Comparing two files in C# [duplicate]

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.