File equals

How can we determine if 2 files are equal? We can compare lengths, dates created and modified. But this is not always enough.

Equal contents

There is no way to know if the contents are the same unless we test them. We demonstrate a method that compares 2 files for equality.

Example

The simplest way to check 2 files for equal contents is to use File.ReadAllBytes on each file. Then we must compare the result arrays.

using System;
using System.IO;

class Program
{
    static bool FileEquals(string path1, string path2)
    {
        byte[] file1 = File.ReadAllBytes(path1);
        byte[] file2 = File.ReadAllBytes(path2);
        if (file1.Length == file2.Length)
        {
            for (int i = 0; i < file1.Length; i++)
            {
                if (file1[i] != file2[i])
                {
                    return false;
                }
            }
            return true;
        }
        return false;
    }

    static void Main()
    {
        bool a = FileEquals("C:\\stage\\htmlmeta", "C:\\stage\\htmlmeta-aspnet");
        bool b = FileEquals("C:\\stage\\htmllink", "C:\\stage\\htmlmeta-aspnet");
        Console.WriteLine(a);
        Console.WriteLine(b);
    }
}
True
False

FileInfo note

If the common case in your program is that the files are not equal, then using a FileInfo struct and the Length property to compare lengths first would be faster.

Usage

Suppose we are trying to update a directory of files but some files may not be different. Instead of rewriting files, we can test them for equality first.

Summary

We can guess whether two files are equal by testing their dates and lengths. But we cannot know if every single byte is equal unless we test every single byte.