Enable nullable reference types (#2069)

<!--- Provide a general summary of your changes in the Title above -->

## Description
<!--- Describe your changes in detail -->

This PR will enable `Nullable` to help catch potential null reference
issues at compile time, improve code clarity, and align the samples with
modern .NET best practices.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here. -->

Closes #2066.

## How Has This Been Tested?
<!--- Please describe in detail how you tested your changes. -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->

## Screenshots (if appropriate):

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

## Issues not Directly Related to This PR
I'll use this section to note any unrelated issues I encounter while
working on this PR.

- Scratch Pad page: Clicking the Load button throws an exception when no
local settings are present.
- AnimatedIcon page: Clicking the button in the first sample and then
closing the app results in an exception.
- PullToRefresh page: Performing a pull-to-refresh on the second sample,
then navigating to another page, triggers an exception.
- RichEditBox page: Menu_Opening method contains duplicate code.

---------

Co-authored-by: Marcel W. <marcel.alex.wagner@outlook.com>

Author: AndrewKeepCoding

WinUIGallery/App.xaml.cs

@@ -116,10 +116,11 @@ private async void EnsureWindow()
         var targetPageArguments = string.Empty;
 
         AppActivationArguments eventArgs = AppInstance.GetCurrent().GetActivatedEventArgs();
-        if (eventArgs != null && eventArgs.Kind == ExtendedActivationKind.Protocol && eventArgs.Data is ProtocolActivatedEventArgs)
+        if (eventArgs != null &&
+            eventArgs.Kind == ExtendedActivationKind.Protocol &&
+            eventArgs.Data is ProtocolActivatedEventArgs protocolArgs)
         {
-            var ProtocolArgs = eventArgs.Data as ProtocolActivatedEventArgs;
-            string uri = ProtocolArgs.Uri.LocalPath.Replace("/", "");
+            string uri = protocolArgs.Uri.LocalPath.Replace("/", "");
             targetPageArguments = uri;
 
             if (uri == "AllControls")
@@ -162,15 +163,15 @@ private void HandleExceptions(object sender, Microsoft.UI.Xaml.UnhandledExceptio
         if (NativeMethods.IsAppPackaged)
         {
             e.Handled = true; //Don't crash the app.
-        
+
             //Create the notification.
             var notification = new AppNotificationBuilder()
                 .AddText("An exception was thrown.")
                 .AddText($"Type: {e.Exception.GetType()}")
                 .AddText($"Message: {e.Message}\r\n" +
                          $"HResult: {e.Exception.HResult}")
                 .BuildNotification();
-        
+
             //Show the notification
             AppNotificationManager.Default.Show(notification);
         }

WinUIGallery/Controls/ControlExample.xaml.cs

@@ -19,12 +19,12 @@ namespace WinUIGallery.Controls;
 /// </summary>
 public sealed class ControlExampleSubstitution : DependencyObject
 {
-    public event TypedEventHandler<ControlExampleSubstitution, object> ValueChanged;
+    public event TypedEventHandler<ControlExampleSubstitution, object?>? ValueChanged;
 
-    public string Key { get; set; }
+    public string Key { get; set; } = string.Empty;
 
-    private object _value = null;
-    public object Value
+    private object? _value = null;
+    public object? Value
     {
         get { return _value; }
         set
@@ -52,20 +52,15 @@ public string ValueAsString()
             return string.Empty;
         }
 
-        object value = Value;
+        object? value = Value;
 
         // For solid color brushes, use the underlying color.
         if (value is SolidColorBrush)
         {
             value = ((SolidColorBrush)value).Color;
         }
 
-        if (value == null)
-        {
-            return string.Empty;
-        }
-
-        return value.ToString();
+        return value?.ToString() ?? string.Empty;
     }
 }
 
@@ -193,15 +188,19 @@ private enum SyntaxHighlightLanguage { Xml, CSharp };
 
     private void SelectorBarItem_Loaded(object sender, RoutedEventArgs e)
     {
-        var item = sender as SelectorBarItem;
-        if (item == null)
+        if (sender is not SelectorBarItem item)
             return;
 
