Merge pull request #7210 from AvaloniaUI/fixes/avalonia-layout-nullability

Added nullable annotations to Avalonia.Layout.

src/Avalonia.Layout/AttachedLayout.cs

@@ -12,17 +12,17 @@ namespace Avalonia.Layout
     /// </summary>
     public abstract class AttachedLayout : AvaloniaObject
     {
-        public string LayoutId { get; set; }
+        public string? LayoutId { get; set; }
 
         /// <summary>
         /// Occurs when the measurement state (layout) has been invalidated.
         /// </summary>
-        public event EventHandler MeasureInvalidated;
+        public event EventHandler? MeasureInvalidated;
 
         /// <summary>
         /// Occurs when the arrange state (layout) has been invalidated.
         /// </summary>
-        public event EventHandler ArrangeInvalidated;
+        public event EventHandler? ArrangeInvalidated;
 
         /// <summary>
         /// Initializes any per-container state the layout requires when it is attached to an

src/Avalonia.Layout/Avalonia.Layout.csproj

@@ -9,4 +9,5 @@
   </ItemGroup>  
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>

src/Avalonia.Layout/ElementManager.cs

@@ -13,10 +13,10 @@ namespace Avalonia.Layout
 {
     internal class ElementManager
     {
-        private readonly List<ILayoutable> _realizedElements = new List<ILayoutable>();
+        private readonly List<ILayoutable?> _realizedElements = new List<ILayoutable?>();
         private readonly List<Rect> _realizedElementLayoutBounds = new List<Rect>();
         private int _firstRealizedDataIndex;
-        private VirtualizingLayoutContext _context;
+        private VirtualizingLayoutContext? _context;
 
         private bool IsVirtualizingContext
         {
@@ -58,7 +58,7 @@ namespace Avalonia.Layout
                         // Make sure there is enough space for the bounds.
                         // Note: We could optimize when the count becomes smaller, but keeping
                         // it always up to date is the simplest option for now.
-                        _realizedElementLayoutBounds.Resize(count);
+                        _realizedElementLayoutBounds.Resize(count, default);
                     }
                 }
             }
@@ -66,34 +66,32 @@ namespace Avalonia.Layout
 
         public int GetRealizedElementCount()
         {
-            return IsVirtualizingContext ? _realizedElements.Count : _context.ItemCount;
+            return IsVirtualizingContext ? _realizedElements.Count : _context!.ItemCount;
         }
 
         public ILayoutable GetAt(int realizedIndex)
         {
-            ILayoutable element;
+            ILayoutable? element;
 
             if (IsVirtualizingContext)
             {
-                if (_realizedElements[realizedIndex] == null)
+                element = _realizedElements[realizedIndex];
+
+                if (element == null)
                 {
                     // Sentinel. Create the element now since we need it.
                     int dataIndex = GetDataIndexFromRealizedRangeIndex(realizedIndex);
                     Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "Creating element for sentinal with data index {Index}", dataIndex);
-                    element = _context.GetOrCreateElementAt(
+                    element = _context!.GetOrCreateElementAt(
                         dataIndex,
                         ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
                     _realizedElements[realizedIndex] = element;
                 }
-                else
-                {
-                    element = _realizedElements[realizedIndex];
-                }
             }
             else
             {
                 // realizedIndex and dataIndex are the same (everything is realized)
-                element = _context.GetOrCreateElementAt(
+                element = _context!.GetOrCreateElementAt(
                     realizedIndex,
                     ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
             }
@@ -112,7 +110,7 @@ namespace Avalonia.Layout
             _realizedElementLayoutBounds.Add(default);
         }
 
-        public void Insert(int realizedIndex, int dataIndex, ILayoutable element)
+        public void Insert(int realizedIndex, int dataIndex, ILayoutable? element)
         {
             if (realizedIndex == 0)
             {
@@ -136,7 +134,7 @@ namespace Avalonia.Layout
 
                 if (elementRef != null)
                 {
-                    _context.RecycleElement(elementRef);
+                    _context!.RecycleElement(elementRef);
                 }
             }
 
@@ -203,26 +201,26 @@ namespace Avalonia.Layout
             else
             {
                 // Non virtualized - everything is realized
-                return index >= 0 && index < _context.ItemCount;
+                return index >= 0 && index < _context!.ItemCount;
             }
         }
 
-        public bool IsIndexValidInData(int currentIndex) => (uint)currentIndex < _context.ItemCount;
+        public bool IsIndexValidInData(int currentIndex) => (uint)currentIndex < _context!.ItemCount;
 
-        public ILayoutable GetRealizedElement(int dataIndex)
+        public ILayoutable? GetRealizedElement(int dataIndex)
         {
             return IsVirtualizingContext ?
                 GetAt(GetRealizedRangeIndexFromDataIndex(dataIndex)) :
-                _context.GetOrCreateElementAt(
+                _context!.GetOrCreateElementAt(
                     dataIndex,
                     ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
         }
 
