Answer by Noctis for How to change Unix Line feed to Windows Line Feed with C#

Because you are writing the string and overwriting it.

Try this:

string[] filePaths = Directory.GetFiles(@"c:\textfiles\", "*.txt");

foreach (string file in filePaths)
{
    string[] lines = File.ReadAllLines(file);
        List<string> list_of_string = new List<string>();
    foreach (string line in lines)
    {
        list_of_string.Add( line.Replace("\n", "\r\n"));
    }
    File.WriteAllLines(file, list_of_string);
}