C# MD5 hasher example

Question

Edit: I've retitled this to an example as the code works as expected.

I am trying to copy a file, get a MD5 hash, then delete the copy. I am doing this to avoid process locks on the original file, which another app writes to. However, I am getting a lock on the file I've copied.

File.Copy(pathSrc, pathDest, true);

String md5Result;
StringBuilder sb = new StringBuilder();
MD5 md5Hasher = MD5.Create();

using (FileStream fs = File.OpenRead(pathDest))
{
    foreach(Byte b in md5Hasher.ComputeHash(fs))
        sb.Append(b.ToString("x2").ToLower());
}

md5Result = sb.ToString();

File.Delete(pathDest);

I am then getting a 'process cannot access the file' exception on File.Delete()'.

I would expect that with the using statement, the filestream would be closed nicely. I have also tried declaring the filestream separately, removing using, and putting fs.Close() and fs.Dispose() after the read.

After this, I commented out the actually md5 computation, and the code excutes, with the file being deleted, so it looks like it's something to do with ComputeHash(fs).

Answer 1

I took your code put it in a console app and ran it with no errors, got the hash and the test file is deleted at the end of execution? I just used the .pdb from my test app as the file.

What version of .NET are you running?

I am putting the code that I have that works here, and if you put this in a console app in VS2008 .NET 3.5 sp1 it runs with no errors (at least for me).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Security.Cryptography;
using System.IO;

namespace lockTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string hash = GetHash("lockTest.pdb");

            Console.WriteLine("Hash: {0}", hash);

            Console.ReadKey();
        }

        public static string GetHash(string pathSrc)
        {
            string pathDest = "copy_" + pathSrc;

            File.Copy(pathSrc, pathDest, true);

            String md5Result;
            StringBuilder sb = new StringBuilder();
            MD5 md5Hasher = MD5.Create();

            using (FileStream fs = File.OpenRead(pathDest))
            {
                foreach (Byte b in md5Hasher.ComputeHash(fs))
                    sb.Append(b.ToString("x2").ToLower());
            }

            md5Result = sb.ToString();

            File.Delete(pathDest);

            return md5Result;
        }
    }
}

Answer 2

Import the name space

using System.Security.Cryptography;

Here is the function that returns you md5 hash code. You need to pass the string as parameter.

public static string GetMd5Hash(string input)
{
        MD5 md5Hash = MD5.Create();
        // Convert the input string to a byte array and compute the hash.
        byte[] data = md5Hash.ComputeHash(Encoding.UTF8.GetBytes(input));

        // Create a new Stringbuilder to collect the bytes
        // and create a string.
        StringBuilder sBuilder = new StringBuilder();

        // Loop through each byte of the hashed data 
        // and format each one as a hexadecimal string.
        for (int i = 0; i < data.Length; i++)
        {
            sBuilder.Append(data[i].ToString("x2"));
        }

        // Return the hexadecimal string.
        return sBuilder.ToString();
}

Answer 3

Did you try wrapping your MD5 object in a using() too? From the docs, MD5 is Disposable. That might make it let go of the file.

Answer 4

md5hasher.Clear() after your loop might do the trick.

Answer 5

Have you tried setting md5Hasher to null before deleting the file? It probably has a handle still attached to the FileStream (memory leak perhaps).

Answer 6

Why not open the file with FileShare.ReadWrite?