Use Filter in SaveFileDialog to filter file extension

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using System;
using System.Windows.Forms;

class MainClass {
    static void Main(string[] args) {
        SaveFileDialog dlg = new SaveFileDialog();
        dlg.Filter = "RichText Files (*.rtf)|*.RTF|Text Files (*.txt)|*.TXT" +
          "|All files (*.*)|*.*";

        if (dlg.ShowDialog() == DialogResult.OK) {
            Console.WriteLine(dlg.FileName);
        }
    }
}