using static System.Console;

public class Program {

    public static void Main() {

        WriteLine(123.456789M.Arredondar(2));

        WriteLine(((decimal?)123.456789M)?.Arredondar(3));

        WriteLine(decimal.Round(123.456789M, 4));

        WriteLine(((decimal?)null)?.Arredondar(5));

    }

}

​

public static class DecimnalExt {

    public static decimal Arredondar(this decimal valor, int casasDecimais) {

        return decimal.Round(valor, casasDecimais);

    }

}