using System;

using static System.Console;

using static System.Math;

using System.Diagnostics;

public class Program {

    public static void Main() {

        var tempo = new Stopwatch();

        WriteLine("Decimal");

        tempo.Start();

        for (var valor = -10000M; valor <= 10000M; valor += 0.05M) valor.RoundMidPoint();

        tempo.Stop();

        WriteLine("Arredondando em ms: {0}", tempo.ElapsedMilliseconds);

        WriteLine("Double");

        tempo.Start();

        for (var valor = -10000.0; valor <= 10000.0; valor += 0.05) valor.RoundMidPoint();

        tempo.Stop();

        WriteLine("Arredondando em ms: {0}", tempo.ElapsedMilliseconds);

        WriteLine("Decimal Alternativo");

        tempo.Start();

        for (var valor = -10000M; valor <= 10000M; valor += 0.05M) valor.RoundMidPointAlt();

        tempo.Stop();

        WriteLine("Arredondando em ms: {0}", tempo.ElapsedMilliseconds);

        WriteLine("Double Alternativo");

        tempo.Start();

        for (var valor = -10000.0; valor <= 10000.0; valor += 0.05) valor.RoundMidPointAlt();

        tempo.Stop();

        WriteLine("Arredondando em ms: {0}", tempo.ElapsedMilliseconds);

    }

}

​

public static class RoundUtil {

    public static Decimal RoundMidPoint(this Decimal value) => Sign(value) * Ceiling(Abs(value) * 2) / 2;

    public static double RoundMidPoint(this double value) => Sign(value) * Ceiling(Abs(value) * 2) / 2;

    public static Decimal RoundMidPointAlt(this Decimal value) {

        int intPart = (int)value;

        decimal decimalPart = value - intPart;

        if (0 < decimalPart && decimalPart <= 0.5M) return intPart + 0.5M;

        else if (0.5M < decimalPart) return intPart + 1;

        else return intPart;

    }

​

    public static double RoundMidPointAlt(this double value) {

        int intPart = (int)value;

        double decimalPart = value - intPart;

        if (0 < decimalPart && decimalPart <= 0.5) return intPart + 0.5;

        else if (0.5 < decimalPart) return intPart + 1;

        else return intPart;

    }

}