Merge branch 'master' into fixes/Samples/RenderDemo

samples/ControlCatalog/MainView.xaml.cs

@@ -22,8 +22,8 @@ namespace ControlCatalog
 
             if (AvaloniaLocator.Current?.GetService<IRuntimePlatform>()?.GetRuntimeInfo().IsDesktop == true)
             {
-                IList tabItems = ((IList)sideBar.Items);
-                tabItems.Add(new TabItem()
+                var tabItems = (sideBar.Items as IList);
+                tabItems?.Add(new TabItem()
                 {
                     Header = "Screens",
                     Content = new ScreenPage()
@@ -36,7 +36,7 @@ namespace ControlCatalog
             {
                 if (themes.SelectedItem is CatalogTheme theme)
                 {
-                    var themeStyle = Application.Current.Styles[0];
+                    var themeStyle = Application.Current!.Styles[0];
                     if (theme == CatalogTheme.FluentLight)
                     {
                         if (App.Fluent.Mode != FluentThemeMode.Light)

samples/ControlCatalog/Models/Person.cs

@@ -85,7 +85,7 @@ namespace ControlCatalog.Models
             }
             else
             {
-                if (_errorLookup.TryGetValue(propertyName, out List<string> errorList))
+                if (_errorLookup.TryGetValue(propertyName, out var errorList))
                 {
                     errorList.Clear();
                     errorList.Add(error!);
@@ -114,12 +114,12 @@ namespace ControlCatalog.Models
             PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
         }
 
-        public IEnumerable? GetErrors(string propertyName)
+        public IEnumerable GetErrors(string? propertyName)
         {
-            if (_errorLookup.TryGetValue(propertyName, out List<string> errorList))
+            if (propertyName is { } && _errorLookup.TryGetValue(propertyName, out var errorList))
                 return errorList;
             else
-                return null;
+                return Array.Empty<object>();
         }
     }
 }

samples/ControlCatalog/Pages/AutoCompleteBoxPage.xaml

@@ -28,7 +28,7 @@
       </StackPanel>
       <StackPanel>
         <TextBlock Text="MinimumPopulateDelay: 1s" />
-        <AutoCompleteBox MinimumPopulateDelay="1" />
+        <AutoCompleteBox MinimumPopulateDelay="00:00:01" />
       </StackPanel>
       <StackPanel>
         <TextBlock Text="MaxDropDownHeight: 60" />

samples/ControlCatalog/Pages/AutoCompleteBoxPage.xaml.cs

@@ -1,8 +1,6 @@
 using Avalonia.Controls;
 using Avalonia.LogicalTree;
-using Avalonia.Markup;
 using Avalonia.Markup.Xaml;
-using Avalonia.Markup.Data;
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -161,23 +159,23 @@ namespace ControlCatalog.Pages
         private bool LastWordContains(string? searchText, string? item)
         {
             var words = searchText?.Split(' ') ?? Array.Empty<string>();
-            var options = Sentences.Select(x => x.First).ToArray();
+            var options = Sentences.Select(x => x.First)
+                .ToArray<LinkedListNode<string>?>();
             for (var i = 0; i < words.Length; ++i)
             {
                 var word = words[i];
                 for (var j = 0; word is { } && j < options.Length; ++j)
                 {
-                    var option = options[j];
-                    if (option == null)
-                        continue;
-
-                    if (i == words.Length - 1)
-                    {
-                        options[j] = option.Value.ToLower().Contains(word.ToLower()) ? option : null;
-                    }
-                    else
+                    if (options[i] is { } option)
                     {
-                        options[j] = option.Value.Equals(word, StringComparison.InvariantCultureIgnoreCase) ? option.Next : null;
+                        if (i == words.Length - 1)
+                        {
+                            options[j] = option.Value.ToLower().Contains(word.ToLower()) ? option : null;
+                        }
+                        else
+                        {
+                            options[j] = option.Value.Equals(word, StringComparison.InvariantCultureIgnoreCase) ? option.Next : null;
+                        }
                     }
                 }
             }

samples/ControlCatalog/Pages/ButtonSpinnerPage.xaml.cs

@@ -21,20 +21,23 @@ namespace ControlCatalog.Pages
         public void OnSpin(object sender, SpinEventArgs e)
         {
             var spinner = (ButtonSpinner)sender;
-            var txtBox = (TextBlock)spinner.Content;
 
-            int value = Array.IndexOf(_mountains, txtBox?.Text);
-            if (e.Direction == SpinDirection.Increase)
-                value++;
-            else
-                value--;
+            if (spinner.Content is TextBlock txtBox)
+            {
+                int value = Array.IndexOf(_mountains, txtBox.Text);
+                if (e.Direction == SpinDirection.Increase)
+                    value++;
+                else
+                    value--;
 
-            if (value < 0)
-                value = _mountains.Length - 1;
-            else if (value >= _mountains.Length)
-                value = 0;
+                if (value < 0)
+                    value = _mountains.Length - 1;
+                else if (value >= _mountains.Length)
+                    value = 0;
+
+                txtBox.Text = _mountains[value];
+            }
 
-            txtBox.Text = _mountains[value];
         }
 
         private readonly string[] _mountains = new[]

samples/ControlCatalog/Pages/ButtonsPage.xaml.cs

@@ -19,7 +19,7 @@ namespace ControlCatalog.Pages
             AvaloniaXamlLoader.Load(this);
         }
 
-        public void OnRepeatButtonClick(object sender, object args)
+        public void OnRepeatButtonClick(object? sender, object args)
         {
             repeatButtonClickCount++;
             var textBlock = this.Get<TextBlock>("RepeatButtonTextBlock");

samples/ControlCatalog/Pages/CarouselPage.xaml.cs

@@ -33,7 +33,7 @@ namespace ControlCatalog.Pages
 
         }
 
-        private void TransitionChanged(object sender, SelectionChangedEventArgs e)
+        private void TransitionChanged(object? sender, SelectionChangedEventArgs e)
         {
             switch (_transition.SelectedIndex)
             {

samples/ControlCatalog/Pages/ClipboardPage.xaml.cs

@@ -23,55 +23,79 @@ namespace ControlCatalog.Pages
             AvaloniaXamlLoader.Load(this);
         }
 
