Make BmpFileHeader readonly

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -581,13 +581,12 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
             this.currentStream.Read(data, 0, BmpFileHeader.Size);
 
-            this.fileHeader = new BmpFileHeader
-            {
-                Type = BitConverter.ToInt16(data, 0),
-                FileSize = BitConverter.ToInt32(data, 2),
-                Reserved = BitConverter.ToInt32(data, 6),
-                Offset = BitConverter.ToInt32(data, 10)
-            };
+            this.fileHeader = new BmpFileHeader(
+                type: BitConverter.ToInt16(data, 0),
+                fileSize: BitConverter.ToInt32(data, 2),
+                reserved: BitConverter.ToInt32(data, 6),
+                offset: BitConverter.ToInt32(data, 10)
+            );
         }
 
         /// <summary>

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -55,9 +55,9 @@ namespace SixLabors.ImageSharp.Formats.Bmp
             this.padding = bytesPerLine - (image.Width * (int)this.bitsPerPixel);
 
             // Do not use IDisposable pattern here as we want to preserve the stream.
-            EndianBinaryWriter writer = new EndianBinaryWriter(Endianness.LittleEndian, stream);
+            var writer = new EndianBinaryWriter(Endianness.LittleEndian, stream);
 
-            BmpInfoHeader infoHeader = new BmpInfoHeader
+            var infoHeader = new BmpInfoHeader
             {
                 HeaderSize = BmpInfoHeader.BitmapInfoHeaderSize,
                 Height = image.Height,
@@ -69,12 +69,12 @@ namespace SixLabors.ImageSharp.Formats.Bmp
                 ClrImportant = 0
             };
 
-            BmpFileHeader fileHeader = new BmpFileHeader
-            {
-                Type = 19778, // BM
-                Offset = 54,
-                FileSize = 54 + infoHeader.ImageSize
-            };
+            var fileHeader = new BmpFileHeader(
+                type: 19778, // BM
+                offset: 54,
+                reserved: 0,
+                fileSize: 54 + infoHeader.ImageSize
+            );
 
             WriteHeader(writer, fileHeader);
             this.WriteInfo(writer, infoHeader);
@@ -92,7 +92,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
         /// <param name="fileHeader">
         /// The <see cref="BmpFileHeader"/> containing the header data.
         /// </param>
-        private static void WriteHeader(EndianBinaryWriter writer, BmpFileHeader fileHeader)
+        private static void WriteHeader(EndianBinaryWriter writer, in BmpFileHeader fileHeader)
         {
             writer.Write(fileHeader.Type);
             writer.Write(fileHeader.FileSize);

src/ImageSharp/Formats/Bmp/BmpFileHeader.cs

@@ -13,35 +13,43 @@ namespace SixLabors.ImageSharp.Formats.Bmp
     /// All of the other integer values are stored in little-endian format
     /// (i.e. least-significant byte first).
     /// </remarks>
-    internal sealed class BmpFileHeader
+    internal readonly struct BmpFileHeader
     {
         /// <summary>
         /// Defines of the data structure in the bitmap file.
         /// </summary>
         public const int Size = 14;
 
+        public BmpFileHeader(short type, int fileSize, int reserved, int offset)
+        {
+            this.Type = type;
+            this.FileSize = fileSize;
+            this.Reserved = reserved;
+            this.Offset = offset;
+        }
+
         /// <summary>
-        /// Gets or sets the Bitmap identifier.
+        /// Gets the Bitmap identifier.
         /// The field used to identify the bitmap file: 0x42 0x4D
         /// (Hex code points for B and M)
         /// </summary>
-        public short Type { get; set; }
+        public short Type { get; }
 
         /// <summary>
-        /// Gets or sets the size of the bitmap file in bytes.
+        /// Gets the size of the bitmap file in bytes.
         /// </summary>
-        public int FileSize { get; set; }
+        public int FileSize { get; }
 
         /// <summary>
-        /// Gets or sets any reserved data; actual value depends on the application
+        /// Gets any reserved data; actual value depends on the application
         /// that creates the image.
         /// </summary>
-        public int Reserved { get; set; }
+        public int Reserved { get;  }
 
         /// <summary>
-        /// Gets or sets the offset, i.e. starting address, of the byte where
+        /// Gets the offset, i.e. starting address, of the byte where
         /// the bitmap data can be found.
         /// </summary>
-        public int Offset { get; set; }
+        public int Offset { get; }
     }
-}
+}