Fix win forms TryGetPlatformHandle usage + add missing docs and adjustments

src/Avalonia.Controls/TopLevel.cs

@@ -383,7 +383,7 @@ namespace Avalonia.Controls
         /// An <see cref="IPlatformHandle"/> describing the window handle, or null if the handle
         /// could not be retrieved.
         /// </returns>
-        public IPlatformHandle? TryGetPlatformHandle() => ((IWindowBaseImpl?) PlatformImpl)?.Handle;
+        public IPlatformHandle? TryGetPlatformHandle() => (PlatformImpl as IWindowBaseImpl)?.Handle;
 
         /// <summary>
         /// Gets the renderer for the window.

src/Windows/Avalonia.Win32.Interop/Avalonia.Win32.Interop.csproj

@@ -1,10 +1,9 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net461</TargetFramework>
+    <TargetFrameworks>net461;net6.0-windows</TargetFrameworks>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <UseWpf>true</UseWpf>
     <UseWindowsForms>true</UseWindowsForms>
-    <ExtrasEnableImplicitWinFormsReferences>true</ExtrasEnableImplicitWinFormsReferences>
     <UseDirect3D9>true</UseDirect3D9>
     <PackageId>Avalonia.Win32.Interoperability</PackageId>
   </PropertyGroup>

src/Windows/Avalonia.Win32.Interop/WinForms/WinFormsAvaloniaControlHost.cs

@@ -2,38 +2,45 @@
 using System.ComponentModel;
 using System.Windows.Forms;
 using Avalonia.Controls.Embedding;
-using Avalonia.Input;
-using Avalonia.VisualTree;
-using Avalonia.Win32.Interop;
 using WinFormsControl = System.Windows.Forms.Control;
 
-namespace Avalonia.Win32.Embedding
+namespace Avalonia.Win32.Interop
 {
+    /// <summary>
+    /// An element that allows you to host a Avalonia control on a Windows Forms page.
+    /// </summary>
     [ToolboxItem(true)]
     public class WinFormsAvaloniaControlHost : WinFormsControl
     {
-        private readonly EmbeddableControlRoot _root = new EmbeddableControlRoot();
+        private readonly EmbeddableControlRoot _root = new();
 
-        private IntPtr WindowHandle => ((WindowImpl) _root?.PlatformImpl)?.Handle?.Handle ?? IntPtr.Zero;
+        private IntPtr WindowHandle => _root?.TryGetPlatformHandle()?.Handle ?? IntPtr.Zero;
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="WinFormsAvaloniaControlHost"/> class.
+        /// </summary>
         public WinFormsAvaloniaControlHost()
         {
             SetStyle(ControlStyles.AllPaintingInWmPaint, true);
             UnmanagedMethods.SetParent(WindowHandle, Handle);
             _root.Prepare();
             if (_root.IsFocused)
-                _root.FocusManager.ClearFocus();
+                _root.FocusManager?.ClearFocus();
             _root.GotFocus += RootGotFocus;
 
             FixPosition();
         }
 
+        /// <summary>
+        /// Gets or sets the Avalonia control hosted by the <see cref="WinFormsAvaloniaControlHost"/> element.
+        /// </summary>
         public Avalonia.Controls.Control Content
         {
-            get { return (Avalonia.Controls.Control)_root.Content; }
-            set { _root.Content = value; }
+            get => (Avalonia.Controls.Control)_root.Content;
+            set => _root.Content = value;
         }
 
+        /// <inheritdoc />
         protected override void Dispose(bool disposing)
         {
             if (disposing)
@@ -46,27 +53,27 @@ namespace Avalonia.Win32.Embedding
             UnmanagedMethods.SetFocus(WindowHandle);
         }
 
+        /// <inheritdoc />
         protected override void OnGotFocus(EventArgs e)
         {
             if (_root != null)
                 UnmanagedMethods.SetFocus(WindowHandle);
         }
-
-
-        void FixPosition()
+        
+        private void FixPosition()
         {
             if (_root != null && Width > 0 && Height > 0)
                 UnmanagedMethods.MoveWindow(WindowHandle, 0, 0, Width, Height, true);
         }
-
-
-
+        
+        /// <inheritdoc />
         protected override void OnResize(EventArgs e)
         {
             FixPosition();
             base.OnResize(e);
         }
 