-        private async void CopyText(object sender, RoutedEventArgs args)
+        private async void CopyText(object? sender, RoutedEventArgs args)
         {
-            await Application.Current.Clipboard.SetTextAsync(ClipboardContent.Text);
+            if (Application.Current!.Clipboard is { } clipboard && ClipboardContent is { } clipboardContent)
+                await clipboard.SetTextAsync(clipboardContent.Text ?? String.Empty);
         }
 
-        private async void PasteText(object sender, RoutedEventArgs args)
+        private async void PasteText(object? sender, RoutedEventArgs args)
         {
-            ClipboardContent.Text = await Application.Current.Clipboard.GetTextAsync();
+            if(Application.Current!.Clipboard is { } clipboard)
+            {
+                ClipboardContent.Text = await clipboard.GetTextAsync();
+            }
         }
 
-        private async void CopyTextDataObject(object sender, RoutedEventArgs args)
+        private async void CopyTextDataObject(object? sender, RoutedEventArgs args)
         {
-            var dataObject = new DataObject();
-            dataObject.Set(DataFormats.Text, ClipboardContent.Text ?? string.Empty);
-            await Application.Current.Clipboard.SetDataObjectAsync(dataObject);
+            if (Application.Current!.Clipboard is { } clipboard)
+            {
+                var dataObject = new DataObject();
+                dataObject.Set(DataFormats.Text, ClipboardContent.Text ?? string.Empty);
+                await clipboard.SetDataObjectAsync(dataObject);
+            }
         }
 
-        private async void PasteTextDataObject(object sender, RoutedEventArgs args)
+        private async void PasteTextDataObject(object? sender, RoutedEventArgs args)
         {
-            ClipboardContent.Text = await Application.Current.Clipboard.GetDataAsync(DataFormats.Text) as string ?? string.Empty;
+            if (Application.Current!.Clipboard is { } clipboard)
+            {
+                ClipboardContent.Text = await clipboard.GetDataAsync(DataFormats.Text) as string ?? string.Empty;
+            }
         }
 
-        private async void CopyFilesDataObject(object sender, RoutedEventArgs args)
+        private async void CopyFilesDataObject(object? sender, RoutedEventArgs args)
         {
-            var files = ClipboardContent.Text.Split(new[] { Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries);
-            if (files.Length == 0)
+            if (Application.Current!.Clipboard is { } clipboard)
             {
-                return;
+                var files = (ClipboardContent.Text ?? String.Empty)
+                .Split(new[] { Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries);
+                if (files.Length == 0)
+                {
+                    return;
+                }
+                var dataObject = new DataObject();
+                dataObject.Set(DataFormats.FileNames, files);
+                await clipboard.SetDataObjectAsync(dataObject);
             }
-            var dataObject = new DataObject();
-            dataObject.Set(DataFormats.FileNames, files);
-            await Application.Current.Clipboard.SetDataObjectAsync(dataObject);
         }
 
-        private async void PasteFilesDataObject(object sender, RoutedEventArgs args)
+        private async void PasteFilesDataObject(object? sender, RoutedEventArgs args)
         {
-            var fiels = await Application.Current.Clipboard.GetDataAsync(DataFormats.FileNames) as IEnumerable<string>;
-            ClipboardContent.Text = fiels != null ? string.Join(Environment.NewLine, fiels) : string.Empty;
+            if (Application.Current!.Clipboard is { } clipboard)
+            {
+                var fiels = await clipboard.GetDataAsync(DataFormats.FileNames) as IEnumerable<string>;
+                ClipboardContent.Text = fiels != null ? string.Join(Environment.NewLine, fiels) : string.Empty;
+            }
         }
 
         private async void GetFormats(object sender, RoutedEventArgs args)
         {
-            var formats = await Application.Current.Clipboard.GetFormatsAsync();
-            ClipboardContent.Text = string.Join(Environment.NewLine, formats);
+            if (Application.Current!.Clipboard is { } clipboard)
+            {
+                var formats = await clipboard.GetFormatsAsync();
+                ClipboardContent.Text = string.Join(Environment.NewLine, formats);
+            }
         }
 
         private async void Clear(object sender, RoutedEventArgs args)
         {
-            await Application.Current.Clipboard.ClearAsync();
+            if (Application.Current!.Clipboard is { } clipboard)
+            {
+                await clipboard.ClearAsync();
+            }
+                
         }
     }
 }

samples/ControlCatalog/Pages/ComboBoxPage.xaml.cs

@@ -17,7 +17,7 @@ namespace ControlCatalog.Pages
         private void InitializeComponent()
         {
             AvaloniaXamlLoader.Load(this);
-            var fontComboBox = this.Find<ComboBox>("fontComboBox");
+            var fontComboBox = this.Get<ComboBox>("fontComboBox");
             fontComboBox.Items = FontManager.Current.GetInstalledFontFamilyNames().Select(x => new FontFamily(x));
             fontComboBox.SelectedIndex = 0;
         }

samples/ControlCatalog/Pages/CompositionPage.axaml.cs

@@ -1,14 +1,8 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
-using System.Numerics;
-using System.Threading.Tasks;
 using Avalonia;
 using Avalonia.Controls;
-using Avalonia.Controls.Primitives;
-using Avalonia.Interactivity;
 using Avalonia.Markup.Xaml;
-using Avalonia.Markup.Xaml.Templates;
 using Avalonia.Media;
 using Avalonia.Rendering.Composition;
 using Avalonia.Rendering.Composition.Animations;
@@ -18,7 +12,7 @@ namespace ControlCatalog.Pages;
 
 public partial class CompositionPage : UserControl
 {
-    private ImplicitAnimationCollection _implicitAnimations;
+    private ImplicitAnimationCollection? _implicitAnimations;
 
     public CompositionPage()
     {
@@ -28,7 +22,7 @@ public partial class CompositionPage : UserControl
     protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
     {
         base.OnAttachedToVisualTree(e);
-        this.FindControl<ItemsControl>("Items").Items = CreateColorItems();
+        this.Get<ItemsControl>("Items").Items = CreateColorItems();
     }
 
