Comparing two files in C#

Question

I want to compare two files in C# and see if they are different. They have the same file names and they are the exact same size when different. I was just wondering if there is a fast way to do this without having to manually go in and read the file.

Thanks

Answer 1

Depending on how far you're looking to take it, you can take a look at Diff.NET

Here's a simple file comparison function:

// This method accepts two strings the represent two files to 
// compare. A return value of 0 indicates that the contents of the files
// are the same. A return value of any other value indicates that the 
// files are not the same.
private bool FileCompare(string file1, string file2)
{
     int file1byte;
     int file2byte;
     FileStream fs1;
     FileStream fs2;

     // Determine if the same file was referenced two times.
     if (file1 == file2)
     {
          // Return true to indicate that the files are the same.
          return true;
     }

     // Open the two files.
     fs1 = new FileStream(file1, FileMode.Open, FileAccess.Read);
     fs2 = new FileStream(file2, FileMode.Open, FileAccess.Read);

     // Check the file sizes. If they are not the same, the files 
        // are not the same.
     if (fs1.Length != fs2.Length)
     {
          // Close the file
          fs1.Close();
          fs2.Close();

          // Return false to indicate files are different
          return false;
     }

     // Read and compare a byte from each file until either a
     // non-matching set of bytes is found or until the end of
     // file1 is reached.
     do 
     {
          // Read one byte from each file.
          file1byte = fs1.ReadByte();
          file2byte = fs2.ReadByte();
     }
     while ((file1byte == file2byte) && (file1byte != -1));

     // Close the files.
     fs1.Close();
     fs2.Close();

     // Return the success of the comparison. "file1byte" is 
     // equal to "file2byte" at this point only if the files are 
     // the same.
     return ((file1byte - file2byte) == 0);
}

Answer 2

I was just wondering if there is a fast way to do this without having to manually go in and read the file.

Not really.

If the files came with hashes, you could compare the hashes, and if they are different you can conclude the files are different (same hashes, however, does not mean the files are the same and so you will still have to do a byte by byte comparison).

However, hashes use all the bytes in the file, so no matter what, you at some point have to read the files byte for byte. And in fact, just a straight byte by byte comparison will be faster than computing a hash. This is because a hash reads all the bytes just like comparing byte-by-byte does, but hashes do some other computations that add time. Additionally, a byte-by-byte comparison can terminate early on the first pair of non-equal bytes.

Finally, you can not avoid the need for a byte-by-byte read. If the hashes are equal, that doesn't mean the files are equal. In this case you still have to compare byte-by-byte.

Answer 3

Well, I'm not sure if you can in the file write timestamps. If not, your unique alternative, is comparing the content of the files.

A simple approach is comparing the files byte-to-byte, but if you're going to compare a file several times with others, you can calculate the hashcode of the files and compare it.

The following code snippet shows how you can do it:

    public static string CalcHashCode(string filename)
    {
        FileStream stream = new FileStream(
            filename,
            System.IO.FileMode.Open,
            System.IO.FileAccess.Read,
            System.IO.FileShare.ReadWrite);

        try
        {
            return CalcHashCode(stream);
        }
        finally
        {
            stream.Close();
        }
    }

    public static string CalcHashCode(FileStream file)
    {
        MD5CryptoServiceProvider md5Provider = new MD5CryptoServiceProvider();
        Byte[] hash = md5Provider.ComputeHash(file);
        return Convert.ToBase64String(hash);
    }

If you're going to compare a file with others more that one time, you can save the file hash and compare it. For a single comparison, the byte-to-byte comparison is better. You need also to recompute hash when the file changes, but if you're going to do massive comparisons (more than one time), I recommend using the hash approach.

Answer 4

If the filenames are the same, and the file sizes are the same, then, no, there is no way to know if they have different content without examining the content.

Answer 5

Read the file into a stream, then hash the stream. That should give you a reliable result for comparing.

byte[] fileHash1, fileHash2;

using (SHA256Managed sha = new SHA256Managed())
{
    fileHash1 = sha.ComputeHash(streamforfile1);
    fileHash2 = sha.ComputeHash(streamforfile2);
}

for (int i = 0; (i < fileHash1.Length) && (i < fileHash2.Length); i++)
    {
        if (fileHash[i] != fileHash2[i]) 
        { 
             //files are not the same
             break; 
        }
    }

Answer 6

If they are not complied files then use a diff tool like KDiff or WinMerge. It will highlight were they are different.

http://kdiff3.sourceforge.net/

http://winmerge.org/

Answer 7

pass each file stream through an MD5 hasher and compare the hashes.