Change implementation to be via different classes so as to not break invariants and pixel measurements.

src/Windows/Avalonia.Direct2D1/Avalonia.Direct2D1.csproj

@@ -65,8 +65,10 @@
     <Compile Include="Media\BrushImpl.cs" />
     <Compile Include="Media\BrushWrapper.cs" />
     <Compile Include="Media\DrawingContext.cs" />
-    <Compile Include="Media\Imaging\RenderTargetBitmapImpl.cs" />
     <Compile Include="Media\Imaging\BitmapImpl.cs" />
+    <Compile Include="Media\Imaging\D2DBitmapImpl.cs" />
+    <Compile Include="Media\Imaging\RenderTargetBitmapImpl.cs" />
+    <Compile Include="Media\Imaging\WicBitmapImpl.cs" />
     <Compile Include="Media\RadialGradientBrushImpl.cs" />
     <Compile Include="Media\LinearGradientBrushImpl.cs" />
     <Compile Include="Media\AvaloniaTextRenderer.cs" />

src/Windows/Avalonia.Direct2D1/Direct2D1Platform.cs

@@ -46,7 +46,7 @@ namespace Avalonia.Direct2D1
 
         public IBitmapImpl CreateBitmap(int width, int height)
         {
-            return new BitmapImpl(s_imagingFactory, width, height);
+            return new WicBitmapImpl(s_imagingFactory, width, height);
         }
 
         public IFormattedTextImpl CreateFormattedText(
@@ -92,12 +92,12 @@ namespace Avalonia.Direct2D1
 
         public IBitmapImpl LoadBitmap(string fileName)
         {
-            return new BitmapImpl(s_imagingFactory, fileName);
+            return new WicBitmapImpl(s_imagingFactory, fileName);
         }
 
         public IBitmapImpl LoadBitmap(Stream stream)
         {
-            return new BitmapImpl(s_imagingFactory, stream);
+            return new WicBitmapImpl(s_imagingFactory, stream);
         }
     }
 }

src/Windows/Avalonia.Direct2D1/Media/DrawingContext.cs

@@ -76,7 +76,7 @@ namespace Avalonia.Direct2D1.Media
         /// <param name="destRect">The rect in the output to draw to.</param>
         public void DrawImage(IBitmap source, double opacity, Rect sourceRect, Rect destRect)
         {
-            BitmapImpl impl = (BitmapImpl)source.PlatformImpl;
+            var impl = (BitmapImpl)source.PlatformImpl;
             Bitmap d2d = impl.GetDirect2DBitmap(_renderTarget);
             _renderTarget.DrawBitmap(
                 d2d,

src/Windows/Avalonia.Direct2D1/Media/Imaging/BitmapImpl.cs

@@ -1,150 +1,24 @@
-// Copyright (c) The Avalonia Project. All rights reserved.
-// Licensed under the MIT license. See licence.md file in the project root for full license information.
-
-using System;
+using System;
+using System.Collections.Generic;
 using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
 using Avalonia.Platform;
-using SharpDX.WIC;
+using SharpDX.Direct2D1;
 
