Calculate MD5 checksum for a file
.net, c#, hash, md5
Solution
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();
}
}
}
Problem
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.