     private List<CompositionPageColorItem> CreateColorItems()
@@ -115,7 +109,6 @@ public partial class CompositionPage : UserControl
 
     public static void SetEnableAnimations(Border border, bool value)
     {
-        
         var page = border.FindAncestorOfType<CompositionPage>();
         if (page == null)
         {
@@ -127,8 +120,11 @@ public partial class CompositionPage : UserControl
             return;
 
         page.EnsureImplicitAnimations();
-        ElementComposition.GetElementVisual((Visual)border.GetVisualParent()).ImplicitAnimations =
-            page._implicitAnimations;
+        if (border.GetVisualParent() is Visual visualParent 
+            && ElementComposition.GetElementVisual(visualParent) is CompositionVisual compositionVisual)
+        {
+            compositionVisual.ImplicitAnimations = page._implicitAnimations;
+        }
     }
 }
 
@@ -150,4 +146,4 @@ public class CompositionPageColorItem
     {
         Color = color;
     }
-}
+}

samples/ControlCatalog/Pages/ContextFlyoutPage.xaml.cs

@@ -52,13 +52,13 @@ namespace ControlCatalog.Pages
             base.OnDataContextChanged(e);
         }
 
-        private void ContextFlyoutPage_Closing(object sender, CancelEventArgs e)
+        private void ContextFlyoutPage_Closing(object? sender, CancelEventArgs e)
         {
             var cancelCloseCheckBox = this.FindControl<CheckBox>("CancelCloseCheckBox");
             e.Cancel = cancelCloseCheckBox?.IsChecked ?? false;
         }
 
-        private void ContextFlyoutPage_Opening(object sender, EventArgs e)
+        private void ContextFlyoutPage_Opening(object? sender, EventArgs e)
         {
             if (e is CancelEventArgs cancelArgs)
             {
@@ -67,20 +67,20 @@ namespace ControlCatalog.Pages
             }
         }
 
-        private void CloseFlyout(object sender, RoutedEventArgs e)
+        private void CloseFlyout(object? sender, RoutedEventArgs e)
         {
             _textBox.ContextFlyout?.Hide();
         }
 
-        public void CustomContextRequested(object sender, ContextRequestedEventArgs e)
+        public void CustomContextRequested(object? sender, ContextRequestedEventArgs e)
         {
-            var border = (Border)sender;
-            var textBlock = (TextBlock)border.Child;
-
-            textBlock.Text = e.TryGetPosition(border, out var point)
-                ? $"Context was requested with pointer at: {point.X:N0}, {point.Y:N0}"
-                : "Context was requested without pointer";
-            e.Handled = true;
+            if (sender is Border border && border.Child is TextBlock textBlock)
+            {
+                textBlock.Text = e.TryGetPosition(border, out var point)
+                    ? $"Context was requested with pointer at: {point.X:N0}, {point.Y:N0}"
+                    : "Context was requested without pointer";
+                e.Handled = true;
+            }
         }
 
         private void InitializeComponent()

samples/ControlCatalog/Pages/ContextMenuPage.xaml.cs

@@ -35,30 +35,31 @@ namespace ControlCatalog.Pages
             base.OnDataContextChanged(e);
         }
 
-        private void ContextFlyoutPage_Closing(object sender, CancelEventArgs e)
+        private void ContextFlyoutPage_Closing(object? sender, CancelEventArgs e)
         {
             var cancelCloseCheckBox = this.FindControl<CheckBox>("CancelCloseCheckBox");
-            e.Cancel = cancelCloseCheckBox.IsChecked ?? false;
+            e.Cancel = cancelCloseCheckBox?.IsChecked ?? false;
         }
 
-        private void ContextFlyoutPage_Opening(object sender, EventArgs e)
+        private void ContextFlyoutPage_Opening(object? sender, EventArgs e)
         {
             if (e is CancelEventArgs cancelArgs)
             {
                 var cancelCloseCheckBox = this.FindControl<CheckBox>("CancelOpenCheckBox");
-                cancelArgs.Cancel = cancelCloseCheckBox.IsChecked ?? false;
+                cancelArgs.Cancel = cancelCloseCheckBox?.IsChecked ?? false;
             }
         }
 
-        public void CustomContextRequested(object sender, ContextRequestedEventArgs e)
+        public void CustomContextRequested(object? sender, ContextRequestedEventArgs e)
         {
-            var border = (Border)sender;
-            var textBlock = (TextBlock)border.Child;
+            if (sender is Border border && border.Child is TextBlock textBlock)
+            {
+                textBlock.Text = e.TryGetPosition(border, out var point)
+                    ? $"Context was requested with pointer at: {point.X:N0}, {point.Y:N0}"
+                    : "Context was requested without pointer";
+                e.Handled = true;
+            }
 
-            textBlock.Text = e.TryGetPosition(border, out var point)
-                ? $"Context was requested with pointer at: {point.X:N0}, {point.Y:N0}"
-                : "Context was requested without pointer";
-            e.Handled = true;
         }
 
         private void InitializeComponent()

samples/ControlCatalog/Pages/DataGridPage.xaml.cs

@@ -62,7 +62,7 @@ namespace ControlCatalog.Pages
             addButton.Click += (a, b) => collectionView3.AddNew();
         }
 