-        if (item.Tag.ToString().Equals("Xaml", StringComparison.OrdinalIgnoreCase))
+        PrepareSelectorBarItem(item);
+    }
+
+    private void PrepareSelectorBarItem(SelectorBarItem item)
+    {
+        if (item.Tag.ToString()?.Equals("Xaml", StringComparison.OrdinalIgnoreCase) is true)
         {
             item.Visibility = string.IsNullOrEmpty(Xaml) && string.IsNullOrEmpty(XamlSource) ? Visibility.Collapsed : Visibility.Visible;
         }
-        else if (item.Tag.ToString().Equals("CSharp", StringComparison.OrdinalIgnoreCase))
+        else if (item.Tag.ToString()?.Equals("CSharp", StringComparison.OrdinalIgnoreCase) is true)
         {
             item.Visibility = string.IsNullOrEmpty(CSharp) && string.IsNullOrEmpty(CSharpSource) ? Visibility.Collapsed : Visibility.Visible;
         }
@@ -217,19 +216,18 @@ private void SelectorBarItem_Loaded(object sender, RoutedEventArgs e)
 
     private static void OnXamlChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
     {
-        var ctrl = (ControlExample)d;
-        if (ctrl != null)
-        {
-            ctrl.SelectorBarItem_Loaded(ctrl.SelectorBarXamlItem, null);
-        }
+        if (d is not ControlExample ctrl)
+            return;
+
+        ctrl.PrepareSelectorBarItem(ctrl.SelectorBarXamlItem);
     }
+
     private static void OnCSharpChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
     {
-        var ctrl = (ControlExample)d;
-        if (ctrl != null)
-        {
-            ctrl.SelectorBarItem_Loaded(ctrl.SelectorBarCSharpItem, null);
-        }
+        if (d is not ControlExample ctrl)
+            return;
+
+        ctrl.PrepareSelectorBarItem(ctrl.SelectorBarCSharpItem);
     }
 
     private void SelectorBarControl_SelectionChanged(SelectorBar sender, SelectorBarSelectionChangedEventArgs args)
@@ -242,12 +240,12 @@ private void HandlePresenterVisibility()
         var selectedItem = SelectorBarControl.SelectedItem;
         if (selectedItem != null)
         {
-            if (selectedItem.Tag.ToString().Equals("Xaml", StringComparison.OrdinalIgnoreCase))
+            if (selectedItem.Tag.ToString()?.Equals("Xaml", StringComparison.OrdinalIgnoreCase) is true)
             {
                 XamlContentPresenter.Visibility = Visibility.Visible;
                 CSharpContentPresenter.Visibility = Visibility.Collapsed;
             }
-            else if (selectedItem.Tag.ToString().Equals("CSharp", StringComparison.OrdinalIgnoreCase))
+            else if (selectedItem.Tag.ToString()?.Equals("CSharp", StringComparison.OrdinalIgnoreCase) is true)
             {
                 CSharpContentPresenter.Visibility = Visibility.Visible;
                 XamlContentPresenter.Visibility = Visibility.Collapsed;

WinUIGallery/Controls/InlineColorPicker.xaml.cs

@@ -40,7 +40,7 @@ public SolidColorBrush ColorBrush
     public static readonly DependencyProperty ColorBrushProperty =
         DependencyProperty.Register("ColorBrush", typeof(SolidColorBrush), typeof(InlineColorPicker), new PropertyMetadata(new SolidColorBrush(Microsoft.UI.Colors.White)));
 
-    public event EventHandler<Color> ColorChanged;
+    public event EventHandler<Color>? ColorChanged;
 
     public InlineColorPicker()
     {

WinUIGallery/Controls/PageHeader.xaml.cs

@@ -21,17 +21,17 @@ public Visibility ThemeButtonVisibility
     public static readonly DependencyProperty ThemeButtonVisibilityProperty =
         DependencyProperty.Register("ThemeButtonVisibility", typeof(Visibility), typeof(PageHeader), new PropertyMetadata(Visibility.Visible));
 
-    public string PageName { get; set; }
-    public Action CopyLinkAction { get; set; }
-    public Action ToggleThemeAction { get; set; }
+    public string PageName { get; set; } = string.Empty;
+    public Action? CopyLinkAction { get; set; }
+    public Action? ToggleThemeAction { get; set; }
 
-    public ControlInfoDataItem Item
+    public ControlInfoDataItem? Item
     {
         get { return _item; }
         set { _item = value; }
     }
 
