单元测试代码取决于DateTime.Now

Question

The Codewars.com description below.

Create a function which passes in a birthday and returns how old a person is in years.

The Test Cases have already been developed for you. When the test case was created the programmer assumed there would be a shared interface between a SystemClock class and StaticClock class.

public class SystemClock : IClock { ... }

public class StaticClock : IClock { ... }

The SystemClock class needs to implement a DateTime method that returns DateTime.Now

The StaticClock method needs to implement a DateTime method that returns a DateTime value that was passed to its constructor.

The StaticClock implementation of IClock is used in our UnitTest to provide a constant time value for our unittest. Considering that time is always in motion...if we were to simply create unit test based on DateTime.Now then our unit test will fail over time.

With the StaticClock we can inject what value our IClock.Now method should return so our unit test will continue to pass even as time moves on.

My code below.

using System;

namespace AgeFromDOB
{
    public class Program
    {

        public class Kata 
        {
            private DateTime staticClock;

            public Kata(StaticClock staticClock)
            {
                this.staticClock = staticClock.Now;
            }

            public int GetAgeFromDOB(DateTime birthday)
            {
                return staticClock.Year - birthday.Year;
            }
        }

        public interface IClock
        {
            DateTime Now { get; }
        }

        public class SystemClock : IClock
        {
            public DateTime Now { get; }

            public  DateTime Method(DateTime Now)
            {
                return Now;
            }
        }

        public class StaticClock : SystemClock, IClock
        {
            public StaticClock(DateTime Now)
            {
                 Method(Now);
            }
        }

        static void Main(string[] args)
        {
            Kata kata;
            kata = new Kata(new StaticClock(new DateTime(2008,09,3)));
            kata.GetAgeFromDOB(new DateTime(1984,04,23));
            Console.ReadKey();
        }
    }
}

when I try to pass in new Kata(new StaticClock(new DateTime(2008,09,3)));

the object is not being passed correctly and I keep getting the wrong outcome.

I would like to know where to research or what pattern I should look at to solve this problem ?

Answer 1

Kata is dependent on an implementation concretion when it should be dependent on an abstraction. That would allow Kata the flexibility at accept different implementations of the abstraction.

public class Kata {
    private readonly IClock clock;

    public Kata(IClock clock) {
        this.clock = clock;
    }

    public int GetAgeFromDOB(DateTime birthday) {
        var now = clock.Now;    
        var age = now.Year - birthday.Year;
        if (birthday > now.AddYears(-age)) age--;
        return age;
    }
}

public interface IClock {
    DateTime Now { get; }
}

The two implementations of IClock can be kept simple based on the requirements stated in the question.

The SystemClock class needs to implement a DateTime method that returns DateTime.Now

public class SystemClock : IClock {
    public DateTime Now { get { return DateTime.Now; } }
}

The StaticClock method needs to implement a DateTime method that returns a DateTime value that was passed to its constructor.

public class StaticClock : IClock {
    public StaticClock(DateTime now) {
         this.Now = now;
    }

    public DateTime Now { get; private set; }
}

Which should allow you to get the desired results.