-        private void Dg1_LoadingRow(object sender, DataGridRowEventArgs e)
+        private void Dg1_LoadingRow(object? sender, DataGridRowEventArgs e)
         {
             e.Row.Header = e.Row.GetIndex() + 1;
         }
@@ -74,7 +74,7 @@ namespace ControlCatalog.Pages
 
         private class ReversedStringComparer : IComparer<object>, IComparer
         {
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
                 if (x is string left && y is string right)
                 {

samples/ControlCatalog/Pages/DialogsPage.xaml.cs

@@ -111,9 +111,16 @@ namespace ControlCatalog.Pages
                     Title = "Select folder",
                     Directory = lastSelectedDirectory?.TryGetUri(out var path) == true ? path.LocalPath : null
                 }.ShowAsync(GetWindow());
-                lastSelectedDirectory = new BclStorageFolder(new System.IO.DirectoryInfo(result));
-                results.Items = new [] { result };
-                resultsVisible.IsVisible = result != null;
+                if (string.IsNullOrEmpty(result))
+                {
+                    resultsVisible.IsVisible = false;
+                }
+                else
+                {
+                    lastSelectedDirectory = new BclStorageFolder(new System.IO.DirectoryInfo(result));
+                    results.Items = new[] { result };
+                    resultsVisible.IsVisible = true;
+                }
             };
             this.Get<Button>("OpenBoth").Click += async delegate
             {

samples/ControlCatalog/Pages/DragAndDropPage.xaml.cs

@@ -1,9 +1,9 @@
-using Avalonia.Controls;
-using Avalonia.Input;
-using Avalonia.Markup.Xaml;
-using System;
+using System;
 using System.Linq;
 using System.Reflection;
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Markup.Xaml;
 
 namespace ControlCatalog.Pages
 {
@@ -27,9 +27,9 @@ namespace ControlCatalog.Pages
         void SetupDnd(string suffix, Action<DataObject> factory, DragDropEffects effects)
         {
             var dragMe = this.Get<Border>("DragMe" + suffix);
-            var dragState = this.Get<TextBlock>("DragState"+suffix);
+            var dragState = this.Get<TextBlock>("DragState" + suffix);
 
-            async void DoDrag(object sender, Avalonia.Input.PointerPressedEventArgs e)
+            async void DoDrag(object? sender, Avalonia.Input.PointerPressedEventArgs e)
             {
                 var dragData = new DataObject();
                 factory(dragData);
@@ -55,7 +55,7 @@ namespace ControlCatalog.Pages
                 }
             }
 
-            void DragOver(object sender, DragEventArgs e)
+            void DragOver(object? sender, DragEventArgs e)
             {
                 if (e.Source is Control c && c.Name == "MoveTarget")
                 {
@@ -73,7 +73,7 @@ namespace ControlCatalog.Pages
                     e.DragEffects = DragDropEffects.None;
             }
 
-            void Drop(object sender, DragEventArgs e)
+            void Drop(object? sender, DragEventArgs e)
             {
                 if (e.Source is Control c && c.Name == "MoveTarget")
                 {
@@ -83,11 +83,11 @@ namespace ControlCatalog.Pages
                 {
                     e.DragEffects = e.DragEffects & (DragDropEffects.Copy);
                 }
-                
+
                 if (e.Data.Contains(DataFormats.Text))
                     _DropState.Text = e.Data.GetText();
                 else if (e.Data.Contains(DataFormats.FileNames))
-                    _DropState.Text = string.Join(Environment.NewLine, e.Data.GetFileNames());
+                    _DropState.Text = string.Join(Environment.NewLine, e.Data.GetFileNames() ?? Array.Empty<string>());
                 else if (e.Data.Contains(CustomFormat))
                     _DropState.Text = "Custom: " + e.Data.Get(CustomFormat);
             }

samples/ControlCatalog/Pages/FlyoutsPage.axaml.cs

@@ -20,7 +20,7 @@ namespace ControlCatalog.Pages
             SetXamlTexts();
         }
 
-        private void Afp_DoubleTapped(object sender, RoutedEventArgs e)
+        private void Afp_DoubleTapped(object? sender, RoutedEventArgs e)
         {
             if (sender is Panel p)
             {

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml.cs

@@ -123,7 +123,7 @@ namespace ControlCatalog.Pages
             element.BringIntoView();
         }
 
-        private void RepeaterClick(object sender, PointerPressedEventArgs e)
+        private void RepeaterClick(object? sender, PointerPressedEventArgs e)
         {
             if ((e.Source as TextBlock)?.DataContext is ItemsRepeaterPageViewModel.Item item)
             {
@@ -132,7 +132,7 @@ namespace ControlCatalog.Pages
             }
         }
 
-        private void RepeaterOnKeyDown(object sender, KeyEventArgs e)
+        private void RepeaterOnKeyDown(object? sender, KeyEventArgs e)
         {
             if (e.Key == Key.F5)
             {
@@ -140,17 +140,17 @@ namespace ControlCatalog.Pages
             }
         }
 
-        private void scrollToLast_Click(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+        private void scrollToLast_Click(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
         {
             ScrollTo(_viewModel.Items.Count - 1);
         }
 
-        private void scrollToRandom_Click(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+        private void scrollToRandom_Click(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
         {
             ScrollTo(_random.Next(_viewModel.Items.Count - 1));
         }
 
-        private void scrollToSelected_Click(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+        private void scrollToSelected_Click(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
         {
             ScrollTo(_selectedIndex);
         }

samples/ControlCatalog/Pages/NumericUpDownPage.xaml.cs

@@ -2,9 +2,7 @@
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
-using Avalonia;
 using Avalonia.Controls;
-using Avalonia.Controls.Primitives;
 using Avalonia.Markup.Xaml;
 using MiniMvvm;
 
@@ -29,7 +27,7 @@ namespace ControlCatalog.Pages
     public class NumbersPageViewModel : ViewModelBase
     {
         private IList<FormatObject>? _formats;
-        private FormatObject _selectedFormat;
+        private FormatObject? _selectedFormat;
         private IList<Location>? _spinnerLocations;
 
         private double _doubleValue;
@@ -89,7 +87,7 @@ namespace ControlCatalog.Pages
             .Where(c => new[] { "en-US", "en-GB", "fr-FR", "ar-DZ", "zh-CH", "cs-CZ" }.Contains(c.Name))
             .ToArray();
 
-        public FormatObject SelectedFormat
+        public FormatObject? SelectedFormat
         {
             get { return _selectedFormat; }
             set { this.RaiseAndSetIfChanged(ref _selectedFormat, value); }

samples/ControlCatalog/Pages/OpenGlPage.xaml.cs

@@ -68,7 +68,7 @@ namespace ControlCatalog.Pages
             set => SetAndRaise(DiscoProperty, ref _disco, value);
         }
 