 namespace Avalonia.Direct2D1.Media
 {
-    /// <summary>
-    /// A Direct2D implementation of a <see cref="Avalonia.Media.Imaging.Bitmap"/>.
-    /// </summary>
-    public class BitmapImpl : IBitmapImpl
+    public abstract class BitmapImpl : IBitmapImpl, IDisposable
     {
-        private readonly ImagingFactory _factory;
-
-        private SharpDX.Direct2D1.Bitmap _direct2D;
-
-        /// <summary>
-        /// Initializes a new instance of the <see cref="BitmapImpl"/> class.
-        /// </summary>
-        /// <param name="factory">The WIC imaging factory to use.</param>
-        /// <param name="fileName">The filename of the bitmap to load.</param>
-        public BitmapImpl(ImagingFactory factory, string fileName)
-        {
-            _factory = factory;
-
-            using (BitmapDecoder decoder = new BitmapDecoder(factory, fileName, DecodeOptions.CacheOnDemand))
-            {
-                WicImpl = new Bitmap(factory, decoder.GetFrame(0), BitmapCreateCacheOption.CacheOnDemand);
-            }
-        }
-
-        /// <summary>
-        /// Initializes a new instance of the <see cref="BitmapImpl"/> class.
-        /// </summary>
-        /// <param name="factory">The WIC imaging factory to use.</param>
-        /// <param name="stream">The stream to read the bitmap from.</param>
-        public BitmapImpl(ImagingFactory factory, Stream stream)
-        {
-            _factory = factory;
-
-            using (BitmapDecoder decoder = new BitmapDecoder(factory, stream, DecodeOptions.CacheOnLoad))
-            {
-                WicImpl = new Bitmap(factory, decoder.GetFrame(0), BitmapCreateCacheOption.CacheOnLoad);
-            }
-        }
-
-        /// <summary>
-        /// Initializes a new instance of the <see cref="BitmapImpl"/> class.
-        /// </summary>
-        /// <param name="factory">The WIC imaging factory to use.</param>
-        /// <param name="width">The width of the bitmap.</param>
-        /// <param name="height">The height of the bitmap.</param>
-        public BitmapImpl(ImagingFactory factory, int width, int height)
-        {
-            _factory = factory;
-            WicImpl = new Bitmap(
-                factory,
-                width,
-                height,
-                PixelFormat.Format32bppPBGRA,
-                BitmapCreateCacheOption.CacheOnLoad);
-        }
-
-        /// <summary>
-        /// Initialize a new instance of the <see cref="BitmapImpl"/> class
-        /// with a bitmap backed by GPU memory.
-        /// </summary>
-        /// <param name="d2DBitmap">The GPU bitmap.</param>
-        /// <remarks>
-        /// This bitmap must be either from the same render target,
-        /// or if the render target is a <see cref="SharpDX.Direct2D1.DeviceContext"/>,
-        /// the device associated with this context, to be renderable.
-        /// </remarks>
-        public BitmapImpl(SharpDX.Direct2D1.Bitmap d2DBitmap)
-        {
-            _direct2D = d2DBitmap;
-        }
-
-        /// <summary>
-        /// Gets the width of the bitmap, in pixels.
-        /// </summary>
-        public int PixelWidth => WicImpl.Size.Width;
-
-        /// <summary>
-        /// Gets the height of the bitmap, in pixels.
-        /// </summary>
-        public int PixelHeight => WicImpl.Size.Height;
+        public abstract Bitmap GetDirect2DBitmap(SharpDX.Direct2D1.RenderTarget target);
+        public abstract int PixelWidth { get; }
+        public abstract int PixelHeight { get; }
+        public abstract void Save(string fileName);
+        public abstract void Save(Stream stream);
 
         public virtual void Dispose()
         {
-            WicImpl.Dispose();
-            _direct2D?.Dispose();
-        }
-
-        /// <summary>
-        /// Gets the WIC implementation of the bitmap.
-        /// </summary>
-        public Bitmap WicImpl { get; }
-
-        /// <summary>
-        /// Gets a Direct2D bitmap to use on the specified render target.
-        /// </summary>
-        /// <param name="renderTarget">The render target.</param>
-        /// <returns>The Direct2D bitmap.</returns>
-        public SharpDX.Direct2D1.Bitmap GetDirect2DBitmap(SharpDX.Direct2D1.RenderTarget renderTarget)
-        {
-            if (_direct2D == null)
-            {
-                FormatConverter converter = new FormatConverter(_factory);
-                converter.Initialize(WicImpl, PixelFormat.Format32bppPBGRA);
-                _direct2D = SharpDX.Direct2D1.Bitmap.FromWicBitmap(renderTarget, converter);
-            }
-
-            return _direct2D;
-        }
-
-        /// <summary>
-        /// Saves the bitmap to a file.
-        /// </summary>
-        /// <param name="fileName">The filename.</param>
-        public void Save(string fileName)
-        {
-            if (Path.GetExtension(fileName) != ".png")
-            {
-                // Yeah, we need to support other formats.
-                throw new NotSupportedException("Use PNG, stoopid.");
-            }
-
-            using (FileStream s = new FileStream(fileName, FileMode.Create))
-            {
-                Save(s);
-            }
-        }
-
-        public void Save(Stream stream)
-        {
-            PngBitmapEncoder encoder = new PngBitmapEncoder(_factory);
-            encoder.Initialize(stream);
-
-            BitmapFrameEncode frame = new BitmapFrameEncode(encoder);
-            frame.Initialize();
-            frame.WriteSource(WicImpl);
-            frame.Commit();
-            encoder.Commit();
         }
     }
 }

