C# - Neue Zeilen hinzufügen

public static void addNewLines(string mainFile, string newLinesFile)
        {
            if (Path.HasExtension(mainFile) && Path.HasExtension(newLinesFile))
            {
                List<string> MainFile = new List<string>(System.IO.File.ReadAllLines(mainFile.ToString()));
                List<string> NewLines = new List<string>(System.IO.File.ReadAllLines(newLinesFile.ToString()));
                foreach (string newLine in NewLines)
                    if (!MainFile.Contains(newLine))
                        MainFile.Add(newLine);
                System.IO.File.WriteAllLines(mainFile, MainFile);
            }
            else
                Console.WriteLine("File not found");
        }