C# 中读取图片头判断图片宽高

直接上代码，很好理解~~~ 

public static class ImageHeader
        {
            const string errorMessage = "Could not recognise image format.";

            private static Dictionary<byte[], Func<BinaryReader, Size>> imageFormatDecoders = new Dictionary<byte[], Func<BinaryReader, Size>>()
            { 
                { new byte[] { 0x42, 0x4D }, DecodeBitmap }, 
                { new byte[] { 0x47, 0x49, 0x46, 0x38, 0x37, 0x61 }, DecodeGif }, 
                { new byte[] { 0x47, 0x49, 0x46, 0x38, 0x39, 0x61 }, DecodeGif }, 
                { new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }, DecodePng },
                { new byte[] { 0xff, 0xd8 }, DecodeJfif }, 
            };

            /// <summary>        
            /// Gets the dimensions of an image.        
            /// </summary>        
            /// <param name="path">The path of the image to get the dimensions of.</param>        
            /// <returns>The dimensions of the specified image.</returns>        
            /// <exception cref="ArgumentException">The image was of an unrecognised format.</exception>        
            public static Size GetDimensions(string path)
            {
                try
                {
                    using (BinaryReader binaryReader = new BinaryReader(File.OpenRead(path)))
                    {
                        try
                        {
                            return GetDimensions(binaryReader);
                        }
                        catch (ArgumentException e)
                        {
                            string newMessage = string.Format("{0} file: '{1}' ", errorMessage, path);

                            throw new ArgumentException(newMessage, "path", e);
                        }
                    }
                }
                catch (ArgumentException)
                {
                    using (Bitmap b = new Bitmap(path))
                    {
                        return b.Size;
                    }
                }
            }

            /// <summary>        
            /// Gets the dimensions of an image.        
            /// </summary>        
            /// <param name="path">The path of the image to get the dimensions of.</param>        
            /// <returns>The dimensions of the specified image.</returns>        
            /// <exception cref="ArgumentException">The image was of an unrecognised format.</exception>            
            public static Size GetDimensions(BinaryReader binaryReader)
            {
                int maxMagicBytesLength = imageFormatDecoders.Keys.OrderByDescending(x => x.Length).First().Length;
                byte[] magicBytes = new byte[maxMagicBytesLength];
                for (int i = 0; i < maxMagicBytesLength; i += 1)
                {
                    magicBytes[i] = binaryReader.ReadByte();
                    foreach (var kvPair in imageFormatDecoders)
                    {
                        if (StartsWith(magicBytes, kvPair.Key))
                        {
                            return kvPair.Value(binaryReader);
                        }
                    }
                }

                throw new ArgumentException(errorMessage, "binaryReader");
            }

            private static bool StartsWith(byte[] thisBytes, byte[] thatBytes)
            {
                for (int i = 0; i < thatBytes.Length; i += 1)
                {
                    if (thisBytes[i] != thatBytes[i])
                    {
                        return false;
                    }
                }

                return true;
            }

            private static short ReadLittleEndianInt16(BinaryReader binaryReader)
            {
                byte[] bytes = new byte[sizeof(short)];

                for (int i = 0; i < sizeof(short); i += 1)
                {
                    bytes[sizeof(short) - 1 - i] = binaryReader.ReadByte();
                }
                return BitConverter.ToInt16(bytes, 0);
            }

            private static ushort ReadLittleEndianUInt16(BinaryReader binaryReader)
            {
                byte[] bytes = new byte[sizeof(ushort)];

                for (int i = 0; i < sizeof(ushort); i += 1)
                {
                    bytes[sizeof(ushort) - 1 - i] = binaryReader.ReadByte();
                }
                return BitConverter.ToUInt16(bytes, 0);
            }

            private static int ReadLittleEndianInt32(BinaryReader binaryReader)
            {
                byte[] bytes = new byte[sizeof(int)];
                for (int i = 0; i < sizeof(int); i += 1)
                {
                    bytes[sizeof(int) - 1 - i] = binaryReader.ReadByte();
                }
                return BitConverter.ToInt32(bytes, 0);
            }

            private static Size DecodeBitmap(BinaryReader binaryReader)
            {
                binaryReader.ReadBytes(16);
                int width = binaryReader.ReadInt32();
                int height = binaryReader.ReadInt32();
                return new Size(width, height);
            }

            private static Size DecodeGif(BinaryReader binaryReader)
            {
                int width = binaryReader.ReadInt16();
                int height = binaryReader.ReadInt16();
                return new Size(width, height);
            }

            private static Size DecodePng(BinaryReader binaryReader)
            {
                binaryReader.ReadBytes(8);
                int width = ReadLittleEndianInt32(binaryReader);
                int height = ReadLittleEndianInt32(binaryReader);
                return new Size(width, height);
            }

            private static Size DecodeJfif(BinaryReader binaryReader)
            {
                while (binaryReader.ReadByte() == 0xff)
                {
                    byte marker = binaryReader.ReadByte();
                    short chunkLength = ReadLittleEndianInt16(binaryReader);
                    if (marker == 0xc0)
                    {
                        binaryReader.ReadByte();
                        int height = ReadLittleEndianInt16(binaryReader);
                        int width = ReadLittleEndianInt16(binaryReader);
                        return new Size(width, height);
                    }

                    if (chunkLength < 0)
                    {
                        ushort uchunkLength = (ushort)chunkLength;
                        binaryReader.ReadBytes(uchunkLength - 2);
                    }
                    else
                    {
                        binaryReader.ReadBytes(chunkLength - 2);
                    }
                }

                throw new ArgumentException(errorMessage);
            }
        }