Numbers.cs

using System.Collections.Generic;
using System.Linq;
using System;

namespace Application
{
    public class Numbers
    {
        private List<float> _numbers = new List<float>();
        private void DisplayMenu()
        {
            Console.Clear();
            Console.CursorVisible = false;
            Console.ForegroundColor = ConsoleColor.White;
            Console.WriteLine("1. Add");
            Console.WriteLine("2. Show");
            Console.WriteLine("3. Find");
            Console.WriteLine("4. Stats");
            RedText("5. Extit");
            Console.WriteLine();
        }

        private void GreenText(string text)
        {
            Console.ForegroundColor = ConsoleColor.Green;
            Console.WriteLine(text);
            Console.ForegroundColor = ConsoleColor.White;

        }

        private void RedText(string text)
        {
            Console.ForegroundColor = ConsoleColor.Red;
            Console.WriteLine(text);
            Console.ForegroundColor = ConsoleColor.White;

        }

        private void DisplayStats()
        {
            Console.WriteLine($"Total number of entries {_numbers.Count}");
            Console.WriteLine($"Sum: {_numbers.Sum()}");
            Console.WriteLine($"Średnia: {_numbers.Sum() / _numbers.Count}");
            Console.WriteLine($"Minimalna liczba: {_numbers.Min()}");
            Console.WriteLine($"Maksymalna liczba: {_numbers.Max()}");
        }

        private void DebugMode()
        {
            Random rnd = new Random();
            for (int i = 0; i < 50; i++)
            {
                _numbers.Add(rnd.Next(200));
            }
        }

        public void Initialize(string[] args)
        {
            ConsoleKeyInfo key;
            char choice;

            //Debug
            if (args.Length > 0 && args[0] == "-d")
            {
                DebugMode();
            }

            DisplayMenu();

            do
            {
                key = Console.ReadKey();
                Console.Clear();
                choice = key.KeyChar;

                Console.CursorVisible = true;

                switch (choice)
                {
                    case '1': //add

                        GreenText("Type number:");
                        string newNumber = Console.ReadLine();
                        float takenNumber = float.Parse(newNumber, System.Globalization.CultureInfo.InvariantCulture);
                        _numbers.Add(takenNumber);
                        DisplayMenu();
                        Console.ForegroundColor = ConsoleColor.Green;
                        Console.WriteLine($"Added number {takenNumber}");
                        break;

                    case '2': //show

                        DisplayMenu();
                        GreenText("List of entries (scroll down to see all):");

                        if (_numbers.Count() == 0)
                        {
                            RedText("No entries!");
                            break;
                        } 
                        else 
                        {
                            string bar = '+' + new string('-', 3) + '+' + new string('-', 14) + '+';

                            Console.WriteLine(bar);
                            for (int i = 0; i < _numbers.Count(); i++)
                            {
                                string formatedDisplay = String.Format("|{0, -3}|{1, -14}|", i + 1, _numbers[i]);
                                Console.WriteLine(formatedDisplay);
                            }
                            Console.WriteLine(bar);
                            Console.SetCursorPosition(0, 0);
                            break;
                        }

                        

                    case '3': //search

                        GreenText("Type number to find: ");
                        float findNumber = Convert.ToSingle(Console.ReadLine());

                        DisplayMenu();

                        if(_numbers.Contains(findNumber))
                        {
                            GreenText($"Found number '{findNumber}' {_numbers.Count(x => x == findNumber)} times!");
                        }
                        else
                        {
                            RedText($"Not found any {findNumber}'s");
                        }
                        break;

                    case '4': //statistics                       

                        DisplayMenu();
                        DisplayStats();

                        break;
                    case '5': //close

                        RedText("Exiting program");
                        break;

                    default:

                        DisplayMenu();
                        RedText("ERROR! Pick option from 1 to 5");

                        break;
                }
            }
            while (choice != '5');
        }

    }
}

Program.cs

using System;


namespace Application
{
    class Program
    {
        static void Main(string[] args)
        {
            var numbers = new Numbers();
            numbers.Initialize(args);
        }
    }
}