System.FormattableString.ToString C# (CSharp) Method

        public override string ToString() => ToString(CultureInfo.CurrentCulture);
        protected FormattableString() { }

 public static string Invariant(FormattableString formattable)
 {
     if (formattable == null)
     {
         throw new ArgumentNullException(nameof(formattable));
     }
     return formattable.ToString(CultureInfo.InvariantCulture);
 }