-        public void EnsureElementRealized(bool forward, int dataIndex, string layoutId)
+        public void EnsureElementRealized(bool forward, int dataIndex, string? layoutId)
         {
             if (IsDataIndexRealized(dataIndex) == false)
             {
-                var element = _context.GetOrCreateElementAt(
+                var element = _context!.GetOrCreateElementAt(
                     dataIndex,
                     ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
 
@@ -273,14 +271,14 @@ namespace Avalonia.Layout
                 {
                     case NotifyCollectionChangedAction.Add:
                         {
-                            OnItemsAdded(args.NewStartingIndex, args.NewItems.Count);
+                            OnItemsAdded(args.NewStartingIndex, args.NewItems!.Count);
                         }
                         break;
 
                     case NotifyCollectionChangedAction.Replace:
                         {
-                            int oldSize = args.OldItems.Count;
-                            int newSize = args.NewItems.Count;
+                            int oldSize = args.OldItems!.Count;
+                            int newSize = args.NewItems!.Count;
                             int oldStartIndex = args.OldStartingIndex;
                             int newStartIndex = args.NewStartingIndex;
 
@@ -301,7 +299,7 @@ namespace Avalonia.Layout
 
                                     if (elementRef != null)
                                     {
-                                        _context.RecycleElement(elementRef);
+                                        _context!.RecycleElement(elementRef);
                                         _realizedElements[realizedIndex] = null;
                                     }
                                 }

src/Avalonia.Layout/FlowLayoutAlgorithm.cs

@@ -16,8 +16,8 @@ namespace Avalonia.Layout
         private Size _lastAvailableSize;
         private double _lastItemSpacing;
         private bool _collectionChangePending;
-        private VirtualizingLayoutContext _context;
-        private IFlowLayoutAlgorithmDelegates _algorithmCallbacks;
+        private VirtualizingLayoutContext? _context;
+        private IFlowLayoutAlgorithmDelegates? _algorithmCallbacks;
         private Rect _lastExtent;
         private int _firstRealizedDataIndexInsideRealizationWindow = -1;
         private int _lastRealizedDataIndexInsideRealizationWindow = -1;
@@ -46,7 +46,7 @@ namespace Avalonia.Layout
             }
         }
 
-        private Rect RealizationRect => IsVirtualizingContext ? _context.RealizationRect : new Rect(Size.Infinity);
+        private Rect RealizationRect => IsVirtualizingContext ? _context!.RealizationRect : new Rect(Size.Infinity);
 
         public void InitializeForContext(VirtualizingLayoutContext context, IFlowLayoutAlgorithmDelegates callbacks)
         {
@@ -76,7 +76,7 @@ namespace Avalonia.Layout
             int maxItemsPerLine,
             ScrollOrientation orientation,
             bool disableVirtualization,
-            string layoutId)
+            string? layoutId)
         {
             _orientation.ScrollOrientation = orientation;
 
@@ -87,7 +87,7 @@ namespace Avalonia.Layout
                 layoutId,
                 realizationRect);
 
-            var suggestedAnchorIndex = _context.RecommendedAnchorIndex;
+            var suggestedAnchorIndex = _context!.RecommendedAnchorIndex;
             if (_elementManager.IsIndexValidInData(suggestedAnchorIndex))
             {
                 var anchorRealized = _elementManager.IsDataIndexRealized(suggestedAnchorIndex);
@@ -124,7 +124,7 @@ namespace Avalonia.Layout
             VirtualizingLayoutContext context,
             bool isWrapping,
             LineAlignment lineAlignment,
-            string layoutId)
+            string? layoutId)
         {
             Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: ArrangeLayout", layoutId);
             ArrangeVirtualizingLayout(finalSize, lineAlignment, isWrapping, layoutId);
@@ -149,7 +149,7 @@ namespace Avalonia.Layout
             Size availableSize,
             VirtualizingLayoutContext context)
         {
-            var measureSize = _algorithmCallbacks.Algorithm_GetMeasureSize(index, availableSize, context);
+            var measureSize = _algorithmCallbacks!.Algorithm_GetMeasureSize(index, availableSize, context);
             element.Measure(measureSize);
             var provisionalArrangeSize = _algorithmCallbacks.Algorithm_GetProvisionalArrangeSize(index, measureSize, element.DesiredSize, context);
             _algorithmCallbacks.Algorithm_OnElementMeasured(element, index, availableSize, measureSize, element.DesiredSize, provisionalArrangeSize, context);
@@ -161,7 +161,7 @@ namespace Avalonia.Layout
             Size availableSize,
             bool isWrapping,
             double minItemSpacing,
-            string layoutId)
+            string? layoutId)
         {
             int anchorIndex = -1;
             var anchorPosition= new Point();
@@ -170,7 +170,7 @@ namespace Avalonia.Layout
             if (!IsVirtualizingContext)
             {
                 // Non virtualizing host, start generating from the element 0
-                anchorIndex = context.ItemCount > 0 ? 0 : -1;
+                anchorIndex = context!.ItemCount > 0 ? 0 : -1;
             }
             else
             {       
@@ -183,7 +183,7 @@ namespace Avalonia.Layout
                     _lastItemSpacing != minItemSpacing ||
                     _collectionChangePending);
 
