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Home Interviews How do you generate a file hash (SHA256, MD5) to detect changes? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 18 May 2025
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To generate a file hash (e.g., SHA256 or MD5) in C#, you can use the System.Security.Cryptography namespace. This is useful for detecting file changes by comparing hash values before and after modifications.

Generate File Hash in C#

Example: SHA256

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

public static class FileHasher
{
    public static string GetSHA256Hash(string filePath)
    {
        using (var sha256 = SHA256.Create())
        using (var stream = File.OpenRead(filePath))
        {
            byte[] hashBytes = sha256.ComputeHash(stream);
            return BitConverter.ToString(hashBytes).Replace("-", "").ToLowerInvariant();
        }
    }
}

Example: MD5

public static string GetMD5Hash(string filePath)
{
    using (var md5 = MD5.Create())
    using (var stream = File.OpenRead(filePath))
    {
        byte[] hashBytes = md5.ComputeHash(stream);
        return BitConverter.ToString(hashBytes).Replace("-", "").ToLowerInvariant();
    }
}

When to Use

  1. SHA256 is more secure and recommended for integrity checks.
  2. MD5 is faster but vulnerable to collisions — fine for quick change detection but not security.

Compare Hashes to Detect Changes

string originalHash = FileHasher.GetSHA256Hash("original.txt");
string newHash = FileHasher.GetSHA256Hash("downloaded.txt");

if (originalHash != newHash)
{
    Console.WriteLine("File has changed.");
}
else
{
    Console.WriteLine("File is unchanged.");
}
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