Make PlatformImpl nullable and always check for null value

samples/interop/Direct3DInteropSample/MainWindow.cs

@@ -58,7 +58,7 @@ namespace Direct3DInteropSample
                    new ModeDescription((int)ClientSize.Width, (int)ClientSize.Height,
                             new Rational(60, 1), Format.R8G8B8A8_UNorm),
                 IsWindowed = true,
-                OutputHandle = PlatformImpl.Handle.Handle,
+                OutputHandle = PlatformImpl?.Handle.Handle ?? IntPtr.Zero,
                 SampleDescription = new SampleDescription(1, 0),
                 SwapEffect = SwapEffect.Discard,
                 Usage = Usage.RenderTargetOutput

src/Avalonia.Controls/Embedding/EmbeddableControlRoot.cs

@@ -4,6 +4,7 @@ using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Platform;
 using Avalonia.Styling;
+using JetBrains.Annotations;
 
 namespace Avalonia.Controls.Embedding
 {
@@ -18,6 +19,7 @@ namespace Avalonia.Controls.Embedding
         {
         }
 
+        [CanBeNull]
         public new IEmbeddableWindowImpl PlatformImpl => (IEmbeddableWindowImpl) base.PlatformImpl;
 
         public void Prepare()
@@ -39,8 +41,9 @@ namespace Avalonia.Controls.Embedding
 
         protected override Size MeasureOverride(Size availableSize)
         {
-            base.MeasureOverride(PlatformImpl.ClientSize);
-            return PlatformImpl.ClientSize;
+            var cs = PlatformImpl?.ClientSize ?? default(Size);
+            base.MeasureOverride(cs);
+            return cs;
         }
 
         private readonly NameScope _nameScope = new NameScope();
@@ -63,9 +66,6 @@ namespace Avalonia.Controls.Embedding
         public void Unregister(string name) => _nameScope.Unregister(name);
 
         Type IStyleable.StyleKey => typeof(EmbeddableControlRoot);
-        public void Dispose()
-        {
-            PlatformImpl.Dispose();
-        }
+        public void Dispose() => PlatformImpl?.Dispose();
     }
 }

src/Avalonia.Controls/Primitives/PopupRoot.cs

@@ -9,6 +9,7 @@ using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.Platform;
 using Avalonia.VisualTree;
+using JetBrains.Annotations;
 
 namespace Avalonia.Controls.Primitives
 {
@@ -49,6 +50,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets the platform-specific window implementation.
         /// </summary>
+        [CanBeNull]
         public new IPopupImpl PlatformImpl => (IPopupImpl)base.PlatformImpl;
 
         /// <summary>
@@ -65,10 +67,7 @@ namespace Avalonia.Controls.Primitives
         IVisual IHostedVisualTreeRoot.Host => Parent;
 
         /// <inheritdoc/>
-        public void Dispose()
-        {
-            this.PlatformImpl.Dispose();
-        }
+        public void Dispose() => PlatformImpl?.Dispose();
 
         /// <inheritdoc/>
         protected override void OnTemplateApplied(TemplateAppliedEventArgs e)

src/Avalonia.Controls/TopLevel.cs

@@ -14,6 +14,7 @@ using Avalonia.Platform;
 using Avalonia.Rendering;
 using Avalonia.Styling;
 using Avalonia.VisualTree;
+using JetBrains.Annotations;
 
 namespace Avalonia.Controls
 {
@@ -92,13 +93,13 @@ namespace Avalonia.Controls
             var rendererFactory = TryGetService<IRendererFactory>(dependencyResolver);
             Renderer = rendererFactory?.CreateRenderer(this, renderLoop);
 
-            PlatformImpl.SetInputRoot(this);
+            impl.SetInputRoot(this);
 
-            PlatformImpl.Closed = HandleClosed;
-            PlatformImpl.Input = HandleInput;
-            PlatformImpl.Paint = HandlePaint;
-            PlatformImpl.Resized = HandleResized;
-            PlatformImpl.ScalingChanged = HandleScalingChanged;
+            impl.Closed = HandleClosed;
+            impl.Input = HandleInput;
+            impl.Paint = HandlePaint;
+            impl.Resized = HandleResized;
+            impl.ScalingChanged = HandleScalingChanged;
 
