string[] linesOld = File.ReadAllLines(Path.Combine(directory, oldFileName));
string[] linesNew = File.ReadAllLines(Path.Combine(directory, newFileName));

for (int i = 1; i < linesNew.Length; i++)
{
    if (linesNew[i] != linesOld[i])
    {
        Console.WriteLine("Line: {0}, Old: {1}, New: {2}", i, linesOld[i], linesNew[i]);
    }
}