /// <param name="pvAttribute">A pointer to an object containing the attribute value to set.</param>
+        /// <param name="cbAttribute">The size, in bytes, of the attribute value being set via the <c>pvAttribute</c> parameter.</param>
+        /// <returns>If the function succeeds, it returns <c>S_OK</c>. Otherwise, it returns an <c>HRESULT</c> error code.</returns>
+        [DllImport("dwmapi.dll")]
+        public static extern int DwmSetWindowAttribute(IntPtr hWnd, DWMWINDOWATTRIBUTE dwAttribute, ref int pvAttribute,
+            int cbAttribute);
+
+        /// <summary>
+        /// The feature is not included in the Microsoft documentation. Reads Desktop Window Manager (DWM) color information.
+        /// </summary>
+        /// <param name="dwParameters">A pointer to a reference value that will hold the color information.</param>
+        [DllImport("dwmapi.dll", EntryPoint = "#127", PreserveSig = false, CharSet = CharSet.Unicode)]
+        public static extern void DwmGetColorizationParameters(out DWMCOLORIZATIONPARAMS dwParameters);
+    }
+}

ModernWpf.MessageBox/Extensions/MessageBoxImageExtensions.cs

@@ -1,10 +1,14 @@
 using System;
 using System.Windows;
 
-namespace ModernWpf.Extensions {
-    internal static class MessageBoxImageExtensions {
-        public static SymbolGlyph ToSymbol(this MessageBoxImage image) {
-            return image switch {
+namespace ModernWpf.Extensions
+{
+    internal static class MessageBoxImageExtensions
+    {
+        public static SymbolGlyph ToSymbol(this MessageBoxImage image)
+        {
+            return image switch
+            {
                 MessageBoxImage.Error => SymbolGlyph.Error,
                 MessageBoxImage.Information => SymbolGlyph.Info,
                 MessageBoxImage.Warning => SymbolGlyph.Warning,

ModernWpf.MessageBox/Extensions/OSVersionHelper.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Diagnostics;
+using System.Runtime.InteropServices;
+
+namespace ModernWpf
+{
+    internal static class OSVersionHelper
+    {
+        public static readonly Version OSVersion = GetOSVersion();
+
+        public static bool IsWindowsNT { get; } = Environment.OSVersion.Platform == PlatformID.Win32NT;
+
+        public static bool IsWindowsVistaOrGreater { get; } = IsWindowsNT && OSVersion >= new Version(6, 0);
+
+        public static bool IsWindows7OrGreater { get; } = IsWindowsNT && OSVersion >= new Version(6, 1);
+
+        public static bool IsWindows8OrGreater { get; } = IsWindowsNT && OSVersion >= new Version(6, 2);
+
+        public static bool IsWindows81OrGreater { get; } = IsWindowsNT && OSVersion >= new Version(6, 2);
+
+        public static bool IsWindows10OrGreater { get; } = IsWindowsNT && OSVersion >= new Version(10, 0);
+
+        public static bool IsWindows11OrGreater { get; } = IsWindowsNT && OSVersion >= new Version(10, 0, 21996);
+
+        private static Version GetOSVersion()
+        {
+            var osv = new RTL_OSVERSIONINFOEX();
+            osv.dwOSVersionInfoSize = (uint)Marshal.SizeOf(osv);
+            int ret = RtlGetVersion(out osv);
+            Debug.Assert(ret == 0);
+            return new Version((int)osv.dwMajorVersion, (int)osv.dwMinorVersion, (int)osv.dwBuildNumber);
+        }
+
+        [DllImport("ntdll.dll")]
+        private static extern int RtlGetVersion(out RTL_OSVERSIONINFOEX lpVersionInformation);
+
+        [StructLayout(LayoutKind.Sequential)]
+        private struct RTL_OSVERSIONINFOEX
+        {
+            internal uint dwOSVersionInfoSize;
+            internal uint dwMajorVersion;
+            internal uint dwMinorVersion;
+            internal uint dwBuildNumber;
+            internal uint dwPlatformId;
+            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
+            internal string szCSDVersion;
+        }
+    }
+}

ModernWpf.MessageBox/Extensions/SymbolExtensions.cs

@@ -1,7 +1,9 @@
 using ModernWpf.Controls;
 
-namespace ModernWpf.Extensions {
-    internal static class SymbolExtensions {
+namespace ModernWpf.Extensions
+{
+    internal static class SymbolExtensions
+    {
         public static string ToGlyph(this Symbol symbol) =>
             char.ConvertFromUtf32((int)symbol);
     }

ModernWpf.MessageBox/Extensions/SymbolGlyphExtensions.cs

@@ -1,5 +1,7 @@
-namespace ModernWpf.Extensions {
-    internal static class SymbolGlyphExtensions {
+namespace ModernWpf.Extensions
+{
+    internal static class SymbolGlyphExtensions
+    {
         public static string ToGlyph(this SymbolGlyph symbol) =>
             char.ConvertFromUtf32((int)symbol);
     }