 
             _keyboardNavigationHandler?.SetOwner(this);
@@ -110,7 +111,7 @@ namespace Avalonia.Controls
             this.GetObservable(PointerOverElementProperty)
                 .Select(
                     x => (x as InputElement)?.GetObservable(CursorProperty) ?? Observable.Empty<Cursor>())
-                .Switch().Subscribe(cursor => PlatformImpl.SetCursor(cursor?.PlatformCursor));
+                .Switch().Subscribe(cursor => PlatformImpl?.SetCursor(cursor?.PlatformCursor));
 
             if (_applicationLifecycle != null)
             {
@@ -135,10 +136,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the platform-specific window implementation.
         /// </summary>
-        public ITopLevelImpl PlatformImpl
-        {
-            get;
-        }
+        [CanBeNull]
+        public ITopLevelImpl PlatformImpl { get; private set; }
         
         /// <summary>
         /// Gets the renderer for the window.
@@ -177,7 +176,7 @@ namespace Avalonia.Controls
         Size ILayoutRoot.MaxClientSize => Size.Infinity;
 
         /// <inheritdoc/>
-        double ILayoutRoot.LayoutScaling => PlatformImpl.Scaling;
+        double ILayoutRoot.LayoutScaling => PlatformImpl?.Scaling ?? 1;
 
         IStyleHost IStyleHost.StylingParent
         {
@@ -189,25 +188,27 @@ namespace Avalonia.Controls
         /// <inheritdoc/>
         protected virtual IRenderTarget CreateRenderTarget()
         {
+            if(PlatformImpl == null)
+                throw new InvalidOperationException("PlatformImpl isn't available");
             return _renderInterface.CreateRenderTarget(PlatformImpl.Surfaces);
         }
 
         /// <inheritdoc/>
         void IRenderRoot.Invalidate(Rect rect)
         {
-            PlatformImpl.Invalidate(rect);
+            PlatformImpl?.Invalidate(rect);
         }
 
         /// <inheritdoc/>
         Point IRenderRoot.PointToClient(Point p)
         {
-            return PlatformImpl.PointToClient(p);
+            return PlatformImpl?.PointToClient(p) ?? default(Point);
         }
 
         /// <inheritdoc/>
         Point IRenderRoot.PointToScreen(Point p)
         {
-            return PlatformImpl.PointToScreen(p);
+            return PlatformImpl?.PointToScreen(p) ?? default(Point);
         }
 
         /// <summary>
@@ -224,6 +225,8 @@ namespace Avalonia.Controls
         /// </summary>
         protected virtual void HandleClosed()
         {
+            PlatformImpl = null;
+
             Closed?.Invoke(this, EventArgs.Empty);
             Renderer?.Dispose();
             Renderer = null;

src/Avalonia.Controls/Window.cs

@@ -12,6 +12,7 @@ using Avalonia.Platform;
 using Avalonia.Styling;
 using System.Collections.Generic;
 using System.Linq;
+using JetBrains.Annotations;
 
 namespace Avalonia.Controls
 {
@@ -87,11 +88,11 @@ namespace Avalonia.Controls
         static Window()
         {
             BackgroundProperty.OverrideDefaultValue(typeof(Window), Brushes.White);
-            TitleProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl.SetTitle((string)e.NewValue));
+            TitleProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl?.SetTitle((string)e.NewValue));
             HasSystemDecorationsProperty.Changed.AddClassHandler<Window>(
-                (s, e) => s.PlatformImpl.SetSystemDecorations((bool) e.NewValue));
+                (s, e) => s.PlatformImpl?.SetSystemDecorations((bool) e.NewValue));
 
-            IconProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl.SetIcon(((WindowIcon)e.NewValue).PlatformImpl));
+            IconProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl?.SetIcon(((WindowIcon)e.NewValue).PlatformImpl));
         }
 
         /// <summary>
@@ -109,7 +110,7 @@ namespace Avalonia.Controls
         public Window(IWindowImpl impl)
             : base(impl)
         {
-            _maxPlatformClientSize = this.PlatformImpl.MaxClientSize;
+            _maxPlatformClientSize = PlatformImpl?.MaxClientSize ?? default(Size);
         }
 
