How to compare 2 files fast using .NET?

The slowest possible method is to compare two files byte by byte. The fastest I've been able to come up with is a similar comparison, but instead of one byte at a time, you would use an array of bytes sized to Int64, and then compare the resulting numbers.

Here's what I came up with:

    const int BYTES_TO_READ = sizeof(Int64);

    static bool FilesAreEqual(FileInfo first, FileInfo second)
    {
        if (first.Length != second.Length)
            return false;

        if (string.Equals(first.FullName, second.FullName, StringComparison.OrdinalIgnoreCase))
            return true;

        int iterations = (int)Math.Ceiling((double)first.Length / BYTES_TO_READ);

        using (FileStream fs1 = first.OpenRead())
        using (FileStream fs2 = second.OpenRead())
        {
            byte[] one = new byte[BYTES_TO_READ];
            byte[] two = new byte[BYTES_TO_READ];

            for (int i = 0; i < iterations; i++)
            {
                 fs1.Read(one, 0, BYTES_TO_READ);
                 fs2.Read(two, 0, BYTES_TO_READ);

                if (BitConverter.ToInt64(one,0) != BitConverter.ToInt64(two,0))
                    return false;
            }
        }

        return true;
    }

In my testing, I was able to see this outperform a straightforward ReadByte() scenario by almost 3:1. Averaged over 1000 runs, I got this method at 1063ms, and the method below (straightforward byte by byte comparison) at 3031ms. Hashing always came back sub-second at around an average of 865ms. This testing was with an ~100MB video file.

Here's the ReadByte and hashing methods I used, for comparison purposes:

    static bool FilesAreEqual_OneByte(FileInfo first, FileInfo second)
    {
        if (first.Length != second.Length)
            return false;

        if (string.Equals(first.FullName, second.FullName, StringComparison.OrdinalIgnoreCase))
            return true;

        using (FileStream fs1 = first.OpenRead())
        using (FileStream fs2 = second.OpenRead())
        {
            for (int i = 0; i < first.Length; i++)
            {
                if (fs1.ReadByte() != fs2.ReadByte())
                    return false;
            }
        }

        return true;
    }

    static bool FilesAreEqual_Hash(FileInfo first, FileInfo second)
    {
        byte[] firstHash = MD5.Create().ComputeHash(first.OpenRead());
        byte[] secondHash = MD5.Create().ComputeHash(second.OpenRead());

        for (int i=0; i<firstHash.Length; i++)
        {
            if (firstHash[i] != secondHash[i])
                return false;
        }
        return true;
    }

A checksum comparison will most likely be slower than a byte-by-byte comparison.

In order to generate a checksum, you'll need to load each byte of the file, and perform processing on it. You'll then have to do this on the second file. The processing will almost definitely be slower than the comparison check.

As for generating a checksum: You can do this easily with the cryptography classes. Here's a short example of generating an MD5 checksum with C#.

However, a checksum may be faster and make more sense if you can pre-compute the checksum of the "test" or "base" case. If you have an existing file, and you're checking to see if a new file is the same as the existing one, pre-computing the checksum on your "existing" file would mean only needing to do the DiskIO one time, on the new file. This would likely be faster than a byte-by-byte comparison.

How to compare 2 files fast using .NET?

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