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Home Forums How can you detect file type using byte-level inspection (e.g., IsPdf, IsImage)? – MindStick
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How can you detect file type using byte-level inspection (e.g., IsPdf, IsImage)?

ICSM Computer 765 16 May 2025

How can you detect file type using byte-level inspection (e.g., IsPdf, IsImage)?

c# c#
ICSM Computer
Ask by ICSM Computer
Updated 25 May 2025

1 Answers

Anubhav Sharma
Anubhav Sharma 25 May 2025
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Detecting file types using byte-level inspection (also called magic number checking) involves reading the beginning bytes of a file (the "magic number") to determine its format. This is more reliable than checking file extensions.

Here's how you can implement file type detection (e.g., IsPdf, IsImage, etc.) by inspecting the byte content:

Step-by-Step Overview

  1. Read the header bytes of the file (usually the first 4–8 bytes).
  2. Match the byte signature against known file type "magic numbers."
  3. Optionally, allow some fuzziness for formats like image types with common variations.

Common File Type Magic Numbers

File Type Magic Number (Hex) ASCII Description
PDF 25 50 44 46 %PDF PDF files start with %PDF
PNG 89 50 4E 47 0D 0A 1A 0A — PNG image
JPEG FF D8 FF — JPEG image
GIF 47 49 46 38 GIF8 GIF image (GIF87a/GIF89a)
ZIP 50 4B 03 04 PK.. ZIP archives (also DOCX, XLSX, etc.)

Example: Detect PDF and Image Types in C#

public static class FileTypeDetector
{
    public static bool IsPdf(byte[] bytes)
    {
        // Check if the first 4 bytes are "%PDF"
        return bytes.Length >= 4 &&
               bytes[0] == 0x25 && // %
               bytes[1] == 0x50 && // P
               bytes[2] == 0x44 && // D
               bytes[3] == 0x46;   // F
    }

    public static bool IsPng(byte[] bytes)
    {
        byte[] pngSignature = new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A };
        return bytes.Take(pngSignature.Length).SequenceEqual(pngSignature);
    }

    public static bool IsJpeg(byte[] bytes)
    {
        return bytes.Length >= 3 &&
               bytes[0] == 0xFF &&
               bytes[1] == 0xD8 &&
               bytes[2] == 0xFF;
    }

    public static bool IsGif(byte[] bytes)
    {
        return bytes.Length >= 4 &&
               bytes[0] == 0x47 && // G
               bytes[1] == 0x49 && // I
               bytes[2] == 0x46 && // F
               bytes[3] == 0x38;   // 8
    }

    public static bool IsZip(byte[] bytes)
    {
        return bytes.Length >= 4 &&
               bytes[0] == 0x50 && // P
               bytes[1] == 0x4B && // K
               bytes[2] == 0x03 &&
               bytes[3] == 0x04;
    }

    public static bool IsImage(byte[] bytes)
    {
        return IsPng(bytes) || IsJpeg(bytes) || IsGif(bytes);
    }
}

Usage Example

byte[] fileBytes = File.ReadAllBytes("example.pdf");

if (FileTypeDetector.IsPdf(fileBytes))
{
    Console.WriteLine("This is a PDF file.");
}
else if (FileTypeDetector.IsImage(fileBytes))
{
    Console.WriteLine("This is an image file.");
}

Benefits

  • Works regardless of file extension.
  • Safer when dealing with user-uploaded files.
  • Can easily be extended to support more file types.
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