-    private ControlInfoDataItem _item;
+    private ControlInfoDataItem? _item;
 
     public PageHeader()
     {
@@ -86,6 +86,11 @@ private void OnCopyDontShowAgainButtonClick(TeachingTip sender, object args)
 
     private void OnCopyLink()
     {
+        if (this.Item is null)
+        {
+            return;
+        }
+
         ProtocolActivationClipboardHelper.Copy(this.Item);
     }
     public async void OnFeedBackButtonClick(object sender, RoutedEventArgs e)
@@ -107,12 +112,12 @@ private void UserControl_Loaded(object sender, RoutedEventArgs e)
 
     private string GetFavoriteGlyph(bool? isFavorite)
     {
-        return (bool)isFavorite ? "\uE735" : "\uE734";
+        return isFavorite is true ? "\uE735" : "\uE734";
     }
 
     private string GetFavoriteToolTip(bool? isFavorite)
     {
-        return (bool)isFavorite ? "Remove from favorites" : "Add to favorites";
+        return isFavorite is true ? "Remove from favorites" : "Add to favorites";
     }
 
     private void FavoriteButton_Click(object sender, RoutedEventArgs e)

WinUIGallery/Controls/SampleCodePresenter.xaml.cs

@@ -60,7 +60,7 @@ public IList<ControlExampleSubstitution> Substitutions
 
     private string actualCode = "";
     private static Regex SubstitutionPattern = new Regex(@"\$\(([^\)]+)\)");
-    private RichTextBlock sampleCodeRTB;
+    private RichTextBlock? sampleCodeRTB;
 
     public SampleCodePresenter()
     {
@@ -126,7 +126,7 @@ private void SampleCodePresenter_ActualThemeChanged(FrameworkElement sender, obj
         GenerateSyntaxHighlightedContent();
     }
 
-    private void OnValueChanged(ControlExampleSubstitution sender, object e)
+    private void OnValueChanged(ControlExampleSubstitution sender, object? _)
     {
         GenerateSyntaxHighlightedContent();
     }
@@ -166,8 +166,7 @@ private async void FormatAndRenderSampleFromFile(string sourceRelativePath, Cont
     {
         if (sourceRelativePath != null && sourceRelativePath.EndsWith("txt"))
         {
-            string sampleString = null;
-            StorageFile file = null;
+            StorageFile? file = null;
             if (!NativeMethods.IsAppPackaged)
             {
                 var relativePath = GetDerivedSourceUnpackaged(sourceRelativePath);
@@ -182,7 +181,7 @@ private async void FormatAndRenderSampleFromFile(string sourceRelativePath, Cont
                 file = await StorageFile.GetFileFromApplicationUriAsync(derivedSource);
             }
 
-            sampleString = await FileIO.ReadTextAsync(file);
+            string sampleString = await FileIO.ReadTextAsync(file);
 
             FormatAndRenderSampleFromString(sampleString, presenter, highlightLanguage);
         }
@@ -319,7 +318,7 @@ private void CopyCodeButton_Click(object sender, RoutedEventArgs e)
 
     private void CodeScrollViewer_Loaded(object sender, RoutedEventArgs e)
     {
-        ScrollBar horizontalScrollBar = FindHorizontalScrollBar(CodeScrollViewer);
+        ScrollBar? horizontalScrollBar = FindHorizontalScrollBar(CodeScrollViewer);
         if (horizontalScrollBar != null)
         {
             // Create a timer and store it in the ScrollBar's Tag property.
@@ -361,7 +360,7 @@ private void HorizontalScrollBar_Scroll(object sender, ScrollEventArgs e)
         }
     }
 
-    private ScrollBar FindHorizontalScrollBar(DependencyObject element)
+    private ScrollBar? FindHorizontalScrollBar(DependencyObject element)
     {
         if (element is ScrollBar sb && sb.Orientation == Orientation.Horizontal)
         {

WinUIGallery/Converters/MenuItemTemplateSelector.cs

@@ -12,14 +12,14 @@ namespace WinUIGallery.Converters;
 [ContentProperty(Name = "ItemTemplate")]
 partial class MenuItemTemplateSelector : DataTemplateSelector
 {
-    public DataTemplate ItemTemplate { get; set; }
+    public DataTemplate? ItemTemplate { get; set; }
 
-    protected override DataTemplate SelectTemplateCore(object item)
+    protected override DataTemplate? SelectTemplateCore(object item)
     {
         return item is Separator ? SeparatorTemplate : item is Header ? HeaderTemplate : ItemTemplate;
     }
 
