.NET: Identify GZip Compressed Files

See also: Identify Brotli-compressed Files.

Determine if a file is compressed with the GZipStream class by reading the file's header bytes and checking for the specific GZip signature. GZip files have a unique "magic number" at the beginning: the first two bytes are 0x1F and 0x8B, which identify the file as being in GZip format.

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

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

Example Code

using System;
using System.IO;

public class Program
{
	public static bool IsGZipFile(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 GZip file
			}

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

			// Check for the GZip signature (0x1F, 0x8B)
			return byte1 == 0x1F && byte2 == 0x8B;
		}
	}

	public static void Main()
	{
		string path = "file.gz";
		bool isGZip = IsGZipFile(path);

		Console.WriteLine(isGZip ? "The file is a GZip file." : "The file is not a GZip file.");
	}
}

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