-                var suggestedAnchorIndex = _context.RecommendedAnchorIndex;
+                var suggestedAnchorIndex = _context!.RecommendedAnchorIndex;
 
                 var isAnchorSuggestionValid = suggestedAnchorIndex >= 0 &&
                     _elementManager.IsDataIndexRealized(suggestedAnchorIndex);
@@ -191,10 +191,10 @@ namespace Avalonia.Layout
                 if (isAnchorSuggestionValid)
                 {
                     Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Using suggested anchor {Anchor}", layoutId, suggestedAnchorIndex);
-                    anchorIndex = _algorithmCallbacks.Algorithm_GetAnchorForTargetElement(
+                    anchorIndex = _algorithmCallbacks!.Algorithm_GetAnchorForTargetElement(
                         suggestedAnchorIndex,
                         availableSize,
-                        context).Index;
+                        context!).Index;
 
                     if (_elementManager.IsDataIndexRealized(anchorIndex))
                     {
@@ -235,7 +235,7 @@ namespace Avalonia.Layout
 
                     // The anchor is based on the realization window because a connected ItemsRepeater might intersect the realization window
                     // but not the visible window. In that situation, we still need to produce a valid anchor.
-                    var anchorInfo = _algorithmCallbacks.Algorithm_GetAnchorForRealizationRect(availableSize, context);
+                    var anchorInfo = _algorithmCallbacks!.Algorithm_GetAnchorForRealizationRect(availableSize, context!);
                     anchorIndex = anchorInfo.Index;
                     anchorPosition = _orientation.MinorMajorPoint(0, anchorInfo.Offset);
                 }
@@ -259,12 +259,12 @@ namespace Avalonia.Layout
                     Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId} Disconnected Window - throwing away all realized elements", layoutId);
                     _elementManager.ClearRealizedRange();
 
-                    var anchor = _context.GetOrCreateElementAt(anchorIndex, ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
+                    var anchor = _context!.GetOrCreateElementAt(anchorIndex, ElementRealizationOptions.ForceCreate | ElementRealizationOptions.SuppressAutoRecycle);
                     _elementManager.Add(anchor, anchorIndex);
                 }
 
                 var anchorElement = _elementManager.GetRealizedElement(anchorIndex);
-                var desiredSize = MeasureElement(anchorElement, anchorIndex, availableSize, _context);
+                var desiredSize = MeasureElement(anchorElement!, anchorIndex, availableSize, _context!);
                 var layoutBounds = new Rect(anchorPosition.X, anchorPosition.Y, desiredSize.Width, desiredSize.Height);
                 _elementManager.SetLayoutBoundsForDataIndex(anchorIndex, layoutBounds);
 
@@ -296,7 +296,7 @@ namespace Avalonia.Layout
             double lineSpacing,
             int maxItemsPerLine,
             bool disableVirtualization,
-            string layoutId)
+            string? layoutId)
         {
             if (anchorIndex != -1)
             {
@@ -322,7 +322,7 @@ namespace Avalonia.Layout
                     // Ensure layout element.
                     _elementManager.EnsureElementRealized(direction == GenerateDirection.Forward, currentIndex, layoutId);
                     var currentElement = _elementManager.GetRealizedElement(currentIndex);
-                    var desiredSize = MeasureElement(currentElement, currentIndex, availableSize, _context);
+                    var desiredSize = MeasureElement(currentElement!, currentIndex, availableSize, _context!);
                     ++count;
 
