Read All Files to The Array from the Folder
var files = from file in Directory.GetFiles(@"Folder Path")
orderby file descending
select file;
int count=files.Count();
string[] array= files.ToArray();
Read The Required File in your button CLick event
TextReader objstream = new StreamReader(array[0]);
richTextBox1.Text = objstream.ReadToEnd();
objstream.Close();