SHA1 Hash Checker

Question

How do I make this go faster? It currently takes about 30 seconds on a 3gb file. I am using this to get a checksum from the file to compare it to Microsoft's official checksums. I am wondering if using a buffer would help and if so how should I do that?

public static String GetSHA1Hash(string fileName)
            {
                string strResult = String.Empty;
                string strHashData = String.Empty;
                byte[] arrbytHashValue;
                System.IO.FileStream oFileStream = null;
                System.Security.Cryptography.SHA1CryptoServiceProvider oSHAHasher =
                    new System.Security.Cryptography.SHA1CryptoServiceProvider();

                try
                {
                    oFileStream = GetFileStream(fileName);
                    arrbytHashValue = oSHAHasher.ComputeHash(oFileStream);
                    oFileStream.Close();

                    strHashData = BitConverter.ToString(arrbytHashValue);
                    strHashData = strHashData.Replace("-", String.Empty);
                    strResult = strHashData;
                }
                catch (Exception ex) { return ex.Message; }
                finally { ((IDisposable)oSHAHasher).Dispose(); }
                return (strResult);
            }

Answer 1

Your code is full of boilerplate and what seems to be misunderstood bits of the language as well as outmoded naming schemes. What I will show below does the following:

1. Eliminates a number of intermediate variables, as well as their unused initializations, not needed, in order to reduce noise from what the method is attempting to do.

2. use the using construct instead of try..finally. Very nice and compact way to deal with IDisposable resources.

3. Followed common camelCase naming convention of local variables without any Hungarian notation.

Et, voila:

public static string GetSHA1Hash(string fileName)
{
    try
    {
        using (var shaHasher = new System.Security.Cryptography.SHA1CryptoServiceProvider())
        using (var fileStream = GetFileStream(fileName))
        {
            return BitConverter.ToString(shaHasher.ComputeHash(fileStream)).Replace("-", string.Empty);
        }
    }
    catch (Exception ex)
    {
        return ex.Message;
    }
}

However, I would also caution you to not return the exception message as a string. In fact, get rid of the try..except completely. This method clearly doesn't know how to deal with the exception, so let the caller catch it and provide whatever processing necessary.

As for speed, profile the code. Seriously. Find where the bottleneck is. I'm betting it's purely the I/O subsystem. You can control a lot of the read buffering and caching if you wish in code, but a file on spinning rust (or coming over a network!) is slow-going in any case.

Good luck!