src/Windows/Avalonia.Direct2D1/Media/Imaging/D2DBitmapImpl.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Avalonia.Platform;
+using SharpDX.Direct2D1;
+
+namespace Avalonia.Direct2D1.Media
+{
+    /// <summary>
+    /// A Direct2D Bitmap implementation that uses a GPU memory bitmap as its image.
+    /// </summary>
+    public class D2DBitmapImpl : BitmapImpl
+    {
+        private Bitmap _direct2D;
+
+        /// <summary>
+        /// Initialize a new instance of the <see cref="BitmapImpl"/> class
+        /// with a bitmap backed by GPU memory.
+        /// </summary>
+        /// <param name="d2DBitmap">The GPU bitmap.</param>
+        /// <remarks>
+        /// This bitmap must be either from the same render target,
+        /// or if the render target is a <see cref="SharpDX.Direct2D1.DeviceContext"/>,
+        /// the device associated with this context, to be renderable.
+        /// </remarks>
+        public D2DBitmapImpl(Bitmap d2DBitmap)
+        {
+            if (d2DBitmap == null) throw new ArgumentNullException(nameof(d2DBitmap));
+
+            _direct2D = d2DBitmap;
+        }
+
+        public override Bitmap GetDirect2DBitmap(SharpDX.Direct2D1.RenderTarget target) => _direct2D;
+               
+        public override int PixelWidth => _direct2D.PixelSize.Width;
+        public override int PixelHeight => _direct2D.PixelSize.Height;
+
+        public override void Save(string fileName)
+        {
+            throw new NotImplementedException();
+        }
+
+        public override void Save(Stream stream)
+        {
+            throw new NotImplementedException();
+        }
+
+        public override void Dispose()
+        {
+            base.Dispose();
+            _direct2D.Dispose();
+        }
+    }
+}

src/Windows/Avalonia.Direct2D1/Media/Imaging/RenderTargetBitmapImpl.cs

@@ -10,7 +10,7 @@ using SharpDX.WIC;
 
 namespace Avalonia.Direct2D1.Media
 {
-    public class RenderTargetBitmapImpl : BitmapImpl, IRenderTargetBitmapImpl, IDisposable
+    public class RenderTargetBitmapImpl : WicBitmapImpl, IRenderTargetBitmapImpl
     {
         private readonly WicRenderTarget _target;
 

