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Home Interviews How can you securely delete a file (overwrite before delete)? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 18 May 2025
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To securely delete a file in C#, you should overwrite its content with random or zeroed data before deleting it. This prevents file recovery tools from retrieving the original content from disk.

Step-by-Step: Secure File Deletion

  1. Overwrite the file content
  2. Flush to disk
  3. Delete the file

Example: Overwrite with Random Data, Then Delete

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

public static class SecureFileDelete
{
    public static void SecureDelete(string filePath, int overwritePasses = 1)
    {
        if (!File.Exists(filePath))
            return;

        FileInfo fileInfo = new FileInfo(filePath);
        long fileLength = fileInfo.Length;

        using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Write))
        {
            byte[] buffer = new byte[4096];
            RandomNumberGenerator rng = RandomNumberGenerator.Create();

            for (int pass = 0; pass < overwritePasses; pass++)
            {
                fs.Position = 0;
                long totalWritten = 0;

                while (totalWritten < fileLength)
                {
                    rng.GetBytes(buffer);
                    int bytesToWrite = (int)Math.Min(buffer.Length, fileLength - totalWritten);
                    fs.Write(buffer, 0, bytesToWrite);
                    totalWritten += bytesToWrite;
                }

                fs.Flush(true); // flush to disk
            }
        }

        // Optionally truncate and delete
        File.Delete(filePath);
    }
}

Parameters

  1. overwritePasses: Number of times to overwrite the file (1 is usually enough; 3 for higher security).
  2. Uses RandomNumberGenerator instead of Random for cryptographic quality.

Notes

  1. This works best on traditional hard drives.
  2. On SSDs, the OS and drive firmware may ignore overwrite attempts due to wear leveling and deferred writes — use full-disk encryption or OS-level secure delete for SSDs.
  3. Truncating the file to zero bytes after overwrite is optional, but often unnecessary.
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