I would try something like that:
// List
private List<String> checkedValues = new List<String>();
public int nTextboxChanged = 0;
// Button, just ignore all the crap inside
private void saveChangesButton_Click(object sender, RoutedEventArgs e)
{
if(nTextboxChanged == 1)
{
checkedValues.Add(sWidth.Text);
System.IO.File.WriteAllLines(@System.IO.File.ReadAllText(@System.IO.Directory.GetCurrentDirectory() + "/dir.txt") + "/commandline.txt", checkedValues);
}
}
// TextChanged
private void sWidth_TextChanged(object sender, TextChangedEventArgs e)
{
nTextboxChanged = 1;
}