src/Windows/Avalonia.Direct2D1/Media/Imaging/WicBitmapImpl.cs

@@ -0,0 +1,135 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.IO;
+using Avalonia.Platform;
+using SharpDX.WIC;
+
+namespace Avalonia.Direct2D1.Media
+{
+    /// <summary>
+    /// A WIC implementation of a <see cref="Avalonia.Media.Imaging.Bitmap"/>.
+    /// </summary>
+    public class WicBitmapImpl : BitmapImpl
+    {
+        private readonly ImagingFactory _factory;
+
+        private SharpDX.Direct2D1.Bitmap _direct2D;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="WicBitmapImpl"/> class.
+        /// </summary>
+        /// <param name="factory">The WIC imaging factory to use.</param>
+        /// <param name="fileName">The filename of the bitmap to load.</param>
+        public WicBitmapImpl(ImagingFactory factory, string fileName)
+        {
+            _factory = factory;
+
+            using (BitmapDecoder decoder = new BitmapDecoder(factory, fileName, DecodeOptions.CacheOnDemand))
+            {
+                WicImpl = new Bitmap(factory, decoder.GetFrame(0), BitmapCreateCacheOption.CacheOnDemand);
+            }
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="WicBitmapImpl"/> class.
+        /// </summary>
+        /// <param name="factory">The WIC imaging factory to use.</param>
+        /// <param name="stream">The stream to read the bitmap from.</param>
+        public WicBitmapImpl(ImagingFactory factory, Stream stream)
+        {
+            _factory = factory;
+
+            using (BitmapDecoder decoder = new BitmapDecoder(factory, stream, DecodeOptions.CacheOnLoad))
+            {
+                WicImpl = new Bitmap(factory, decoder.GetFrame(0), BitmapCreateCacheOption.CacheOnLoad);
+            }
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="WicBitmapImpl"/> class.
+        /// </summary>
+        /// <param name="factory">The WIC imaging factory to use.</param>
+        /// <param name="width">The width of the bitmap.</param>
+        /// <param name="height">The height of the bitmap.</param>
+        public WicBitmapImpl(ImagingFactory factory, int width, int height)
+        {
+            _factory = factory;
+            WicImpl = new Bitmap(
+                factory,
+                width,
+                height,
+                PixelFormat.Format32bppPBGRA,
+                BitmapCreateCacheOption.CacheOnLoad);
+        }
+
+        /// <summary>
+        /// Gets the width of the bitmap, in pixels.
+        /// </summary>
+        public override int PixelWidth => WicImpl.Size.Width;
+
+        /// <summary>
+        /// Gets the height of the bitmap, in pixels.
+        /// </summary>
+        public override int PixelHeight => WicImpl.Size.Height;
+
+        public override void Dispose()
+        {
+            WicImpl.Dispose();
+            _direct2D?.Dispose();
+        }
+
+        /// <summary>
+        /// Gets the WIC implementation of the bitmap.
+        /// </summary>
+        public Bitmap WicImpl { get; }
+
+        /// <summary>
+        /// Gets a Direct2D bitmap to use on the specified render target.
+        /// </summary>
+        /// <param name="renderTarget">The render target.</param>
+        /// <returns>The Direct2D bitmap.</returns>
+        public override SharpDX.Direct2D1.Bitmap GetDirect2DBitmap(SharpDX.Direct2D1.RenderTarget renderTarget)
+        {
+            if (_direct2D == null)
+            {
+                FormatConverter converter = new FormatConverter(_factory);
+                converter.Initialize(WicImpl, PixelFormat.Format32bppPBGRA);
+                _direct2D = SharpDX.Direct2D1.Bitmap.FromWicBitmap(renderTarget, converter);
+            }
+
+            return _direct2D;
+        }
+
+        /// <summary>
+        /// Saves the bitmap to a file.
+        /// </summary>
+        /// <param name="fileName">The filename.</param>
+        public override void Save(string fileName)
+        {
+            if (Path.GetExtension(fileName) != ".png")
+            {
+                // Yeah, we need to support other formats.
+                throw new NotSupportedException("Use PNG, stoopid.");
+            }
+
+            using (FileStream s = new FileStream(fileName, FileMode.Create))
+            {
+                Save(s);
+            }
+        }
+
+        public override void Save(Stream stream)
+        {
+            PngBitmapEncoder encoder = new PngBitmapEncoder(_factory);
+            encoder.Initialize(stream);
+
+            BitmapFrameEncode frame = new BitmapFrameEncode(encoder);
+            frame.Initialize();
+            frame.WriteSource(WicImpl);
+            frame.Commit();
+            encoder.Commit();
+        }
+    }
+}