using System;

using static System.Console;

using static System.Math;

​

public class Program {

    public static void Main() {

        WriteLine("Decimal");

        for (var valor = -1M; valor <= 1M; valor += 0.05M) WriteLine("{0:N1} => {1:N1}", valor, valor.RoundMidPoint());

        WriteLine("Double");

        for (var valor = -1.0; valor <= 1.0; valor += 0.05) WriteLine("{0:N1} => {1:N1}", valor, valor.RoundMidPoint());

    }

}

​

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;

}