Sealed some classes and made some classes internal.

src/ImageSharp/Common/Exceptions/ImageFormatException.cs

@@ -11,7 +11,7 @@ namespace ImageSharp
     /// The exception that is thrown when the library tries to load
     /// an image, which has an invalid format.
     /// </summary>
-    public class ImageFormatException : Exception
+    public sealed class ImageFormatException : Exception
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="ImageFormatException"/> class.

src/ImageSharp/Common/Exceptions/ImageProcessingException.cs

@@ -10,7 +10,7 @@ namespace ImageSharp
     /// <summary>
     /// The exception that is thrown when an error occurs when applying a process to an image.
     /// </summary>
-    public class ImageProcessingException : Exception
+    public sealed class ImageProcessingException : Exception
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="ImageProcessingException"/> class.

src/ImageSharp/Formats/Bmp/BmpConfigurationModule.cs

@@ -8,7 +8,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Registers the image encoders, decoders and mime type detectors for the bmp format.
     /// </summary>
-    public class BmpConfigurationModule : IConfigurationModule
+    public sealed class BmpConfigurationModule : IConfigurationModule
     {
         /// <inheritdoc/>
         public void Configure(Configuration config)

src/ImageSharp/Formats/Bmp/BmpDecoder.cs

@@ -26,7 +26,7 @@ namespace ImageSharp.Formats
     /// Formats will be supported in a later releases. We advise always
     /// to use only 24 Bit Windows bitmaps.
     /// </remarks>
-    public class BmpDecoder : IImageDecoder, IBmpDecoderOptions
+    public sealed class BmpDecoder : IImageDecoder, IBmpDecoderOptions
     {
         /// <inheritdoc/>
         public Image<TPixel> Decode<TPixel>(Configuration configuration, Stream stream)

src/ImageSharp/Formats/Bmp/BmpEncoder.cs

@@ -15,7 +15,7 @@ namespace ImageSharp.Formats
     /// Image encoder for writing an image to a stream as a Windows bitmap.
     /// </summary>
     /// <remarks>The encoder can currently only write 24-bit rgb images to streams.</remarks>
-    public class BmpEncoder : IImageEncoder, IBmpEncoderOptions
+    public sealed class BmpEncoder : IImageEncoder, IBmpEncoderOptions
     {
         /// <summary>
         /// Gets or sets the number of bits per pixel.

src/ImageSharp/Formats/Bmp/BmpFileHeader.cs

@@ -15,7 +15,7 @@ namespace ImageSharp.Formats
     /// All of the other integer values are stored in little-endian format
     /// (i.e. least-significant byte first).
     /// </remarks>
-    internal class BmpFileHeader
+    internal sealed class BmpFileHeader
     {
         /// <summary>
         /// Defines of the data structure in the bitmap file.

src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs

@@ -10,7 +10,7 @@ namespace ImageSharp.Formats
     /// the screen.
     /// <see href="https://en.wikipedia.org/wiki/BMP_file_format"/>
     /// </summary>
-    internal class BmpInfoHeader
+    internal sealed class BmpInfoHeader
     {
         /// <summary>
         /// Defines of the data structure in the bitmap file.

src/ImageSharp/Formats/Gif/GifConfigurationModule.cs

@@ -8,7 +8,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Registers the image encoders, decoders and mime type detectors for the gif format.
     /// </summary>
-    public class GifConfigurationModule : IConfigurationModule
+    public sealed class GifConfigurationModule : IConfigurationModule
     {
         /// <inheritdoc/>
         public void Configure(Configuration config)

src/ImageSharp/Formats/Gif/GifDecoder.cs

@@ -14,7 +14,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Decoder for generating an image out of a gif encoded stream.
     /// </summary>
-    public class GifDecoder : IImageDecoder, IGifDecoderOptions
+    public sealed class GifDecoder : IImageDecoder, IGifDecoderOptions
     {
         /// <summary>
         /// Gets or sets a value indicating whether the metadata should be ignored when the image is being decoded.

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -18,7 +18,7 @@ namespace ImageSharp.Formats
     /// Performs the gif decoding operation.
     /// </summary>
     /// <typeparam name="TPixel">The pixel format.</typeparam>
-    internal class GifDecoderCore<TPixel>
+    internal sealed class GifDecoderCore<TPixel>
         where TPixel : struct, IPixel<TPixel>
     {
         /// <summary>

src/ImageSharp/Formats/Gif/GifEncoder.cs

@@ -15,7 +15,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Image encoder for writing image data to a stream in gif format.
     /// </summary>
-    public class GifEncoder : IImageEncoder, IGifEncoderOptions
+    public sealed class GifEncoder : IImageEncoder, IGifEncoderOptions
     {
         /// <summary>
         /// Gets or sets a value indicating whether the metadata should be ignored when the image is being encoded.

