อยากทราบวิธีการ Auto Detect ไฟล์ที่จะ Browse ให้เห็นเฉพาะไฟล์ .CSV และ .TXT

private void button1_Click(object sender, System.EventArgs e)
{
    Stream myStream = null;
    OpenFileDialog openFileDialog1 = new OpenFileDialog();

    openFileDialog1.InitialDirectory = "c:\\" ;
    openFileDialog1.Filter = "txt files (*.txt)|*.txt|All files (*.*)|*.*" ;
    openFileDialog1.FilterIndex = 2 ;
    openFileDialog1.RestoreDirectory = true ;

    if(openFileDialog1.ShowDialog() == DialogResult.OK)
    {
        try
        {
            if ((myStream = openFileDialog1.OpenFile()) != null)
            {
                using (myStream)
                {
                    // Insert code to read the stream here.
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show("Error: Could not read file from disk. Original error: " + ex.Message);
        }
    }
}
  

public static void Main()
{
	//Declare a OpenFileDialog object
	OpenFileDialog objOpenFileDialog = new OpenFileDialog();

	//Set the Open dialog properties
	var _with1 = objOpenFileDialog;
	_with1.Filter = "Text files (*.txt)|*.txt|All files (*.*)|*.*";
	_with1.FilterIndex = 1;
	_with1.Title = "Demo Open File Dialog";

	//Show the Open dialog and if the user clicks the Open button,
	//load the file
	if (objOpenFileDialog.ShowDialog == System.Windows.Forms.DialogResult.OK) {
		string allText = null;
		try {
			//Read the contents of the file
			allText = My.Computer.FileSystem.ReadAllText(objOpenFileDialog.FileName);
			//Display the file contents in the TextBox
			System.Console.WriteLine(allText);
		} catch (Exception fileException) {
			throw fileException;
		}
	}

	//Clean up
	objOpenFileDialog.Dispose();
	objOpenFileDialog = null;
}