How can I read, write, and modify the contents of a notepad (.txt) file, in Winform and WPF? [duplicate]

Easiest is StreamReader and StreamWriter:

    using (var writer = new StreamWriter(@"C:\blah\somefile.txt"))
    {
        writer.WriteLine("Hello!");
    }

    using (var reader = new StreamReader(@"C:\blah\somefile.txt"))
    {
        var line = reader.ReadLine();
    }