                     // Lay it out.
@@ -333,7 +333,7 @@ namespace Avalonia.Layout
                     if (direction == GenerateDirection.Forward)
                     {
                         double remainingSpace = _orientation.Minor(availableSize) - (_orientation.MinorStart(previousElementBounds) + _orientation.MinorSize(previousElementBounds) + minItemSpacing + _orientation.Minor(desiredSize));
-                        if (countInLine >= maxItemsPerLine || _algorithmCallbacks.Algorithm_ShouldBreakLine(currentIndex, remainingSpace))
+                        if (countInLine >= maxItemsPerLine || _algorithmCallbacks!.Algorithm_ShouldBreakLine(currentIndex, remainingSpace))
                         {
                             // No more space in this row. wrap to next row.
                             _orientation.SetMinorStart(ref currentBounds, 0);
@@ -371,7 +371,7 @@ namespace Avalonia.Layout
                     {
                         // Backward 
                         double remainingSpace = _orientation.MinorStart(previousElementBounds) - (_orientation.Minor(desiredSize) + minItemSpacing);
-                        if (countInLine >= maxItemsPerLine || _algorithmCallbacks.Algorithm_ShouldBreakLine(currentIndex, remainingSpace))
+                        if (countInLine >= maxItemsPerLine || _algorithmCallbacks!.Algorithm_ShouldBreakLine(currentIndex, remainingSpace))
                         {
                             // Does not fit, wrap to the previous row
                             var availableSizeMinor = _orientation.Minor(availableSize);
@@ -432,13 +432,13 @@ namespace Avalonia.Layout
                 // account for that element in the indices inside the realization window.
                 if (direction == GenerateDirection.Forward)
                 {
-                    int dataCount = _context.ItemCount;
+                    int dataCount = _context!.ItemCount;
                     _lastRealizedDataIndexInsideRealizationWindow = previousIndex == dataCount - 1 ? dataCount - 1 : previousIndex - 1;
                     _lastRealizedDataIndexInsideRealizationWindow = Math.Max(0, _lastRealizedDataIndexInsideRealizationWindow);
                 }
                 else
                 {
-                    int dataCount = _context.ItemCount;
+                    int dataCount = _context!.ItemCount;
                     _firstRealizedDataIndexInsideRealizationWindow = previousIndex == 0 ? 0 : previousIndex + 1;
                     _firstRealizedDataIndexInsideRealizationWindow = Math.Min(dataCount - 1, _firstRealizedDataIndexInsideRealizationWindow);
                 }
@@ -454,7 +454,7 @@ namespace Avalonia.Layout
         {
             _elementManager.ClearRealizedRange();
             // FlowLayout requires that the anchor is the first element in the row.
-            var internalAnchor = _algorithmCallbacks.Algorithm_GetAnchorForTargetElement(index, availableSize, context);
+            var internalAnchor = _algorithmCallbacks!.Algorithm_GetAnchorForTargetElement(index, availableSize, context);
 
             // No need to set the position of the anchor.
             // (0,0) is fine for now since the extent can
@@ -487,7 +487,7 @@ namespace Avalonia.Layout
             }
             else
             {
-                var realizationRect = _context.RealizationRect;
+                var realizationRect = _context!.RealizationRect;
                 var elementBounds = _elementManager.GetLayoutBoundsForDataIndex(index);
 
                 var elementMajorStart = _orientation.MajorStart(elementBounds);
@@ -510,11 +510,11 @@ namespace Avalonia.Layout
             return shouldContinue;
         }
 
-        private Rect EstimateExtent(Size availableSize, string layoutId)
+        private Rect EstimateExtent(Size availableSize, string? layoutId)
         {
-            ILayoutable firstRealizedElement = null;
+            ILayoutable? firstRealizedElement = null;
             Rect firstBounds = new Rect();
-            ILayoutable lastRealizedElement = null;
+            ILayoutable? lastRealizedElement = null;
             Rect lastBounds = new Rect();
             int firstDataIndex = -1;
             int lastDataIndex = -1;
@@ -531,9 +531,9 @@ namespace Avalonia.Layout
                 lastBounds = _elementManager.GetLayoutBoundsForRealizedIndex(last);
             }
 
-            Rect extent = _algorithmCallbacks.Algorithm_GetExtent(
+            Rect extent = _algorithmCallbacks!.Algorithm_GetExtent(
                 availableSize,
-                _context,
+                _context!,
                 firstRealizedElement,
                 firstDataIndex,
                 firstBounds,
@@ -563,7 +563,7 @@ namespace Avalonia.Layout
                         if (_orientation.MajorStart(currentBounds) != currentLineOffset)
                         {
                             // Staring a new line
-                            _algorithmCallbacks.Algorithm_OnLineArranged(currentDataIndex - countInLine, countInLine, currentLineSize, _context);
+                            _algorithmCallbacks!.Algorithm_OnLineArranged(currentDataIndex - countInLine, countInLine, currentLineSize, _context!);
                             countInLine = 0;
                             currentLineOffset = _orientation.MajorStart(currentBounds);
                             currentLineSize = 0;
@@ -575,7 +575,7 @@ namespace Avalonia.Layout
                     }
 
                     // Raise for the last line.
-                    _algorithmCallbacks.Algorithm_OnLineArranged(_lastRealizedDataIndexInsideRealizationWindow - countInLine + 1, countInLine, currentLineSize, _context);
+                    _algorithmCallbacks!.Algorithm_OnLineArranged(_lastRealizedDataIndexInsideRealizationWindow - countInLine + 1, countInLine, currentLineSize, _context!);
                 }
             }
         }
@@ -584,7 +584,7 @@ namespace Avalonia.Layout
             Size finalSize,
             LineAlignment lineAlignment,
             bool isWrapping,
-            string layoutId)
+            string? layoutId)
         {
             // Walk through the realized elements one line at a time and 
             // align them, Then call element.Arrange with the arranged bounds.
@@ -636,7 +636,7 @@ namespace Avalonia.Layout
             LineAlignment lineAlignment,
             bool isWrapping,
             Size finalSize,
-            string layoutId)
+            string? layoutId)
         {
             for (int rangeIndex = lineStartIndex; rangeIndex < lineStartIndex + countInLine; ++rangeIndex)
             {
@@ -723,11 +723,11 @@ namespace Avalonia.Layout
         {
             if (IsVirtualizingContext)
             {
-                _context.LayoutOrigin = new Point(_lastExtent.X, _lastExtent.Y);
+                _context!.LayoutOrigin = new Point(_lastExtent.X, _lastExtent.Y);
             }
         }
 
