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) => decimal.Round(valor, casasDecimais);
}
 
//https://pt.stackoverflow.com/q/104297/101

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