Calculate MD5 checksum for a file

I'm using iTextSharp to read the text from a PDF file. However, there are times I cannot extract text, because the PDF file is only containing images. I download the same PDF files everyday, and I want to see if the PDF has been modified. If the text and modification date cannot be obtained, is a MD5 checksum the most reliable way to tell if the file has changed?

If it is, some code samples would be appreciated, because I don't have much experience with cryptography.

It's very simple using System.Security.Cryptography.MD5:

using (var md5 = MD5.Create())
{
    using (var stream = File.OpenRead(filename))
    {
        return md5.ComputeHash(stream);
    }
}

(I believe that actually the MD5 implementation used doesn't need to be disposed, but I'd probably still do so anyway.)

How you compare the results afterwards is up to you; you can convert the byte array to base64 for example, or compare the bytes directly. (Just be aware that arrays don't override Equals. Using base64 is simpler to get right, but slightly less efficient if you're really only interested in comparing the hashes.)

If you need to represent the hash as a string, you could convert it to hex using BitConverter:

static string CalculateMD5(string filename)
{
    using (var md5 = MD5.Create())
    {
        using (var stream = File.OpenRead(filename))
        {
            var hash = md5.ComputeHash(stream);
            return BitConverter.ToString(hash).Replace("-", "").ToLowerInvariant();
        }
    }
}

This is how I do it:

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

public string checkMD5(string filename)
{
    using (var md5 = MD5.Create())
    {
        using (var stream = File.OpenRead(filename))
        {
            return Encoding.Default.GetString(md5.ComputeHash(stream));
        }
    }
}