Extra.cs

using System;

namespace Extra
{
    public static class Print
    {
        public static string sentence{ get; set; }
        public static void Talk() => Console.WriteLine(sentence);
    }

    public static class Dated
    {
        public static DateTime actual {set; get;} = DateTime.Now;
    }

    public static class Houred
    {
        public static int actual_hour {set; get;} = Dated.actual.Hour;
    }

    public static class Hello
    {
        public static void Talk(string text = "world")
        {
            Print.sentence = "Good ";
            if(Houred.actual_hour >= 12 && Houred.actual_hour <=20 )
            {
                Print.sentence+="afternoon ";
            }
            else if((Houred.actual_hour >= 20 && Houred.actual_hour <=23) ||(Houred.actual_hour >= 0 && Houred.actual_hour < 6 ))
            {
                Print.sentence+="night ";
            }else if(Houred.actual_hour < 12)
            {
                Print.sentence+="morning ";
            }
            Print.sentence+=text;
            Print.Talk();
        }
    }

}

Program.cs

using System;
using Extra;

namespace App
{
    class Program
    {
        static void Main(string[] args)
        {
            Hello.Talk("paul");
        }
    }
}

UnitTest.cs

using System;
using Xunit;
using NFluent;
using Extra;

namespace Tests
{
    public class UnitTest1
    {
        [Theory]
        [InlineData("Paul")]
        [InlineData("Rimbo")]
        [InlineData("Tibo Inshape")]
        public void should_return_GoodAfternoonValue_when_value(string value)
        {
            Houred.actual_hour = 13;
            Hello.Talk(value);
            Check.That(Print.sentence).IsEqualTo("Good afternoon "+value);

        }

        [Theory]
        [InlineData("Jean")]
        [InlineData("Arthur")]
        [InlineData("Aypierre")]
        public void should_return_GoodMorningValue_when_value(string value)
        {
            Houred.actual_hour = 10;
            Hello.Talk(value);
            Check.That(Print.sentence).IsEqualTo("Good morning "+value);

        }

        [Theory]
        [InlineData("Kevin")]
        [InlineData("Ronald")]
        [InlineData("Sylvio")]
        public void should_return_GoodNightValue_when_value(string value)
        {
            Houred.actual_hour = 2;
            Hello.Talk(value);
            Check.That(Print.sentence).IsEqualTo("Good night "+value);

        }
        
        [Theory]
        [InlineData]
        public void should_return_GoodNightWorld_when_value_not_set()
        {
            Houred.actual_hour = 2;
            Hello.Talk();
            Check.That(Print.sentence).IsEqualTo("Good night world");
        }
    }
}
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