-        public ILayoutable GetElementIfRealized(int dataIndex)
+        public ILayoutable? GetElementIfRealized(int dataIndex)
         {
             if (_elementManager.IsDataIndexRealized(dataIndex))
             {

src/Avalonia.Layout/IFlowLayoutAlgorithmDelegates.cs

@@ -21,10 +21,10 @@ namespace Avalonia.Layout
         Rect Algorithm_GetExtent(
             Size availableSize,
             VirtualizingLayoutContext context,
-            ILayoutable firstRealized,
+            ILayoutable? firstRealized,
             int firstRealizedItemIndex,
             Rect firstRealizedLayoutBounds,
-            ILayoutable lastRealized,
+            ILayoutable? lastRealized,
             int lastRealizedItemIndex,
             Rect lastRealizedLayoutBounds);
         void Algorithm_OnElementMeasured(

src/Avalonia.Layout/LayoutContext.cs

@@ -14,7 +14,7 @@ namespace Avalonia.Layout
         /// <summary>
         /// Gets or sets an object that represents the state of a layout.
         /// </summary>
-        public object LayoutState 
+        public object? LayoutState 
         {
             get => LayoutStateCore;
             set => LayoutStateCore = value;
@@ -23,6 +23,6 @@ namespace Avalonia.Layout
         /// <summary>
         /// Implements the behavior of <see cref="LayoutState"/> in a derived or custom LayoutContext.
         /// </summary>
-        protected virtual object LayoutStateCore { get; set; }
+        protected virtual object? LayoutStateCore { get; set; }
     }
 }

src/Avalonia.Layout/LayoutContextAdapter.cs

@@ -16,7 +16,7 @@ namespace Avalonia.Layout
             _nonVirtualizingContext = nonVirtualizingContext;
         }
 
-        protected override object LayoutStateCore 
+        protected override object? LayoutStateCore 
         { 
             get => _nonVirtualizingContext.LayoutState;
             set => _nonVirtualizingContext.LayoutState = value; 

src/Avalonia.Layout/LayoutHelper.cs

@@ -101,7 +101,7 @@ namespace Avalonia.Layout
 
             if (result == 0 || double.IsNaN(result) || double.IsInfinity(result))
             {
-                throw new Exception($"Invalid LayoutScaling returned from {visualRoot.GetType()}");
+                throw new Exception($"Invalid LayoutScaling returned from {visualRoot!.GetType()}");
             }
 
             return result;

src/Avalonia.Layout/LayoutQueue.cs

@@ -5,6 +5,7 @@ using System.Collections.Generic;
 namespace Avalonia.Layout
 {
     internal class LayoutQueue<T> : IReadOnlyCollection<T>, IDisposable
+        where T : notnull
     {
         private struct Info
         {

src/Avalonia.Layout/Layoutable.cs

@@ -794,7 +794,7 @@ namespace Avalonia.Layout
         /// </summary>
         /// <param name="sender">The sender.</param>
         /// <param name="e">The event args.</param>
-        private void LayoutManagedLayoutUpdated(object sender, EventArgs e) => _layoutUpdated?.Invoke(this, e);
+        private void LayoutManagedLayoutUpdated(object? sender, EventArgs e) => _layoutUpdated?.Invoke(this, e);
 
         /// <summary>
         /// Tests whether any of a <see cref="Rect"/>'s properties include negative values,

src/Avalonia.Layout/NonVirtualizingLayoutContext.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Layout
     /// </summary>
     public abstract class NonVirtualizingLayoutContext : LayoutContext
     {
-        private VirtualizingLayoutContext _contextAdapter;
+        private VirtualizingLayoutContext? _contextAdapter;
 
         /// <summary>
         /// Gets the collection of child controls from the container that provides the context.
@@ -26,6 +26,6 @@ namespace Avalonia.Layout
         protected abstract IReadOnlyList<ILayoutable> ChildrenCore { get; }
 
         internal VirtualizingLayoutContext GetVirtualizingContextAdapter() =>
-            _contextAdapter ?? (_contextAdapter = new LayoutContextAdapter(this));
+            _contextAdapter ??= new LayoutContextAdapter(this);
     }
 }

src/Avalonia.Layout/StackLayout.cs

@@ -78,10 +78,10 @@ namespace Avalonia.Layout
         internal Rect GetExtent(
             Size availableSize,
             VirtualizingLayoutContext context,
-            ILayoutable firstRealized,
+            ILayoutable? firstRealized,
             int firstRealizedItemIndex,
             Rect firstRealizedLayoutBounds,
-            ILayoutable lastRealized,
+            ILayoutable? lastRealized,
             int lastRealizedItemIndex,
             Rect lastRealizedLayoutBounds)
         {
@@ -89,7 +89,7 @@ namespace Avalonia.Layout
 