+        /// <inheritdoc />
         protected override void OnPaint(PaintEventArgs e)
         {
 

src/Windows/Avalonia.Win32.Interop/Wpf/CursorShim.cs

@@ -7,7 +7,7 @@ using System.Windows.Input;
 
 namespace Avalonia.Win32.Interop.Wpf
 {
-    static class CursorShim
+    internal static class CursorShim
     {
         public static Cursor FromHCursor(IntPtr hcursor)
         {
@@ -20,7 +20,7 @@ namespace Avalonia.Win32.Interop.Wpf
             return rv;
         }
 
-        class SafeHandleShim : SafeHandle
+        private class SafeHandleShim : SafeHandle
         {
             public SafeHandleShim(IntPtr hcursor) : base(new IntPtr(-1), false)
             {

src/Windows/Avalonia.Win32.Interop/Wpf/Direct2DImageSurface.cs

@@ -18,9 +18,9 @@ using Usage = SharpDX.Direct3D9.Usage;
 
 namespace Avalonia.Win32.Interop.Wpf
 {
-    class Direct2DImageSurface : IExternalDirect2DRenderTargetSurface, IDisposable
+    internal class Direct2DImageSurface : IExternalDirect2DRenderTargetSurface, IDisposable
     {
-        class SwapBuffer: IDisposable
+        private class SwapBuffer: IDisposable
         {
             private readonly Query _event;
             private readonly SharpDX.Direct3D11.Resource _resource;
@@ -114,7 +114,7 @@ namespace Avalonia.Win32.Interop.Wpf
         [DllImport("user32.dll", SetLastError = false)]
         private static extern IntPtr GetDesktopWindow();
 
-        static void EnsureDirectX()
+        private static void EnsureDirectX()
         {
             if(s_d3DDevice != null)
                 return;
@@ -168,13 +168,13 @@ namespace Avalonia.Win32.Interop.Wpf
             return _backBuffer.Target;
         }
 
-        static void RemoveAndDispose<T>(ref T d) where T : IDisposable
+        private static void RemoveAndDispose<T>(ref T d) where T : IDisposable
         {
             d?.Dispose();
             d = default;
         }
 
-        void Swap()
+        private void Swap()
         {
             _backBuffer.Flush();
             _image.Lock();

src/Windows/Avalonia.Win32.Interop/Wpf/IntSize.cs

@@ -2,7 +2,7 @@
 
 namespace Avalonia.Win32.Interop.Wpf
 {
-    struct IntSize : IEquatable<IntSize>
+    internal struct IntSize : IEquatable<IntSize>
     {
         public bool Equals(IntSize other)
         {

src/Windows/Avalonia.Win32.Interop/Wpf/WpfAvaloniaHost.cs

@@ -2,15 +2,24 @@
 using System.Threading;
 using System.Windows;
 using System.Windows.Markup;
+using Avalonia.Win32.Interop.Wpf;
+using AvControl = Avalonia.Controls.Control;
 
-namespace Avalonia.Win32.Interop.Wpf
+namespace Avalonia.Win32.Interop
 {
+    /// <summary>
+    /// An element that allows you to host a Avalonia control on a WPF page.
+    /// </summary>
     [ContentProperty("Content")]
     public class WpfAvaloniaHost : FrameworkElement, IDisposable, IAddChild
     {
         private WpfTopLevelImpl _impl;
         private readonly SynchronizationContext _sync;
         private bool _hasChildren;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="WpfAvaloniaHost"/> class.
+        /// </summary>
         public WpfAvaloniaHost()
         {
             _sync = SynchronizationContext.Current;
@@ -38,14 +47,17 @@ namespace Avalonia.Win32.Interop.Wpf
             }
         }
 
-        public object Content
+        /// <summary>
+        /// Gets or sets the Avalonia control hosted by the <see cref="WpfAvaloniaHost"/> element.
+        /// </summary>
+        public AvControl Content
         {
-            get => _impl.ControlRoot.Content;
+            get => (AvControl)_impl.ControlRoot.Content;
             set => _impl.ControlRoot.Content = value;
         }
 
         //Separate class is needed to prevent accidental resurrection
-        class Disposer
+        private class Disposer
         {
             private readonly WpfTopLevelImpl _impl;
 
@@ -60,6 +72,7 @@ namespace Avalonia.Win32.Interop.Wpf
             }
         }
 
+        /// <inheritdoc />
         protected override System.Windows.Size MeasureOverride(System.Windows.Size constraint)
         {
             _impl.InvalidateMeasure();
@@ -67,13 +80,17 @@ namespace Avalonia.Win32.Interop.Wpf
             return _impl.DesiredSize;
         }
 
+        /// <inheritdoc />
         protected override System.Windows.Size ArrangeOverride(System.Windows.Size arrangeSize)
         {
             _impl.Arrange(new System.Windows.Rect(arrangeSize));
             return arrangeSize;
         }
         
