Write .ICO files [C#]

Greetings. Here's a console application I wrote, which uses the BinaryWriter to create icon files (.ico image file format). It is based on these specifications.
[Note: .NET ImageFormat.Icon doesn't really write .ico files]

You can drag and drop one or more .NET compatible image files over the app icon (equivalent to console arguments).
The program creates a Bitmap of each image, and checks if it is within the 256x256 pixels limit. Then, the Bitmap data is used by the BinaryWriter to create the converted file.

CreateNew mode on the FileStream avoids file overwriting.
Since the app is targeted to run from its' icon, exceptions are shown via MessageBoxes.

using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Drawing; using System.Windows.Forms; using System.IO; namespace Icon_Maker { class Program { static void Main(string[] args) { foreach (string _file in args) { try { Bitmap _bitmap = (Bitmap)Image.FromFile(_file); if (_bitmap.Width > 256 || _bitmap.Height > 256) throw new ArgumentOutOfRangeException("Image must not exceed 256x256 pixels."); ConvertFile(_file, _bitmap); _bitmap.Dispose(); _bitmap = null; } catch (OutOfMemoryException ex) { MessageBox.Show(ex.Message + "\n" + _file.Substring(_file.LastIndexOf(".")) + " is not a valid image format.", "Icon Maker", MessageBoxButtons.OK, MessageBoxIcon.Error); } catch (ArgumentOutOfRangeException ex) { MessageBox.Show(ex.Message, "Icon Maker", MessageBoxButtons.OK, MessageBoxIcon.Error); } catch (IOException ex) // raised by the FileStream, if target file already exists { MessageBox.Show(ex.Message, "Icon Maker", MessageBoxButtons.OK, MessageBoxIcon.Error); } } } private static void ConvertFile(string _file, Bitmap _bitmap) { FileStream _fileStream = new FileStream(_file.Remove(_file.LastIndexOf(@".")) + ".ico", FileMode.CreateNew); // the mode raises IOException if the file already exists BinaryWriter _binaryWriter = new BinaryWriter(_fileStream); /* Write file header. */ // [2 bytes] Reserved _binaryWriter.Write((ushort)0); // [2 bytes] Type: 1 for .ICO, 2 for .CUR _binaryWriter.Write((ushort)1); // [2 bytes] Number of icons in the file. _binaryWriter.Write((ushort)1); // [1 byte] Icon width in pixels (0 is used for 256 pixels). if (_bitmap.Width < 256) _binaryWriter.Write((byte)_bitmap.Width); else _binaryWriter.Write((byte)0); // [1 byte] Icon height in pixels (0 is used for 256 pixels). if (_bitmap.Height < 256) _binaryWriter.Write((byte)_bitmap.Height); else _binaryWriter.Write((byte)0); // [1 byte] Number of colors (0 is used for non-palette RGB) _binaryWriter.Write((byte)0); // [1 byte] Reserved _binaryWriter.Write((byte)0); // [2 bytes] In .ICO: Color planes. // In .CUR: Horizontal coordinates of the hotspot. _binaryWriter.Write((ushort)1); // [2 bytes] In .ICO format: Bits per pixel (RGB + Alpha is 4 channels of 8 bits each.) // In .CUR format: Vertical coordinates of the hotspot. _binaryWriter.Write((ushort)32); // [4 bytes] Size of the bitmap data in bytes + InfoHeader which equals 40. _binaryWriter.Write((uint)(40 + _bitmap.Width * _bitmap.Height * 32 / 8)); // [4 bytes] The offset where the bitmap InfoHeader starts. _binaryWriter.Write((uint)22); /* Write bitmap InfoHeader */ // [4 bytes] Size of InfoHeader. _binaryWriter.Write((uint)40); // [4 bytes] Bitmap width in pixels. _binaryWriter.Write((uint)_bitmap.Width); // [4 bytes] Bitmap height in pixels * 2 // (the multiplication represents the alpha channel image). _binaryWriter.Write((uint)_bitmap.Height * 2); // Number of planes. _binaryWriter.Write((ushort)1); // Bits per pixel. _binaryWriter.Write((ushort)32); // Type of compression (0 for none). _binaryWriter.Write((uint)0); // Size of image in bytes(0 for uncompressed). _binaryWriter.Write((uint)0); // Unused _binaryWriter.Write((uint)0); _binaryWriter.Write((uint)0); _binaryWriter.Write((uint)0); _binaryWriter.Write((uint)0); /* Write bitmap data */ for (int y = _bitmap.Height - 1; y >= 0; y--) // Pixels are written bottom-up { for (int x = 0; x < _bitmap.Width; x++) // and left-to-right. { // Color values are written GBR. _binaryWriter.Write((byte)(_bitmap.GetPixel(x, y).B)); _binaryWriter.Write((byte)(_bitmap.GetPixel(x, y).G)); _binaryWriter.Write((byte)(_bitmap.GetPixel(x, y).R)); _binaryWriter.Write((byte)(255)); // Transparency (alpha channel) } } _binaryWriter.Close(); _fileStream.Close(); } } }