             // Constants
             int itemsCount = context.ItemCount;
-            var stackState = (StackLayoutState)context.LayoutState;
+            var stackState = (StackLayoutState)context.LayoutState!;
             double averageElementSize = GetAverageElementSize(availableSize, context, stackState) + Spacing;
 
             _orientation.SetMinorSize(ref extent, stackState.MaxArrangeBounds);
@@ -131,7 +131,7 @@ namespace Avalonia.Layout
         {
             if (context is VirtualizingLayoutContext virtualContext)
             {
-                var stackState = (StackLayoutState)virtualContext.LayoutState;
+                var stackState = (StackLayoutState)virtualContext.LayoutState!;
                 var provisionalArrangeSizeWinRt = provisionalArrangeSize;
                 stackState.OnElementMeasured(
                     index,
@@ -177,7 +177,7 @@ namespace Avalonia.Layout
             if (targetIndex >= 0 && targetIndex < itemsCount)
             {
                 index = targetIndex;
-                var state = (StackLayoutState)context.LayoutState;
+                var state = (StackLayoutState)context.LayoutState!;
                 double averageElementSize = GetAverageElementSize(availableSize, context, state) + Spacing;
                 offset = index * averageElementSize + _orientation.MajorStart(state.FlowAlgorithm.LastExtent);
             }
@@ -188,10 +188,10 @@ namespace Avalonia.Layout
         Rect IFlowLayoutAlgorithmDelegates.Algorithm_GetExtent(
             Size availableSize,
             VirtualizingLayoutContext context,
-            ILayoutable firstRealized,
+            ILayoutable? firstRealized,
             int firstRealizedItemIndex,
             Rect firstRealizedLayoutBounds,
-            ILayoutable lastRealized,
+            ILayoutable? lastRealized,
             int lastRealizedItemIndex,
             Rect lastRealizedLayoutBounds)
         {
@@ -234,7 +234,7 @@ namespace Avalonia.Layout
             if (itemsCount > 0)
             {
                 var realizationRect = context.RealizationRect;
-                var state = (StackLayoutState)context.LayoutState;
+                var state = (StackLayoutState)context.LayoutState!;
                 var lastExtent = state.FlowAlgorithm.LastExtent;
 
                 double averageElementSize = GetAverageElementSize(availableSize, context, state) + Spacing;
@@ -279,13 +279,13 @@ namespace Avalonia.Layout
 
         protected internal override void UninitializeForContextCore(VirtualizingLayoutContext context)
         {
-            var stackState = (StackLayoutState)context.LayoutState;
+            var stackState = (StackLayoutState)context.LayoutState!;
             stackState.UninitializeForContext(context);
         }
 
         protected internal override Size MeasureOverride(VirtualizingLayoutContext context, Size availableSize)
         {
-            ((StackLayoutState)context.LayoutState).OnMeasureStart();
+            ((StackLayoutState)context.LayoutState!).OnMeasureStart();
 
             var desiredSize = GetFlowAlgorithm(context).Measure(
                 availableSize,
@@ -358,6 +358,6 @@ namespace Avalonia.Layout
 
         private void InvalidateLayout() => InvalidateMeasure();
 
-        private FlowLayoutAlgorithm GetFlowAlgorithm(VirtualizingLayoutContext context) => ((StackLayoutState)context.LayoutState).FlowAlgorithm;
+        private FlowLayoutAlgorithm GetFlowAlgorithm(VirtualizingLayoutContext context) => ((StackLayoutState)context.LayoutState!).FlowAlgorithm;
     }
 }

src/Avalonia.Layout/UniformGridLayout.cs

@@ -258,7 +258,7 @@ namespace Avalonia.Layout
             Size availableSize,
             VirtualizingLayoutContext context)
         {
-            var gridState = (UniformGridLayoutState)context.LayoutState;
+            var gridState = (UniformGridLayoutState)context.LayoutState!;
             return new Size(gridState.EffectiveItemWidth, gridState.EffectiveItemHeight);
         }
 
@@ -268,7 +268,7 @@ namespace Avalonia.Layout
             Size desiredSize,
             VirtualizingLayoutContext context)
         {
-            var gridState = (UniformGridLayoutState)context.LayoutState;
+            var gridState = (UniformGridLayoutState)context.LayoutState!;
             return new Size(gridState.EffectiveItemWidth, gridState.EffectiveItemHeight);
         }
 
@@ -285,7 +285,7 @@ namespace Avalonia.Layout
             var realizationRect = context.RealizationRect;
             if (itemsCount > 0 && _orientation.MajorSize(realizationRect) > 0)
             {
-                var gridState = (UniformGridLayoutState)context.LayoutState;
+                var gridState = (UniformGridLayoutState)context.LayoutState!;
                 var lastExtent = gridState.FlowAlgorithm.LastExtent;
                 var itemsPerLine = Math.Min( // note use of unsigned ints
                     Math.Max(1u, (uint)(_orientation.Minor(availableSize) / GetMinorSizeWithSpacing(context))),
@@ -324,7 +324,7 @@ namespace Avalonia.Layout
                     Math.Max(1u, _maximumRowsOrColumns));
                 int indexOfFirstInLine = (targetIndex / itemsPerLine) * itemsPerLine;
                 index = indexOfFirstInLine;
-                var state = context.LayoutState as UniformGridLayoutState;
+                var state = (UniformGridLayoutState)context.LayoutState!;
                 offset = _orientation.MajorStart(GetLayoutRectForDataIndex(availableSize, indexOfFirstInLine, state.FlowAlgorithm.LastExtent, context));
             }
 