+        /// <inheritdoc />
         protected override int VisualChildrenCount => 1;
+
+        /// <inheritdoc />
         protected override System.Windows.Media.Visual GetVisualChild(int index) => _impl;
 
         ~WpfAvaloniaHost()
@@ -82,6 +99,7 @@ namespace Avalonia.Win32.Interop.Wpf
                 _sync.Post(new Disposer(_impl).Callback, null);
         }
 
+        /// <inheritdoc />
         public void Dispose()
         {
             if (_impl != null)
@@ -97,9 +115,12 @@ namespace Avalonia.Win32.Interop.Wpf
         void IAddChild.AddChild(object value)
         {
             if (Content == null)
-                Content = value;
+                if (value is AvControl avControl)
+                    Content = avControl;
+                else
+                    throw new InvalidOperationException("WpfAvaloniaHost.Content only accepts value of Avalonia.Controls.Control type.");
             else
-                throw new InvalidOperationException();
+                throw new InvalidOperationException("WpfAvaloniaHost.Content was already set.");
         }
 
         void IAddChild.AddText(string text)

src/Windows/Avalonia.Win32.Interop/Wpf/WpfInteropExtensions.cs

@@ -1,6 +1,6 @@
 namespace Avalonia.Win32.Interop.Wpf
 {
-    static class WpfInteropExtensions
+    internal static class WpfInteropExtensions
     {
         public static System.Windows.Point ToWpfPoint(this Point pt) => new System.Windows.Point(pt.X, pt.Y);
         public static System.Windows.Point ToWpfPoint(this PixelPoint pt) => new System.Windows.Point(pt.X, pt.Y);

src/Windows/Avalonia.Win32.Interop/Wpf/WpfMouseDevice.cs

@@ -5,7 +5,7 @@ using Avalonia.VisualTree;
 
 namespace Avalonia.Win32.Interop.Wpf
 {
-    class WpfMouseDevice : MouseDevice
+    internal class WpfMouseDevice : MouseDevice
     {
         private readonly WpfTopLevelImpl _impl;
 
@@ -14,7 +14,7 @@ namespace Avalonia.Win32.Interop.Wpf
             _impl = impl;
         }
 
-        class WpfMousePointer : Pointer
+        private class WpfMousePointer : Pointer
         {
             private readonly WpfTopLevelImpl _impl;
 
@@ -35,6 +35,5 @@ namespace Avalonia.Win32.Interop.Wpf
                     System.Windows.Input.Mouse.Capture(_impl);
             }
         }
-        
     }
 }

src/Windows/Avalonia.Win32.Interop/Wpf/WpfTopLevelImpl.cs

@@ -19,7 +19,7 @@ using MouseButton = System.Windows.Input.MouseButton;
 
 namespace Avalonia.Win32.Interop.Wpf
 {
-    class WpfTopLevelImpl : FrameworkElement, ITopLevelImpl
+    internal class WpfTopLevelImpl : FrameworkElement, ITopLevelImpl
     {
         private HwndSource _currentHwndSource;
         private readonly HwndSourceHook _hook;
@@ -137,7 +137,7 @@ namespace Avalonia.Win32.Interop.Wpf
 
         protected override void OnLostFocus(RoutedEventArgs e) => LostFocus?.Invoke();
 
-        static RawInputModifiers GetModifiers(MouseEventArgs e)
+        private static RawInputModifiers GetModifiers(MouseEventArgs e)
         {
             var state = Keyboard.Modifiers;
             var rv = default(RawInputModifiers);
@@ -161,7 +161,7 @@ namespace Avalonia.Win32.Interop.Wpf
             return rv;
         }
 
-        void MouseEvent(RawPointerEventType type, MouseEventArgs e)
+        private void MouseEvent(RawPointerEventType type, MouseEventArgs e)
             => _ttl.Input?.Invoke(new RawPointerEventArgs(_mouse, (uint)e.Timestamp, _inputRoot, type,
             e.GetPosition(this).ToAvaloniaPoint(), GetModifiers(e)));
 

src/Windows/Avalonia.Win32.Interop/Wpf/WritableBitmapSurface.cs

@@ -8,7 +8,7 @@ using PixelFormat = Avalonia.Platform.PixelFormat;
 
 namespace Avalonia.Win32.Interop.Wpf
 {
-    class WritableBitmapSurface : IFramebufferPlatformSurface
+    internal class WritableBitmapSurface : IFramebufferPlatformSurface
     {
         private readonly WpfTopLevelImpl _impl;
         private WriteableBitmap _bitmap;