Finding a unique File ID, based on path, in C#

Question

I need a way to determine if a user has copied a file to a new location.

Example: You have two computers and you copy file.txt from C:\Temp\ on computer1 to C:\Temp\ on computer2.

Is there an ID associated with these two files, based on their location, that will help me determine if this file has moved?

Update: After some discussion, here is the resulting code. This determines if a file has been copied by creating a Guid using the file path and creation time. This resulting Guid can be compared to a stored Guid to determine if the file has been copied.

FileInfo fi = new FileInfo("C:\\Temp\\temp.txt"); 
string filePathCreationComposite = String.Format("{0}{1}", Path.GetFullPath(fi.FullName), fi.CreationTime); 

using (MD5 md5 = MD5.Create()) 
{ 
   byte[] hash = md5.ComputeHash(Encoding.Default.GetBytes(filePathCreationCo‌​mposite)); 
   Guid result = new Guid(hash); 
}

Answer 1

If you cannot control the process of the file copy (i.e. know 100% that a user does this under your control, e.g. inside your UI), the only possible way of checking this is compare the contents. As far as I know there are no special trustworthy file ID especially across various machines that even stays the same so that you can compare it.

This is of course not a guarantee that a user copied a file from A to B, since the file with exact same content can be copied from anywhere else, but I think you really need only to verify that the two files in the two different locations are the same.

One easy way to compare the content of a file (and to store it's "id") is to calculate a hash. SHA-256 will fit ideally for that purpose.

This is the example of doing it:

string Hash(string file)
{
    using (FileStream stream = File.OpenRead(file))
    {
        SHA256Managed sha = new SHA256Managed();
        byte[] checksum = sha.ComputeHash(stream);
        return BitConverter.ToString(checksum).Replace("-", String.Empty);
    }
}

Though you don't need a string of course. This is not a very efficient way, you can read more about it here and here. In the first question they suggest that for some reason SHA-512 is faster to calculate, don't know if it's true and this is.

P.S. It's worth noting that if there is a simplest change in the original file, the hashes won't match anymore.

P.P.S. You can use any faster hash like MD5 or some other one (MD5 is not cryptographically secure, but this should not matter for your case).

Answer 2

You can do the following:

• Check if sizes are different. If so, the file has changed.
• If size has not changed, check if creation date/time or modified date/time is different between the two files. If so, the new file is likely different. See https://learn.microsoft.com/en-us/dotnet/api/system.io.fileinfo?view=net-7.0. This is the same method most ftp programs use when uploading (i.e. Filezilla). May be good enough for your use case. If you want more, continue to the next point.
• For extra thoroughness, you could compute the MD5 of the two files. This might be overkill but would be an absolute guarantee of knowing if the files have changed. See https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.md5?view=net-7.0. MD5 is very fast to compute and highly unlikely to have collisions. You could upgrade to SHA-256 but that would be slower to compute and not likely to get you any benefit given that you already did a size check and date/time comparison.
• Unless you have files that are measured in gigabytes for size, MD5 will be very fast.

I will say that using the date/time method is a good way of knowing if a file is newer. If for some reason an older version of a file got placed in the new computer folder, the MD5 compare or size compare would just tell you that it's different, not that it's an older version.

As long as you are MOVING files around, the creation date is usable to determine if a file is newer. If you are COPYING files, you must include the size check followed by an MD5 check for files that are the same size. See https://superuser.com/questions/1748898/how-to-copy-file-folder-and-preserve-their-creation-date-on-windows-10.

Hope this helps :)