How to get the difference between two files in C#

The article that Mehdi Gholam refers you to is an excellent source for "going deep" into analyzing differences.

For the simple purpose of finding what strings in one Text File are not in the strings in another Text File, you can use a pretty simple technique:

// required
using System.IO;
using System.Linq;

string originalPath = @""; // path to valid .txt File
string revisionPath = @""; // path to valid .txt File

private void SomeButton_Click(object sender, EventArgs e)
{
    string[] InRevisionNotInOriginal = LineDiff(originalPath, revisionPath).ToArray();

    string[] InOriginalNotInRevision = LineDiff(revisionPath, originalPath).ToArray();
}

public static IEnumerable<string> LineDiff(string originalPath, string revisionPath)
{
    if (File.Exists(originalPath) && File.Exists(revisionPath))
    {
        return File.ReadAllLines(revisionPath).Except(File.ReadAllLines(originalPath));
    }
    else
    {
        throw new FileNotFoundException("Bad File Path");
    }
}

Where analyzing "difference" gets interesting is when you want to detect when an element in source1 has been moved to a different position in source2; and, when you want to detect changes in the total number of duplicate value elements.