Chalenge solution

Converter.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DegreeConverter
{
    public class TemperatureConverter
    {
        private Convert _convert;

        public TemperatureConverter(Convert convert)
        {
            _convert = convert;
        }

        internal object CelsiusToFahrenheit()
        {
            return $"{_convert._converter.value * 1.8 + 32}f";
        }

        public object FahrenheitToCelsius()
        {
            var s = ((_convert._converter.value - 32)*0.56).ToString("N2", CultureInfo.InvariantCulture);
            if (s.EndsWith(".00"))
            {
                return $"{s.Replace(".00", "")}c";
            }
            return $"{s}c";
        }
    }

    public class DegreeConverter
    {
        private Convert _convert;

        public DegreeConverter(Convert convert)
        {
            _convert = convert;
        }

        internal object DegreesToRadians()
        {
            var s = (_convert._converter.value*(Math.PI/180)).ToString("N2", CultureInfo.InvariantCulture);
            if (s.EndsWith(".00"))
            {
                return $"{s.Replace(".00", "")}r";
            }
            return $"{s}r";
        }

        internal object RadiansToDegrees()
        {
            return $"{System.Convert.ToInt32(_convert._converter.value*(180/Math.PI))}d";
        }
    }

    public class Convert
    {
        internal Converter _converter;
        private readonly TemperatureConverter _temperatureConverter;
        private readonly DegreeConverter _degreeConverter;

        public Convert(Converter converter)
        {
            _converter = converter;
            _temperatureConverter = new TemperatureConverter(this);
            _degreeConverter = new DegreeConverter(this);
        }

        public object Conversion()
        {
            object degrees;
            if (_converter.conversion_instruction[0].Equals('r'))
            {
                degrees = _degreeConverter.RadiansToDegrees();
                return degrees;
            }
            else if (_converter.conversion_instruction[0].Equals('c'))
            {
                degrees = _temperatureConverter.CelsiusToFahrenheit();
                return degrees;
            }
            else if (_converter.conversion_instruction[0].Equals('f'))
            {
                degrees = _temperatureConverter.FahrenheitToCelsius();
                return degrees;
            }
            else
            {
                degrees = _degreeConverter.DegreesToRadians();
                return degrees;
            }
        }
    }

    public class Converter
    {
        internal double value;
        internal string conversion_instruction;
        private readonly Convert _convert;

        public Converter(string v)
        {
            this.conversion_instruction = v.Substring(v.Length - 2);
            var format = new NumberFormatInfo();
            format.NumberGroupSeparator = ".";
            this.value = Double.Parse(v.Replace(conversion_instruction, ""), format);
            _convert = new Convert(this);
        }

        public Convert Convert
        {
            get { return _convert; }
        }
    }
}

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DegreeCmd
{
    class Program
    {
        static void Main(string[] args)
        {
            var app = new DegreeConverter.Converter(args[0]);

            Console.WriteLine(app.Convert.Conversion());
        }
    }
}