         /// <inheritdoc/>
@@ -129,6 +130,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the platform-specific window implementation.
         /// </summary>
+        [CanBeNull]
         public new IWindowImpl PlatformImpl => (IWindowImpl)base.PlatformImpl;
 
         /// <summary>
@@ -164,8 +166,12 @@ namespace Avalonia.Controls
         /// </summary>
         public WindowState WindowState
         {
-            get { return this.PlatformImpl.WindowState; }
-            set { this.PlatformImpl.WindowState = value; }
+            get { return PlatformImpl?.WindowState ?? WindowState.Normal; }
+            set
+            {
+                if (PlatformImpl != null)
+                    PlatformImpl.WindowState = value;
+            }
         }
 
         /// <summary>
@@ -189,7 +195,7 @@ namespace Avalonia.Controls
         public void Close()
         {
             s_windows.Remove(this);
-            PlatformImpl.Dispose();
+            PlatformImpl?.Dispose();
             IsVisible = false;
         }
 
@@ -221,7 +227,7 @@ namespace Avalonia.Controls
         {
             using (BeginAutoSizing())
             {
-                PlatformImpl.Hide();
+                PlatformImpl?.Hide();
             }
 
             IsVisible = false;
@@ -240,7 +246,7 @@ namespace Avalonia.Controls
 
             using (BeginAutoSizing())
             {
-                PlatformImpl.Show();
+                PlatformImpl?.Show();
             }
         }
 
@@ -278,7 +284,7 @@ namespace Avalonia.Controls
                 var activated = affectedWindows.Where(w => w.IsActive).FirstOrDefault();
                 SetIsEnabled(affectedWindows, false);
 
-                var modal = PlatformImpl.ShowDialog();
+                var modal = PlatformImpl?.ShowDialog();
                 var result = new TaskCompletionSource<TResult>();
 
                 Observable.FromEventPattern<EventHandler, EventArgs>(
@@ -287,7 +293,7 @@ namespace Avalonia.Controls
                     .Take(1)
                     .Subscribe(_ =>
                     {
-                        modal.Dispose();
+                        modal?.Dispose();
                         SetIsEnabled(affectedWindows, true);
                         activated?.Activate();
                         result.SetResult((TResult)_dialogResult);

src/Avalonia.Controls/WindowBase.cs

@@ -9,6 +9,7 @@ using Avalonia.Controls.Primitives;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Platform;
+using JetBrains.Annotations;
 
 namespace Avalonia.Controls
 {
@@ -43,10 +44,10 @@ namespace Avalonia.Controls
 
         public WindowBase(IWindowBaseImpl impl, IAvaloniaDependencyResolver dependencyResolver) : base(impl, dependencyResolver)
         {
-            PlatformImpl.Activated = HandleActivated;
-            PlatformImpl.Deactivated = HandleDeactivated;
-            PlatformImpl.PositionChanged = HandlePositionChanged;
-            this.GetObservable(ClientSizeProperty).Skip(1).Subscribe(x => PlatformImpl.Resize(x));
+            impl.Activated = HandleActivated;
+            impl.Deactivated = HandleDeactivated;
+            impl.PositionChanged = HandlePositionChanged;
+            this.GetObservable(ClientSizeProperty).Skip(1).Subscribe(x => PlatformImpl?.Resize(x));
         }
 
         /// <summary>
@@ -64,6 +65,7 @@ namespace Avalonia.Controls
         /// </summary>
         public event EventHandler<PointEventArgs> PositionChanged;
 
+        [CanBeNull]
         public new IWindowBaseImpl PlatformImpl => (IWindowBaseImpl) base.PlatformImpl;
 
         /// <summary>
@@ -80,8 +82,12 @@ namespace Avalonia.Controls
         /// </summary>
         public Point Position
         {
-            get { return PlatformImpl.Position; }
-            set { PlatformImpl.Position = value; }
+            get { return PlatformImpl?.Position ?? default(Point); }
+            set
+            {
+                if (PlatformImpl is IWindowBaseImpl impl)
+                    impl.Position = value;
+            }
         }
 