-        private string _info;
+        private string _info = string.Empty;
 
         public static readonly DirectProperty<OpenGlPageControl, string> InfoProperty =
             AvaloniaProperty.RegisterDirect<OpenGlPageControl, string>("Info", o => o.Info, (o, v) => o.Info = v);
@@ -205,7 +205,7 @@ namespace ControlCatalog.Pages
         public OpenGlPageControl()
         {
             var name = typeof(OpenGlPage).Assembly.GetManifestResourceNames().First(x => x.Contains("teapot.bin"));
-            using (var sr = new BinaryReader(typeof(OpenGlPage).Assembly.GetManifestResourceStream(name)))
+            using (var sr = new BinaryReader(typeof(OpenGlPage).Assembly.GetManifestResourceStream(name)!))
             {
                 var buf = new byte[sr.ReadInt32()];
                 sr.Read(buf, 0, buf.Length);
@@ -345,7 +345,7 @@ namespace ControlCatalog.Pages
                     0.01f, 1000);
 
 
-            var view = Matrix4x4.CreateLookAt(new Vector3(25, 25, 25), new Vector3(), new Vector3(0, -1, 0));
+            var view = Matrix4x4.CreateLookAt(new Vector3(25, 25, 25), new Vector3(), new Vector3(0, 1, 0));
             var model = Matrix4x4.CreateFromYawPitchRoll(_yaw, _pitch, _roll);
             var modelLoc = GL.GetUniformLocationString(_shaderProgram, "uModel");
             var viewLoc = GL.GetUniformLocationString(_shaderProgram, "uView");

samples/ControlCatalog/Pages/PointersPage.xaml.cs

@@ -1,8 +1,6 @@
 using System;
-using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Input;
-using Avalonia.Interactivity;
 using Avalonia.Markup.Xaml;
 
 namespace ControlCatalog.Pages;
@@ -31,28 +29,33 @@ public class PointersPage : UserControl
         border2.PointerExited += Border_PointerUpdated;
     }
 
-    private void Border_PointerUpdated(object sender, PointerEventArgs e)
+    private void Border_PointerUpdated(object? sender, PointerEventArgs e)
     {
-        var textBlock = (TextBlock)((Border)sender).Child;
-        var position = e.GetPosition((Border)sender);
-        textBlock.Text = @$"Type: {e.Pointer.Type}
+        if (sender is Border border && border.Child is TextBlock textBlock)
+        {
+            var position = e.GetPosition(border);
+            textBlock.Text = @$"Type: {e.Pointer.Type}
 Captured: {e.Pointer.Captured == sender}
 PointerId: {e.Pointer.Id}
 Position: {(int)position.X} {(int)position.Y}";
-        e.Handled = true;
+            e.Handled = true;
+        }
     }
 
-    private void Border_PointerCaptureLost(object sender, PointerCaptureLostEventArgs e)
+    private void Border_PointerCaptureLost(object? sender, PointerCaptureLostEventArgs e)
     {
-        var textBlock = (TextBlock)((Border)sender).Child;
-        textBlock.Text = @$"Type: {e.Pointer.Type}
+        if (sender is Border border && border.Child is TextBlock textBlock)
+        {
+            textBlock.Text = @$"Type: {e.Pointer.Type}
 Captured: {e.Pointer.Captured == sender}
 PointerId: {e.Pointer.Id}
 Position: ??? ???";
-        e.Handled = true;
+            e.Handled = true;
+
+        }
     }
 
-    private void Border_PointerReleased(object sender, PointerReleasedEventArgs e)
+    private void Border_PointerReleased(object? sender, PointerReleasedEventArgs e)
     {
         if (e.Pointer.Captured == sender)
         {
@@ -65,9 +68,9 @@ Position: ??? ???";
         }
     }
 
-    private void Border_PointerPressed(object sender, PointerPressedEventArgs e)
+    private void Border_PointerPressed(object? sender, PointerPressedEventArgs e)
     {
-        e.Pointer.Capture((Border)sender);
+        e.Pointer.Capture(sender as Border);
         e.Handled = true;
     }
 

samples/ControlCatalog/ViewModels/TransitioningContentControlPageViewModel.cs

@@ -117,9 +117,9 @@ namespace ControlCatalog.ViewModels
             PageTransitions[3].Transition = new PageSlide(TimeSpan.FromMilliseconds(Duration), PageSlide.SlideAxis.Vertical);
 
             var compositeTransition = new CompositePageTransition();
-            compositeTransition.PageTransitions.Add(PageTransitions[1].Transition);
-            compositeTransition.PageTransitions.Add(PageTransitions[2].Transition);
-            compositeTransition.PageTransitions.Add(PageTransitions[3].Transition);
+            compositeTransition.PageTransitions.Add(PageTransitions[1].Transition!);
+            compositeTransition.PageTransitions.Add(PageTransitions[2].Transition!);
+            compositeTransition.PageTransitions.Add(PageTransitions[3].Transition!);
             PageTransitions[4].Transition = compositeTransition;
 
             PageTransitions[5].Transition = new CustomTransition(TimeSpan.FromMilliseconds(Duration));

samples/IntegrationTestApp/MainWindow.axaml.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using Avalonia;
@@ -31,20 +30,23 @@ namespace IntegrationTestApp
 
         private void InitializeViewMenu()
         {
-            var mainTabs = this.FindControl<TabControl>("MainTabs");
+            var mainTabs = this.Get<TabControl>("MainTabs");
             var viewMenu = (NativeMenuItem)NativeMenu.GetMenu(this).Items[1];
 
-            foreach (TabItem tabItem in mainTabs.Items)
+            if (mainTabs.Items is not null)
             {
-                var menuItem = new NativeMenuItem
+                foreach (TabItem tabItem in mainTabs.Items)
                 {
-                    Header = (string)tabItem.Header!,
-                    IsChecked = tabItem.IsSelected,
-                    ToggleType = NativeMenuItemToggleType.Radio,
-                };
-
-                menuItem.Click += (s, e) => tabItem.IsSelected = true;
-                viewMenu.Menu.Items.Add(menuItem);
+                    var menuItem = new NativeMenuItem
+                    {
+                        Header = (string)tabItem.Header!,
+                        IsChecked = tabItem.IsSelected,
+                        ToggleType = NativeMenuItemToggleType.Radio,
+                    };
+
+                    menuItem.Click += (s, e) => tabItem.IsSelected = true;
+                    viewMenu?.Menu?.Items.Add(menuItem);
+                }
             }
         }
 
