Refactored version of SmartHelloWorld.

Domain_Application.cs

public class Application
    {
        IOutput output;
        public Application(IOutput output)
        {
            this.output = output;
        }
        public void Run(string name)
        {
            if (ApplicationTime.Now.Hour >= 6 && ApplicationTime.Now.Hour < 12 )
            {
                output.Send($"Good morning {name}"); return;
            } else if (ApplicationTime.Now.Hour >= 12 && ApplicationTime.Now.Hour < 20)
            {—
                output.Send($"Good afternood {name}"); return;
            } 

            output.Send($"Good night {name}");
        }
    }

Domain_ApplicationTime.cs

public class ApplicationTime
    {
        public static Func<DateTime> SetNow {get; set;} = () => DateTime.Now;
        //private Func<DateTime>
        public static DateTime Now => SetNow();
    }

Domain_IOutput.cs

public interface IOutput
{
    void Send(string message);
}

Infrastructure_ConsoleOutput.cs

public class ConsoleOutput : IOutput
    {
        public void Send(string message) => Console.WriteLine(message);
    }

Infrastructure_Program.cs

public static void Main(string[] args)
  {
      var name = args.Length > 0 ? args[0] : "World";

      var app = new Application(new ConsoleOutput());
      app.Run(name);
  }

Tests_Domain_tests.cs

public class HelloTests
    {
        Func<StringWriter> GetWriterAndSetConsole = ()=>
        {
            var writer = new StringWriter();
            Console.SetOut(writer);
            Console.SetError(writer);
            return writer;
        };
        
       [Fact]
        public void GoldenMaster_with_all_defaults()
        {
            ApplicationTime.SetNow = () => DateTime.Now;
            using var writer = GetWriterAndSetConsole();
            Program.Main(new string[]{});
            var sut = writer.ToString();
            Check.That(sut).IsEqualTo($"Good afternood World{Environment.NewLine}");
        }

        [Fact]
        public void refactored_should_return_morning_in_the_morning_with_defaults()
        {
           ApplicationTime.SetNow = () => new DateTime(2020,03,10,10,00,00);
           using var writer = GetWriterAndSetConsole();
           Program.Main(new string[]{});
           var sut = writer.ToString();
           
           Check.That(sut).IsEqualTo($"Good morning World{Environment.NewLine}");
        }

        [Fact]
        public void refactored_should_return_morning_in_the_morning_with_name()
        {
            ApplicationTime.SetNow = () => new DateTime(2020,03,10,10,00,00);
           
            var output = new InmemoryOutput();
            var app = new Application(output);
            app.Run("toto");
            var result = output.Value;
            Check.That(result).IsEqualTo("Good morning toto");

        }
    }
    
    public class InmemoryOutput : IOutput
    {
        private string output = "";
        public string Value => output;
        public void Send(string message)
        {
            output += message;
        }
    }