@@ -338,10 +338,10 @@ namespace Avalonia.Layout
         Rect IFlowLayoutAlgorithmDelegates.Algorithm_GetExtent(
             Size availableSize,
             VirtualizingLayoutContext context,
-            ILayoutable firstRealized,
+            ILayoutable? firstRealized,
             int firstRealizedItemIndex,
             Rect firstRealizedLayoutBounds,
-            ILayoutable lastRealized,
+            ILayoutable? lastRealized,
             int lastRealizedItemIndex,
             Rect lastRealizedLayoutBounds)
         {
@@ -421,7 +421,7 @@ namespace Avalonia.Layout
 
         protected internal override void UninitializeForContextCore(VirtualizingLayoutContext context)
         {
-            var gridState = (UniformGridLayoutState)context.LayoutState;
+            var gridState = (UniformGridLayoutState)context.LayoutState!;
             gridState.UninitializeForContext(context);
         }
 
@@ -429,7 +429,7 @@ namespace Avalonia.Layout
         {
             // Set the width and height on the grid state. If the user already set them then use the preset. 
             // If not, we have to measure the first element and get back a size which we're going to be using for the rest of the items.
-            var gridState = (UniformGridLayoutState)context.LayoutState;
+            var gridState = (UniformGridLayoutState)context.LayoutState!;
             gridState.EnsureElementSize(availableSize, context, _minItemWidth, _minItemHeight, _itemsStretch, Orientation, MinRowSpacing, MinColumnSpacing, _maximumRowsOrColumns);
 
             var desiredSize = GetFlowAlgorithm(context).Measure(
@@ -467,7 +467,7 @@ namespace Avalonia.Layout
             // Always invalidate layout to keep the view accurate.
             InvalidateLayout();
 
-            var gridState = (UniformGridLayoutState)context.LayoutState;
+            var gridState = (UniformGridLayoutState)context.LayoutState!;
             gridState.ClearElementOnDataSourceChange(context, args);
         }
 
@@ -517,7 +517,7 @@ namespace Avalonia.Layout
         private double GetMinorSizeWithSpacing(VirtualizingLayoutContext context)
         {
             var minItemSpacing = MinItemSpacing;
-            var gridState = (UniformGridLayoutState)context.LayoutState;
+            var gridState = (UniformGridLayoutState)context.LayoutState!;
             return _orientation.ScrollOrientation == ScrollOrientation.Vertical?
                 gridState.EffectiveItemWidth + minItemSpacing :
                 gridState.EffectiveItemHeight + minItemSpacing;
@@ -526,7 +526,7 @@ namespace Avalonia.Layout
         private double GetMajorSizeWithSpacing(VirtualizingLayoutContext context)
         {
             var lineSpacing = LineSpacing;
-            var gridState = (UniformGridLayoutState)context.LayoutState;
+            var gridState = (UniformGridLayoutState)context.LayoutState!;
             return _orientation.ScrollOrientation == ScrollOrientation.Vertical ?
                 gridState.EffectiveItemHeight + lineSpacing :
                 gridState.EffectiveItemWidth + lineSpacing;
@@ -544,7 +544,7 @@ namespace Avalonia.Layout
             int rowIndex = (int)(index / itemsPerLine);
             int indexInRow = index - (rowIndex * itemsPerLine);
 
-            var gridState = (UniformGridLayoutState)context.LayoutState;
+            var gridState = (UniformGridLayoutState)context.LayoutState!;
             Rect bounds = _orientation.MinorMajorRect(
                 indexInRow * GetMinorSizeWithSpacing(context) + _orientation.MinorStart(lastExtent),
                 rowIndex * GetMajorSizeWithSpacing(context) + _orientation.MajorStart(lastExtent),
@@ -556,6 +556,6 @@ namespace Avalonia.Layout
 
         private void InvalidateLayout() => InvalidateMeasure();
 