@@ -107,8 +109,8 @@ namespace IntegrationTestApp
 
         private void MenuClicked(object? sender, RoutedEventArgs e)
         {
-            var clickedMenuItemTextBlock = this.FindControl<TextBlock>("ClickedMenuItem");
-            clickedMenuItemTextBlock.Text = ((MenuItem)sender!).Header.ToString();
+            var clickedMenuItemTextBlock = this.Get<TextBlock>("ClickedMenuItem");
+            clickedMenuItemTextBlock.Text = (sender as MenuItem)?.Header?.ToString();
         }
 
         private void OnButtonClick(object? sender, RoutedEventArgs e)
@@ -116,13 +118,13 @@ namespace IntegrationTestApp
             var source = e.Source as Button;
 
             if (source?.Name == "ComboBoxSelectionClear")
-                this.FindControl<ComboBox>("BasicComboBox").SelectedIndex = -1;
+                this.Get<ComboBox>("BasicComboBox").SelectedIndex = -1;
             if (source?.Name == "ComboBoxSelectFirst")
-                this.FindControl<ComboBox>("BasicComboBox").SelectedIndex = 0;
+                this.Get<ComboBox>("BasicComboBox").SelectedIndex = 0;
             if (source?.Name == "ListBoxSelectionClear")
-                this.FindControl<ListBox>("BasicListBox").SelectedIndex = -1;
+                this.Get<ListBox>("BasicListBox").SelectedIndex = -1;
             if (source?.Name == "MenuClickedMenuItemReset")
-                this.FindControl<TextBlock>("ClickedMenuItem").Text = "None";
+                this.Get<TextBlock>("ClickedMenuItem").Text = "None";
             if (source?.Name == "ShowWindow")
                 ShowWindow();
             if (source?.Name == "SendToBack")

src/Avalonia.Controls/AutoCompleteBox.cs

@@ -392,6 +392,8 @@ namespace Avalonia.Controls
         private AutoCompleteSelector<object>? _itemSelector;
         private AutoCompleteSelector<string?>? _textSelector;
 
+        private readonly EventHandler _populateDropDownHandler;
+
         public static readonly RoutedEvent<SelectionChangedEventArgs> SelectionChangedEvent =
             RoutedEvent.Register<SelectionChangedEventArgs>(nameof(SelectionChanged), RoutingStrategies.Bubble, typeof(AutoCompleteBox));
 
@@ -668,6 +670,7 @@ namespace Avalonia.Controls
 
                 if (newValue == TimeSpan.Zero)
                 {
+                    _delayTimer.Tick -= _populateDropDownHandler;
                     _delayTimer = null;
                 }
             }
@@ -678,7 +681,7 @@ namespace Avalonia.Controls
                 if (_delayTimer == null)
                 {
                     _delayTimer = new DispatcherTimer();
-                    _delayTimer.Tick += PopulateDropDown;
+                    _delayTimer.Tick += _populateDropDownHandler;
                 }
 
                 // Set the new tick interval
@@ -864,6 +867,7 @@ namespace Avalonia.Controls
         /// </summary>
         public AutoCompleteBox()
         {
+            _populateDropDownHandler = PopulateDropDown;
             ClearView();
         }
 
@@ -1771,10 +1775,7 @@ namespace Avalonia.Controls
         /// <param name="e">The event arguments.</param>
         private void PopulateDropDown(object? sender, EventArgs e)
         {
-            if (_delayTimer != null)
-            {
-                _delayTimer.Stop();
-            }
+            _delayTimer?.Stop();
 
             // Update the prefix/search text.
             SearchText = Text;

src/Avalonia.Controls/Calendar/Calendar.cs

@@ -224,7 +224,7 @@ namespace Avalonia.Controls
     /// </para>
     /// </remarks>
     [TemplatePart(PART_ElementMonth, typeof(CalendarItem))]
-    [TemplatePart(PART_ElementRoot,  typeof(Panel))]
+    [TemplatePart(PART_ElementRoot, typeof(Panel))]
     public class Calendar : TemplatedControl
     {
         internal const int RowsPerMonth = 7;
@@ -338,14 +338,11 @@ namespace Avalonia.Controls
         /// <param name="e">The DependencyPropertyChangedEventArgs.</param>
         private void OnIsTodayHighlightedChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            if (DisplayDate != null)
-            {
-                int i = DateTimeHelper.CompareYearMonth(DisplayDateInternal, DateTime.Today);
+            int i = DateTimeHelper.CompareYearMonth(DisplayDateInternal, DateTime.Today);
 
-                if (i > -2 && i < 2)
-                {
-                    UpdateMonths();
-                }
+            if (i > -2 && i < 2)
+            {
+                UpdateMonths();
             }
         }
 
