Set Filter

   
 
using System;
using System.Windows.Forms;

class MainClass {
    static void Main(string[] args) {

        OpenFileDialog dlg = new OpenFileDialog();
        dlg.Filter = "Rich Text Files (*.rtf)|*.RTF|" +
          "All files (*.*)|*.*";
        if (dlg.ShowDialog() == DialogResult.OK) {
            Console.WriteLine(dlg.FileName);

        }
    }
}