         /// <summary>
@@ -98,7 +104,7 @@ namespace Avalonia.Controls
         /// </summary>
         public void Activate()
         {
-            PlatformImpl.Activate();
+            PlatformImpl?.Activate();
         }
 
         /// <summary>
@@ -110,7 +116,7 @@ namespace Avalonia.Controls
 
             try
             {
-                PlatformImpl.Hide();
+                PlatformImpl?.Hide();
                 IsVisible = false;
             }
             finally
@@ -131,7 +137,7 @@ namespace Avalonia.Controls
                 EnsureInitialized();
                 IsVisible = true;
                 LayoutManager.Instance.ExecuteInitialLayoutPass(this);
-                PlatformImpl.Show();
+                PlatformImpl?.Show();
             }
             finally
             {
@@ -163,10 +169,10 @@ namespace Avalonia.Controls
         {
             using (BeginAutoSizing())
             {
-                PlatformImpl.Resize(finalSize);
+                PlatformImpl?.Resize(finalSize);
             }
 
-            return base.ArrangeOverride(PlatformImpl.ClientSize);
+            return base.ArrangeOverride(PlatformImpl?.ClientSize ?? default(Size));
         }
 
         /// <summary>
@@ -174,7 +180,7 @@ namespace Avalonia.Controls
         /// </summary>
         protected void EnsureInitialized()
         {
-            if (!this.IsInitialized)
+            if (!IsInitialized)
             {
                 var init = (ISupportInitialize)this;
                 init.BeginInit();
@@ -268,12 +274,12 @@ namespace Avalonia.Controls
         /// <summary>
         /// Starts moving a window with left button being held. Should be called from left mouse button press event handler
         /// </summary>
-        public void BeginMoveDrag() => PlatformImpl.BeginMoveDrag();
+        public void BeginMoveDrag() => PlatformImpl?.BeginMoveDrag();
 
         /// <summary>
         /// Starts resizing a window. This function is used if an application has window resizing controls. 
         /// Should be called from left mouse button press event handler
         /// </summary>
-        public void BeginResizeDrag(WindowEdge edge) => PlatformImpl.BeginResizeDrag(edge);
+        public void BeginResizeDrag(WindowEdge edge) => PlatformImpl?.BeginResizeDrag(edge);
     }
 }

src/Avalonia.DesignerSupport/DesignerAssist.cs

@@ -75,7 +75,7 @@ namespace Avalonia.DesignerSupport
         private static void SetScalingFactor(double factor)
         {
             PlatformManager.SetDesignerScalingFactor(factor);
-            s_currentWindow?.PlatformImpl.Resize(s_currentWindow.ClientSize);
+            s_currentWindow?.PlatformImpl?.Resize(s_currentWindow.ClientSize);
         }
 
         static Window s_currentWindow;
@@ -149,6 +149,8 @@ namespace Avalonia.DesignerSupport
             s_currentWindow = window;
             window.Show();
             Design.ApplyDesignerProperties(window, control);
+            // ReSharper disable once PossibleNullReferenceException
+            // Always not null at this point
             Api.OnWindowCreated?.Invoke(window.PlatformImpl.Handle.Handle);
             Api.OnResize?.Invoke();
         }

src/Windows/Avalonia.Win32/Embedding/WinFormsAvaloniaControlHost.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Win32.Embedding
     {
         private readonly EmbeddableControlRoot _root = new EmbeddableControlRoot();
 
-        private IntPtr WindowHandle => ((WindowImpl) _root.PlatformImpl).Handle.Handle;
+        private IntPtr WindowHandle => ((WindowImpl) _root?.PlatformImpl)?.Handle?.Handle ?? IntPtr.Zero;
 
         public WinFormsAvaloniaControlHost()
         {
@@ -25,6 +25,8 @@ namespace Avalonia.Win32.Embedding
             if (_root.IsFocused)
                 FocusManager.Instance.Focus(null);
             _root.GotFocus += RootGotFocus;
+            // ReSharper disable once PossibleNullReferenceException
+            // Always non-null at this point
             _root.PlatformImpl.LostFocus += PlatformImpl_LostFocus;
             FixPosition();
         }