.NET: Identify Brotli Compressed Files

See also: Identify GZip Files.

Determine if a file is compressed with the BrotliStream class by reading the file's header bytes and checking for the specific Brotli signature. Brotli-compressed files have a unique "magic number" at the beginning: the first two bytes are 0xCE and 0xB2, which identify the file as being in Brotli format.

This approach is lightweight and allows for quickly verifying if a file is compressed with BrotliStream without fully decompressing it.

NOTE: One can always try to decompress a Brolti-compressed file, and if it fails, then it is not a Brotli file.

Example Code

using System;
using System.IO;

public class Program
{
	public static bool IsBrotliFile(string filePath)
	{
		// Open the file to check the header
		using (var fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
		{
			if (fileStream.Length < 2)
			{
				return false; // File is too short to be a valid Brotli file
			}

			int byte1 = fileStream.ReadByte();
			int byte2 = fileStream.ReadByte();

			// Check for the Brotli signature (0xCE, 0xB2)
			return byte1 == 0xCE && byte2 == 0xB2;
		}
	}

	public static void Main()
	{
		string path = "file.br";
		bool isBrotli = IsBrotliFile(path);

		Console.WriteLine(isBrotli ? "The file is a Brotli-compressed file." : "The file is not a Brotli-compressed file.");
	}
}

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