-    protected override DataTemplate SelectTemplateCore(object item, DependencyObject container)
+    protected override DataTemplate? SelectTemplateCore(object item, DependencyObject container)
     {
         return item is Separator ? SeparatorTemplate : item is Header ? HeaderTemplate : ItemTemplate;
     }

WinUIGallery/Helpers/ControlInfoDataSource.cs

@@ -54,7 +54,7 @@ public async Task<IEnumerable<ControlInfoDataGroup>> GetGroupsAsync()
         return _instance.Groups;
     }
 
-    public static async Task<ControlInfoDataGroup> GetGroupAsync(string uniqueId)
+    public static async Task<ControlInfoDataGroup?> GetGroupAsync(string uniqueId)
     {
         await _instance.GetControlInfoDataAsync();
         // Simple linear search is acceptable for small data sets
@@ -63,7 +63,7 @@ public static async Task<ControlInfoDataGroup> GetGroupAsync(string uniqueId)
         return null;
     }
 
-    public static async Task<ControlInfoDataItem> GetItemAsync(string uniqueId)
+    public static async Task<ControlInfoDataItem?> GetItemAsync(string uniqueId)
     {
         await _instance.GetControlInfoDataAsync();
         // Simple linear search is acceptable for small data sets
@@ -72,7 +72,7 @@ public static async Task<ControlInfoDataItem> GetItemAsync(string uniqueId)
         return null;
     }
 
-    public static async Task<ControlInfoDataGroup> GetGroupFromItemAsync(string uniqueId)
+    public static async Task<ControlInfoDataGroup?> GetGroupFromItemAsync(string uniqueId)
     {
         await _instance.GetControlInfoDataAsync();
         var matches = _instance.Groups.Where((group) => group.Items.FirstOrDefault(item => item.UniqueId.Equals(uniqueId)) != null);
@@ -93,27 +93,30 @@ private async Task GetControlInfoDataAsync()
         var jsonText = await FileLoader.LoadText("Samples/Data/ControlInfoData.json");
         var controlInfoDataGroup = JsonSerializer.Deserialize(jsonText, typeof(Root), RootContext.Default) as Root;
 
+        if (controlInfoDataGroup is null)
+        {
+            return;
+        }
+
         lock (_lock)
         {
             string pageRoot = "WinUIGallery.ControlPages.";
 
             controlInfoDataGroup.Groups.SelectMany(g => g.Items).ToList().ForEach(item =>
             {
-#nullable enable
-                string? badgeString = item switch
+                string badgeString = item switch
                 {
                     { IsNew: true } => "New",
                     { IsUpdated: true } => "Updated",
                     { IsPreview: true } => "Preview",
-                    _ => null
+                    _ => string.Empty,
                 };
                 string pageString = $"{pageRoot}{item.UniqueId}Page";
                 Type? pageType = Type.GetType(pageString);
 
                 item.BadgeString = badgeString;
                 item.IncludedInBuild = pageType is not null;
                 item.ImagePath ??= "ms-appx:///Assets/ControlImages/Placeholder.png";
-#nullable disable
             });
 
             foreach (var group in controlInfoDataGroup.Groups)

WinUIGallery/Helpers/FileLoader.cs

@@ -14,7 +14,7 @@ internal partial class FileLoader
 {
     public static async Task<string> LoadText(string relativeFilePath)
     {
-        StorageFile file = null;
+        StorageFile? file = null;
         if (!NativeMethods.IsAppPackaged)
         {
             var sourcePath = Path.GetFullPath(Path.Combine(AppContext.BaseDirectory, relativeFilePath));

WinUIGallery/Helpers/IconsDataSource.cs

@@ -31,10 +31,12 @@ public async Task<List<IconData>> LoadIcons()
         var jsonText = await FileLoader.LoadText("Samples/Data/IconsData.json");
         lock (_lock)
         {
-            if (icons.Count == 0)
+            if (icons.Count == 0 &&
+                JsonSerializer.Deserialize(jsonText, typeof(List<IconData>), IconDataListContext.Default) is List<IconData> loadedIcons)
             {
-                icons = JsonSerializer.Deserialize(jsonText, typeof(List<IconData>), IconDataListContext.Default) as List<IconData>;
+                icons = loadedIcons;
             }
+
             return icons;
         }
     }