void TextBoxOnTextChanged(object sender, TextChangedEventArgs args) { double result; btnOk.IsEnabled = Double.TryParse(txtbox[(int)Side.Left].Text, out result) && Double.TryParse(txtbox[(int)Side.Right].Text, out result) && Double.TryParse(txtbox[(int)Side.Top].Text, out result) && Double.TryParse(txtbox[(int)Side.Bottom].Text, out result); }