@@ -655,7 +652,7 @@ namespace Avalonia.Controls
                 SelectedDatesChanged?.Invoke(this, e);
             }
         }
-        
+
         internal Collection<DateTime> RemovedItems { get; set; }
         internal DateTime? LastSelectedDateInternal { get; set; }
         internal DateTime? LastSelectedDate
@@ -914,7 +911,7 @@ namespace Avalonia.Controls
                 o => o.DisplayDateEnd,
                 (o, v) => o.DisplayDateEnd = v,
                 defaultBindingMode: BindingMode.TwoWay);
-        
+
         /// <summary>
         /// Gets or sets the last date to be displayed.
         /// </summary>
@@ -1242,7 +1239,7 @@ namespace Avalonia.Controls
                                     {
                                         b.IsSelected = false;
                                     }
-                                } 
+                                }
                             }
                         }
                     }
@@ -1278,7 +1275,7 @@ namespace Avalonia.Controls
 
         internal void OnPreviousClick()
         {
-            if (DisplayMode == CalendarMode.Month && DisplayDate != null)
+            if (DisplayMode == CalendarMode.Month)
             {
                 DateTime? d = DateTimeHelper.AddMonths(DateTimeHelper.DiscardDayTime(DisplayDate), -1);
                 if (d.HasValue)
@@ -1326,7 +1323,7 @@ namespace Avalonia.Controls
         }
         internal void OnNextClick()
         {
-            if (DisplayMode == CalendarMode.Month && DisplayDate != null)
+            if (DisplayMode == CalendarMode.Month)
             {
                 DateTime? d = DateTimeHelper.AddMonths(DateTimeHelper.DiscardDayTime(DisplayDate), 1);
                 if (d.HasValue)
@@ -1645,7 +1642,7 @@ namespace Avalonia.Controls
         {
             if (DisplayMode == CalendarMode.Month)
             {
-                if (LastSelectedDate.HasValue && DisplayDateInternal != null)
+                if (LastSelectedDate.HasValue)
                 {
                     // If a blackout day is inactive, when clicked on it, the
                     // previous inactive day which is not a blackout day can get
@@ -1897,24 +1894,21 @@ namespace Avalonia.Controls
             {
                 case CalendarMode.Month:
                     {
-                        if (DisplayDate != null)
-                        {
-                            DateTime? selectedDate = new DateTime(DisplayDateInternal.Year, DisplayDateInternal.Month, 1);
+                        DateTime? selectedDate = new DateTime(DisplayDateInternal.Year, DisplayDateInternal.Month, 1);
 
-                            if (DateTimeHelper.CompareYearMonth(DateTime.MaxValue, selectedDate.Value) > 0)
-                            {
-                                // since DisplayDate is not equal to
-                                // DateTime.MaxValue we are sure selectedDate is\
-                                // not null
-                                selectedDate = DateTimeHelper.AddMonths(selectedDate.Value, 1)!.Value;
-                                selectedDate = DateTimeHelper.AddDays(selectedDate.Value, -1)!.Value;
-                            }
-                            else
-                            {
-                                selectedDate = DateTime.MaxValue;
-                            }
-                            ProcessSelection(shift, selectedDate, null);
+                        if (DateTimeHelper.CompareYearMonth(DateTime.MaxValue, selectedDate.Value) > 0)
+                        {
+                            // since DisplayDate is not equal to
+                            // DateTime.MaxValue we are sure selectedDate is\
+                            // not null
+                            selectedDate = DateTimeHelper.AddMonths(selectedDate.Value, 1)!.Value;
+                            selectedDate = DateTimeHelper.AddDays(selectedDate.Value, -1)!.Value;
                         }
+                        else
+                        {
+                            selectedDate = DateTime.MaxValue;
+                        }
+                        ProcessSelection(shift, selectedDate, null);
                         break;
                     }
                 case CalendarMode.Year:
@@ -2026,7 +2020,6 @@ namespace Avalonia.Controls
                             focusDate = DisplayDate;
                             LastSelectedDate = DisplayDate;
                         }
-                        Debug.Assert(focusDate != null, "focusDate should not be null!");
                         FocusButton = FindDayButtonFromDay(focusDate);
 
                         if (FocusButton != null)
@@ -2091,17 +2084,17 @@ namespace Avalonia.Controls
 
         static Calendar()
         {
-            IsEnabledProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnIsEnabledChanged(e));
-            FirstDayOfWeekProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnFirstDayOfWeekChanged(e));
-            IsTodayHighlightedProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnIsTodayHighlightedChanged(e));
-            DisplayModeProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnDisplayModePropertyChanged(e));
-            SelectionModeProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnSelectionModeChanged(e));
-            SelectedDateProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnSelectedDateChanged(e));
-            DisplayDateProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnDisplayDateChanged(e));
-            DisplayDateStartProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnDisplayDateStartChanged(e));
-            DisplayDateEndProperty.Changed.AddClassHandler<Calendar>((x,e) => x.OnDisplayDateEndChanged(e));
-            KeyDownEvent.AddClassHandler<Calendar>((x,e) => x.Calendar_KeyDown(e));
-            KeyUpEvent.AddClassHandler<Calendar>((x,e) => x.Calendar_KeyUp(e));
+            IsEnabledProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnIsEnabledChanged(e));
+            FirstDayOfWeekProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnFirstDayOfWeekChanged(e));
+            IsTodayHighlightedProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnIsTodayHighlightedChanged(e));
+            DisplayModeProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnDisplayModePropertyChanged(e));
+            SelectionModeProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnSelectionModeChanged(e));
+            SelectedDateProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnSelectedDateChanged(e));
+            DisplayDateProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnDisplayDateChanged(e));
+            DisplayDateStartProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnDisplayDateStartChanged(e));
+            DisplayDateEndProperty.Changed.AddClassHandler<Calendar>((x, e) => x.OnDisplayDateEndChanged(e));
+            KeyDownEvent.AddClassHandler<Calendar>((x, e) => x.Calendar_KeyDown(e));
+            KeyUpEvent.AddClassHandler<Calendar>((x, e) => x.Calendar_KeyUp(e));
         }
 
         /// <summary>

src/Avalonia.Controls/Calendar/CalendarItem.cs

@@ -4,7 +4,6 @@
 // All other rights reserved.
 
 using System;
-using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
 using Avalonia.Collections.Pooled;
@@ -353,7 +352,6 @@ namespace Avalonia.Controls.Primitives
         {
             if (Owner != null)
             {
-                Debug.Assert(Owner.DisplayDate != null, "The Owner Calendar's DisplayDate should not be null!");
                 _currentMonth = Owner.DisplayDateInternal;
             }
             else
@@ -361,17 +359,14 @@ namespace Avalonia.Controls.Primitives
                 _currentMonth = DateTime.Today;
             }
 