-        private FlowLayoutAlgorithm GetFlowAlgorithm(VirtualizingLayoutContext context) => ((UniformGridLayoutState)context.LayoutState).FlowAlgorithm;
+        private FlowLayoutAlgorithm GetFlowAlgorithm(VirtualizingLayoutContext context) => ((UniformGridLayoutState)context.LayoutState!).FlowAlgorithm;
     }
 }

src/Avalonia.Layout/UniformGridLayoutState.cs

@@ -18,7 +18,7 @@ namespace Avalonia.Layout
         // If it does not, then we need to do context.GetElement(0) at which point we have requested an element and are on point to clear it.
         // If we are responsible for clearing element 0 we keep m_cachedFirstElement valid. 
         // If we are not (because FlowLayoutAlgorithm is holding it for us) then we just null out this field and use the one from FlowLayoutAlgorithm.
-        private ILayoutable _cachedFirstElement;
+        private ILayoutable? _cachedFirstElement;
 
         internal FlowLayoutAlgorithm FlowAlgorithm { get; } = new FlowLayoutAlgorithm();
         internal double EffectiveItemWidth { get; private set; }

src/Avalonia.Layout/Utils/ListUtils.cs

@@ -23,10 +23,5 @@ namespace Avalonia.Layout.Utils
                 list.AddRange(Enumerable.Repeat(value, size - cur));
             }
         }
-
-        public static void Resize<T>(this List<T> list, int count)
-        {
-            Resize(list, count, default);
-        }
     }
 }

src/Avalonia.Layout/VirtualLayoutContextAdapter.cs

@@ -6,14 +6,14 @@ namespace Avalonia.Layout
     public class VirtualLayoutContextAdapter : NonVirtualizingLayoutContext
     {
         private readonly VirtualizingLayoutContext _virtualizingContext;
-        private ChildrenCollection _children;
+        private ChildrenCollection? _children;
 
         public VirtualLayoutContextAdapter(VirtualizingLayoutContext virtualizingContext)
         {
             _virtualizingContext = virtualizingContext;
         }
 
-        protected override object LayoutStateCore
+        protected override object? LayoutStateCore
         {
             get => _virtualizingContext.LayoutState;
             set => _virtualizingContext.LayoutState = value;

src/Avalonia.Layout/VirtualizingLayoutContext.cs

@@ -43,7 +43,7 @@ namespace Avalonia.Layout
     /// </summary>
     public abstract class VirtualizingLayoutContext : LayoutContext
     {
-        private NonVirtualizingLayoutContext _contextAdapter;
+        private NonVirtualizingLayoutContext? _contextAdapter;
 
         /// <summary>
         /// Gets the number of items in the data.

src/Avalonia.Layout/WrapLayout/UvMeasure.cs

@@ -27,7 +27,7 @@ namespace Avalonia.Layout
             }
         }
 
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             if (obj is UvMeasure measure)
             {

src/Avalonia.Layout/WrapLayout/WrapItem.cs

@@ -20,6 +20,6 @@ namespace Avalonia.Layout
 
         public UvMeasure? Position { get; internal set; }
 
-        public ILayoutable Element { get; internal set; }
+        public ILayoutable? Element { get; internal set; }
     }
 }

src/Avalonia.Layout/WrapLayout/WrapLayout.cs

@@ -88,7 +88,7 @@ namespace Avalonia.Layout
         /// <inheritdoc />
         protected internal override void OnItemsChangedCore(VirtualizingLayoutContext context, object source, NotifyCollectionChangedEventArgs args)
         {
-            var state = (WrapLayoutState)context.LayoutState;
+            var state = (WrapLayoutState)context.LayoutState!;
 
             switch (args.Action)
             {
@@ -126,7 +126,7 @@ namespace Avalonia.Layout
             var realizationBounds = new UvBounds(Orientation, context.RealizationRect);
             var position = UvMeasure.Zero;
 
-            var state = (WrapLayoutState)context.LayoutState;
+            var state = (WrapLayoutState)context.LayoutState!;
             if (state.Orientation != Orientation)
             {
                 state.SetOrientation(Orientation);
@@ -261,7 +261,7 @@ namespace Avalonia.Layout
                 var spacingMeasure = new UvMeasure(Orientation, HorizontalSpacing, VerticalSpacing);
                 var realizationBounds = new UvBounds(Orientation, context.RealizationRect);
 
-                var state = (WrapLayoutState)context.LayoutState;
+                var state = (WrapLayoutState)context.LayoutState!;
                 bool Arrange(WrapItem item, bool isLast = false)
                 {
                     if (item.Measure.HasValue == false)

src/Avalonia.Layout/WrapLayout/WrapLayoutState.cs

@@ -62,8 +62,11 @@ namespace Avalonia.Layout
 
         internal void SetOrientation(Orientation orientation)
         {
-            foreach (var item in _items.Where(i => i.Measure.HasValue))
+            foreach (var item in _items)
             {
+                if (!item.Measure.HasValue)
+                    continue;
+
                 UvMeasure measure = item.Measure.Value;
                 double v = measure.V;
                 measure.V = measure.U;
@@ -120,10 +123,10 @@ namespace Avalonia.Layout
                 }
 
                 lastPosition = item.Position;
-                maxV = Math.Max(maxV, item.Measure.Value.V);
+                maxV = Math.Max(maxV, item.Measure!.Value.V);
             }
 
-            double totalHeight = lastPosition.Value.V + maxV;
+            double totalHeight = lastPosition!.Value.V + maxV;
             if (calculateAverage)
             {
                 return (totalHeight / itemCount) * _context.ItemCount;