c# - How to compare 2 files fast using .NET?

Question

Typical approaches recommend reading the binary via FileStream and comparing it byte-by-byte.

• Would a checksum comparison such as CRC be faster?
• Are there any .NET libraries that can generate a checksum for a file?

Answer 1

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;
    }