.github/workflows/dotnetcore.yml

@@ -15,6 +15,8 @@ jobs:
     - uses: actions/checkout@v2
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.1.101
     - name: Install dependencies
       run: dotnet restore
     - name: Build

App/ContentDialogExample.xaml.cs

@@ -7,9 +7,9 @@ namespace ModernWpfPlayground
         public static readonly DependencyProperty MessageProperty = DependencyProperty.Register(
             "Message", typeof(string), typeof(ContentDialogExample), new PropertyMetadata(default(string)));
 
-        public string? Message
+        public string Message
         {
-            get => (string?) GetValue(MessageProperty);
+            get => (string) GetValue(MessageProperty);
             set => SetValue(MessageProperty, value);
         }
 

App/MainWindowViewModel.cs

@@ -75,7 +75,7 @@ namespace ModernWpfPlayground
 
         public ICommand ShowDialogCommand { get; }
 
-        public string? WelcomeMessage
+        public string WelcomeMessage
         {
             get => GetProperty("Shadow of the empire");
             set => SetProperty(value);
@@ -137,7 +137,7 @@ namespace ModernWpfPlayground
         private void SaveViewModel()
         {
             var contents = _serializer.Serialize(Values);
-            if (Path is null)
+            if (Path == null)
             {
                 var saveFileDialog = new SaveFileDialog {AddExtension = true, DefaultExt = "*.yaml"};
                 var result = saveFileDialog.ShowDialog(Application.Current.MainWindow?.Owner);

App/ModernWpfPlayground.csproj

@@ -15,6 +15,8 @@
     <PackageReference Include="YamlDotNet" Version="9.1.0" />
     <PackageReference Include="System.Drawing.Common" Version="5.0.0" />
     <PackageReference Include="MahApps.Metro.IconPacks.FontAwesome" Version="4.8.0" />
+    <PackageReference Include="Microsoft.Windows.CsWinRT" Version="1.1.0" />
+    <PackageReference Include="Microsoft.Windows.SDK.Contracts" Version="10.0.18362.2005" />
   </ItemGroup>
 
   <ItemGroup>

App/MvvmStuff/BaseViewModel.cs

@@ -14,9 +14,9 @@ namespace ModernWpfPlayground.MvvmStuff
             ObjectAccessor = ObjectAccessor.Create(this);
         }
 
-        private readonly Dictionary<string, object?> _values = new();
+        private readonly Dictionary<string, object?> _values = new Dictionary<string, object?>();
         protected readonly ObjectAccessor ObjectAccessor;
-        private readonly Dictionary<string, object?> _defaultValues = new();
+        private readonly Dictionary<string, object?> _defaultValues = new Dictionary<string, object?>();
 
         protected IReadOnlyDictionary<string, object?> Values => _values;
 
@@ -32,7 +32,7 @@ namespace ModernWpfPlayground.MvvmStuff
             return true;
         }
 
-        protected T? GetProperty<T>(T? defaultValue = default, [CallerMemberName] string? propertyName = null)
+        protected T GetProperty<T>(T defaultValue = default, [CallerMemberName] string? propertyName = null)
         {
             if (propertyName == null) throw new ArgumentNullException(nameof(propertyName));
             if (Values.TryGetValue(propertyName, out var obj))

Controls/Controls.csproj

@@ -1,13 +1,16 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net5.0-windows</TargetFramework>
+    <TargetFramework>net5.0-windows10.0.18362.0</TargetFramework>
     <UseWPF>true</UseWPF>
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.Windows.CsWinRT" Version="1.1.0" />
+    <PackageReference Include="ModernWpfUI" Version="0.9.3-preview.201204.0" />
     <PackageReference Include="System.Drawing.Common" Version="5.0.0" />
+    <PackageReference Include="Microsoft.Windows.SDK.Contracts" Version="10.0.18362.2005" />
   </ItemGroup>
 
 </Project>

Controls/PropertyPresenter/EnumToKeyValueListConverter.cs

@@ -26,12 +26,10 @@ namespace Controls
         /// <returns></returns>
         private static string GetDescription(object value)
         {
-            if (value is not Enum enumValue) return string.Empty;
+            if (!(value is Enum enumValue)) return string.Empty;
             var descriptionAttribute = enumValue.GetType()
-              .GetField(enumValue.ToString())?
-              .GetCustomAttributes(false)
-              .OfType<DescriptionAttribute>()
-              .FirstOrDefault();
+              .GetField(enumValue.ToString())?.GetCustomAttributes(false)
+              .OfType<DescriptionAttribute>().FirstOrDefault();
 
             return descriptionAttribute?.Description ?? value.ToString() ?? string.Empty;
         }

Controls/PropertyPresenter/MagicSymbolConverter.cs

@@ -36,11 +36,11 @@ namespace Controls
             var data = value as string;
             if (string.IsNullOrWhiteSpace(data)) return value; //maybe not a string. eventually something else.
 
-            if (data.StartsWith(NoParseKeyword, StringComparison.Ordinal)) return data[NoParseKeyword.Length..];
+            if (data.StartsWith(NoParseKeyword, StringComparison.Ordinal)) return data.Substring(NoParseKeyword.Length);
 
             if (data.StartsWith(PathKeyword, StringComparison.Ordinal))
             {
-                var path = data[PathKeyword.Length..];
+                var path = data.Substring(PathKeyword.Length);
                 var icon = ObjectImageConverter.GetIcon(Geometry.Parse(path), Brushes.Black);
                 return new Image
                 {
@@ -53,7 +53,7 @@ namespace Controls
 
             if (data.StartsWith(DynResKeyword, StringComparison.Ordinal))
             {
-                var resourceKey = data[DynResKeyword.Length..];
+                var resourceKey = data.Substring(DynResKeyword.Length);
                 //get icon from resource dictionary
                 return new Image
                 {

Controls/PropertyPresenter/ValueConverter.cs

@@ -26,7 +26,7 @@ namespace Controls
                 return System.Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap(bitmap.GetHbitmap(), IntPtr.Zero, Int32Rect.Empty, BitmapSizeOptions.FromWidthAndHeight(16, 16));
             }
 
-            if (value is not string strValue) return Binding.DoNothing;
+            if (!(value is string strValue)) return Binding.DoNothing;
             if (strValue.StartsWith(dynResPrefix, StringComparison.Ordinal))
             {
                 var resource = Application.Current.TryFindResource(strValue.Replace(dynResPrefix, string.Empty , StringComparison.InvariantCulture));