Execício área triangulo utilizando classes, atributos e métodos

triangulo2.cs

//Criando Classe
using System;

namespace Course
{
    class Triangulo
    {
        public double A;
        public double B;
        public double C;
        public double Area()
        {
            double p = (A + B + C) / 2.0;
            return Math.Sqrt(p * (p - A) * (p - B) * (p - C));
        }
    }
}

//_________________________________________________________

using System;
using System.Globalization;

namespace Course
{
    class Program
    {
        static void Main(string[] args)
        {
            Triangulo x, y;

            x = new Triangulo();
            y = new Triangulo();

            Console.WriteLine("Entre com as medidas do triângulo X:");
            x.A = double.Parse(Console.ReadLine(), CultureInfo.InvariantCulture);
            x.B = double.Parse(Console.ReadLine(), CultureInfo.InvariantCulture);
            x.C = double.Parse(Console.ReadLine(), CultureInfo.InvariantCulture);

            Console.WriteLine("Entre com as medidas do triângulo Y:");
            y.A = double.Parse(Console.ReadLine(), CultureInfo.InvariantCulture);
            y.B = double.Parse(Console.ReadLine(), CultureInfo.InvariantCulture);
            y.C = double.Parse(Console.ReadLine(), CultureInfo.InvariantCulture);

            double areaX = x.Area();
            double areaY = y.Area();

            Console.WriteLine("Área de X = " + areaX.ToString("F4", CultureInfo.InvariantCulture));
            Console.WriteLine("Área de Y = " + areaY.ToString("F4", CultureInfo.InvariantCulture));

            if (areaX > areaY)
            {
                Console.WriteLine("Maior área: X");
            }
            else
            {
                Console.WriteLine("Maior área: Y");
            }
        }
    }
}

Hey you, I made some changes in your code in my fork, chek it out:

https://gist.github.com/robsonalves/df50208dbc4d6cd435859c048c99e18c