src/ImageSharp/Formats/Jpeg/JpegConfigurationModule.cs

@@ -8,7 +8,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Registers the image encoders, decoders and mime type detectors for the jpeg format.
     /// </summary>
-    public class JpegConfigurationModule : IConfigurationModule
+    public sealed class JpegConfigurationModule : IConfigurationModule
     {
         /// <inheritdoc/>
         public void Configure(Configuration config)

src/ImageSharp/Formats/Jpeg/JpegDecoder.cs

@@ -14,7 +14,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Image decoder for generating an image out of a jpg stream.
     /// </summary>
-    public class JpegDecoder : IImageDecoder, IJpegDecoderOptions
+    public sealed class JpegDecoder : IImageDecoder, IJpegDecoderOptions
     {
         /// <summary>
         /// Gets or sets a value indicating whether the metadata should be ignored when the image is being decoded.

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -18,7 +18,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Performs the jpeg decoding operation.
     /// </summary>
-    internal unsafe class JpegDecoderCore : IDisposable
+    internal sealed unsafe class JpegDecoderCore : IDisposable
     {
         /// <summary>
         /// The maximum number of color components

src/ImageSharp/Formats/Jpeg/JpegEncoder.cs

@@ -14,7 +14,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Encoder for writing the data image to a stream in jpeg format.
     /// </summary>
-    public class JpegEncoder : IImageEncoder, IJpegEncoderOptions
+    public sealed class JpegEncoder : IImageEncoder, IJpegEncoderOptions
     {
         /// <summary>
         /// Gets or sets a value indicating whether the metadata should be ignored when the image is being decoded.

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -17,7 +17,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Image encoder for writing an image to a stream as a jpeg.
     /// </summary>
-    internal unsafe class JpegEncoderCore
+    internal sealed unsafe class JpegEncoderCore
     {
         /// <summary>
         /// The number of quantization tables.

src/ImageSharp/Formats/Png/PngConfigurationModule.cs

@@ -8,7 +8,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Registers the image encoders, decoders and mime type detectors for the png format.
     /// </summary>
-    public class PngConfigurationModule : IConfigurationModule
+    public sealed class PngConfigurationModule : IConfigurationModule
     {
         /// <inheritdoc/>
         public void Configure(Configuration host)

src/ImageSharp/Formats/Png/PngDecoder.cs

@@ -31,7 +31,7 @@ namespace ImageSharp.Formats
     /// </list>
     /// </para>
     /// </remarks>
-    public class PngDecoder : IImageDecoder, IPngDecoderOptions
+    public sealed class PngDecoder : IImageDecoder, IPngDecoderOptions
     {
         /// <summary>
         /// Gets or sets a value indicating whether the metadata should be ignored when the image is being decoded.

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -20,7 +20,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Performs the png decoding operation.
     /// </summary>
-    internal class PngDecoderCore
+    internal sealed class PngDecoderCore
     {
         /// <summary>
         /// The dictionary of available color types.

src/ImageSharp/Formats/Png/PngEncoder.cs

@@ -14,7 +14,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Image encoder for writing image data to a stream in png format.
     /// </summary>
-    public class PngEncoder : IImageEncoder, IPngEncoderOptions
+    public sealed class PngEncoder : IImageEncoder, IPngEncoderOptions
     {
         /// <summary>
         /// Gets or sets a value indicating whether the metadata should be ignored when the image is being encoded.

src/ImageSharp/Formats/Png/PngHeader.cs

@@ -8,7 +8,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Represents the png header chunk.
     /// </summary>
-    public sealed class PngHeader
+    internal sealed class PngHeader
     {
         /// <summary>
         /// Gets or sets the dimension in x-direction of the image in pixels.

src/ImageSharp/Formats/Png/PngInterlaceMode.cs

@@ -8,7 +8,7 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Provides enumeration of available PNG interlace modes.
     /// </summary>
-    public enum PngInterlaceMode : byte
+    internal enum PngInterlaceMode : byte
     {
         /// <summary>
         /// Non interlaced

src/ImageSharp/Formats/Png/Zlib/IChecksum.cs

@@ -12,7 +12,7 @@ namespace ImageSharp.Formats
     /// <code>Value</code>. The complete checksum object can also be reset
     /// so it can be used again with new data.
     /// </summary>
-    public interface IChecksum
+    internal interface IChecksum
     {
         /// <summary>
         /// Gets the data checksum computed so far.

src/ImageSharp/Formats/Png/Zlib/ZlibInflateStream.cs

@@ -9,7 +9,7 @@
     /// <summary>
     /// Provides methods and properties for deframing streams from PNGs.
     /// </summary>
-    internal class ZlibInflateStream : Stream
+    internal sealed class ZlibInflateStream : Stream
     {
         /// <summary>
         /// The inner raw memory stream