System.FormattableString.ToString C# (CSharp) Method

        public abstract string ToString(IFormatProvider formatProvider);
        string IFormattable.ToString(string ignored, IFormatProvider formatProvider) => ToString(formatProvider);

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