-            if (_currentMonth != null)
-            {
-                SetMonthModeHeaderButton();
-                SetMonthModePreviousButton(_currentMonth);
-                SetMonthModeNextButton(_currentMonth);
+            SetMonthModeHeaderButton();
+            SetMonthModePreviousButton(_currentMonth);
+            SetMonthModeNextButton(_currentMonth);
 
-                if (MonthView != null)
-                {
-                    SetDayTitles();
-                    SetCalendarDayButtons(_currentMonth);
-                }
+            if (MonthView != null)
+            {
+                SetDayTitles();
+                SetCalendarDayButtons(_currentMonth);
             }
         }
         private void SetMonthModeHeaderButton()
@@ -592,7 +587,6 @@ namespace Avalonia.Controls.Primitives
         {
             if (Owner != null)
             {
-                Debug.Assert(Owner.SelectedMonth != null, "The Owner Calendar's SelectedMonth should not be null!");
                 _currentMonth = (DateTime)Owner.SelectedMonth;
             }
             else
@@ -600,16 +594,13 @@ namespace Avalonia.Controls.Primitives
                 _currentMonth = DateTime.Today;
             }
 
-            if (_currentMonth != null)
-            {
-                SetYearModeHeaderButton();
-                SetYearModePreviousButton();
-                SetYearModeNextButton();
+            SetYearModeHeaderButton();
+            SetYearModePreviousButton();
+            SetYearModeNextButton();
 
-                if (YearView != null)
-                {
-                    SetMonthButtonsForYearMode();
-                }
+            if (YearView != null)
+            {
+                SetMonthButtonsForYearMode();
             }
         }
         private void SetYearModeHeaderButton()
@@ -660,7 +651,6 @@ namespace Avalonia.Controls.Primitives
                         childButton.IsCalendarButtonFocused = false;
                     }
 
-                    Debug.Assert(Owner.DisplayDateInternal != null, "The Owner Calendar's DisplayDateInternal should not be null!");
                     childButton.IsSelected = (DateTimeHelper.CompareYearMonth(day, Owner.DisplayDateInternal) == 0);
 
                     if (DateTimeHelper.CompareYearMonth(day, Owner.DisplayDateRangeStart) < 0 || DateTimeHelper.CompareYearMonth(day, Owner.DisplayDateRangeEnd) > 0)
@@ -685,7 +675,6 @@ namespace Avalonia.Controls.Primitives
 
             if (Owner != null)
             {
-                Debug.Assert(Owner.SelectedYear != null, "The owning Calendar's selected year should not be null!");
                 selectedYear = Owner.SelectedYear;
                 _currentMonth = (DateTime)Owner.SelectedMonth;
             }
@@ -695,19 +684,16 @@ namespace Avalonia.Controls.Primitives
                 selectedYear = DateTime.Today;
             }
 
-            if (_currentMonth != null)
-            {
-                int decade = DateTimeHelper.DecadeOfDate(selectedYear);
-                int decadeEnd = DateTimeHelper.EndOfDecade(selectedYear);
+            int decade = DateTimeHelper.DecadeOfDate(selectedYear);
+            int decadeEnd = DateTimeHelper.EndOfDecade(selectedYear);
 
-                SetDecadeModeHeaderButton(decade, decadeEnd);
-                SetDecadeModePreviousButton(decade);
-                SetDecadeModeNextButton(decadeEnd);
+            SetDecadeModeHeaderButton(decade, decadeEnd);
+            SetDecadeModePreviousButton(decade);
+            SetDecadeModeNextButton(decadeEnd);
 
-                if (YearView != null)
-                {
-                    SetYearButtons(decade, decadeEnd);
-                }
+            if (YearView != null)
+            {
+                SetYearButtons(decade, decadeEnd);
             }
         }
         internal void UpdateYearViewSelection(CalendarButton calendarButton)
@@ -822,22 +808,15 @@ namespace Avalonia.Controls.Primitives
                 {
                     if (Owner.DisplayMode == CalendarMode.Month)
                     {
-                        if (Owner.DisplayDate != null)
-                        {
-                            d = Owner.DisplayDateInternal;
-                            Owner.SelectedMonth = new DateTime(d.Year, d.Month, 1);
-                        }
+                        d = Owner.DisplayDateInternal;
+                        Owner.SelectedMonth = new DateTime(d.Year, d.Month, 1);
                         Owner.DisplayMode = CalendarMode.Year;
                     }
                     else
                     {
                         Debug.Assert(Owner.DisplayMode == CalendarMode.Year, "The Owner Calendar's DisplayMode should be Year!");
-
-                        if (Owner.SelectedMonth != null)
-                        {
-                            d = Owner.SelectedMonth;
-                            Owner.SelectedYear = new DateTime(d.Year, d.Month, 1);
-                        }
+                        d = Owner.SelectedMonth;
+                        Owner.SelectedYear = new DateTime(d.Year, d.Month, 1);
                         Owner.DisplayMode = CalendarMode.Decade;
                     }
                 }

src/Avalonia.Controls/Calendar/SelectedDatesCollection.cs

@@ -297,7 +297,7 @@ namespace Avalonia.Controls.Primitives
                 }
                 else
                 {
-                    if (item != null && DateTime.Compare(this[index], item) != 0 && Calendar.IsValidDateSelection(_owner, item))
+                    if (DateTime.Compare(this[index], item) != 0 && Calendar.IsValidDateSelection(_owner, item))
                     {
                         removedItems.Add(this[index]);
                         base.SetItem(index, item);

src/Skia/Avalonia.Skia/Gpu/OpenGl/OpenGlBitmapImpl.cs

@@ -41,7 +41,7 @@ namespace Avalonia.Skia
                         new GRGlTextureInfo(
                             GlConsts.GL_TEXTURE_2D, (uint)_surface.GetTextureId(),
                             (uint)_surface.InternalFormat)))
-                    using (var surface = SKSurface.Create(context.GrContext, backendTexture, GRSurfaceOrigin.TopLeft,
+                    using (var surface = SKSurface.Create(context.GrContext, backendTexture, GRSurfaceOrigin.BottomLeft,
                         SKColorType.Rgba8888))
                     {
                         // Again, silently ignore, if something went wrong it's not our fault