使用DateTimeOffset对象的单元测试类的正确方法？

Question

I would appreciate information or examples about how to correctly test code that uses DateTimeOffset instances. I know the tests have to be deterministic.

So, how would one isolate the application from the DateTimeOffset classes ? I would, of course, like to be able to use a fake DateTimeOffset.Now, etc.

In my tests, should I be using something like:

var myDate = new DateTimeOffset(2016, 3, 29, 12, 20, 35, 93, TimeSpan.FromHours(-3));

Or would I instead be using a wrapper class like MyCustomDateTimeOffset ? Should I not use DateTimeOffset at all in my code and use a wrapper instead?

Answer 1

As the fundamentals theorem says:

We can solve any problem by introducing an extra level of indirection.

You don't really need a wrapper, all you need is to avoid DateTimeOffset.Now / DateTimeOffset.UtcNow .

Here are a few ways you could handle that:

• If you use dependency injection, write an IClock interface which exposes the Now / UtcNow properties.

   public interface IClock { DateTimeOffset Now { get; } DateTimeOffset UtcNow { get; } } internal class Clock : IClock { public DateTimeOffset Now => DateTimeOffset.Now; public DateTimeOffset UtcNow => DateTimeOffset.UtcNow; } 

  In your tests, you just mock the interface as you wish.

• If you'd rather keep using a static property, write a static type, let's say Clock , and use that.

   public static class Clock { internal static Func<DateTimeOffset> DateTimeOffsetProvider { get; set; } = () => DateTimeOffset.Now; public static DateTimeOffset Now => DateTimeOffsetProvider(); public static DateTimeOffset UtcNow => DateTimeOffsetProvider().ToUniversalTime(); } 

  In your tests, you can substitute DateTimeOffsetProvider .

  Here's a .NET 2 version:

   public static class Clock { internal delegate DateTimeOffset DateTimeOffsetProviderDelegate(); internal static DateTimeOffsetProviderDelegate DateTimeOffsetProvider { get; set; } public static DateTimeOffset Now { get { return DateTimeOffsetProvider(); } } public static DateTimeOffset UtcNow { get { return DateTimeOffsetProvider().ToUniversalTime(); } } static Clock() { DateTimeOffsetProvider = delegate() { return DateTimeOffset.Now; }; } } 

Answer 2

As you won't know the value of DateTimeOffSet.Now, then all you can't assert that DateTimeOffSet.Now equals a value.

You probably should refactor to use one of two methods:

• Dependency Injection
• Interface and Wrappers

Dependency Injection (DI)

DI means instead of having the method determine the date, you pass it in.

This method . . .

public void DoSomething()
{
   var now = DateTimeOffSet.Now;
   // Do other stuff with the date
}

. . . would change to this method

public void DoSomething(DateTimeOffSet dtos)
{
   // Do other stuff with the date
}

Interface and Wrapper

Your other option (although in the end you would use DI with this too) is to create an Interface and a Wrapper. Then use you the interface in your object instead of a concrete DateTimeOffSet, so you can then use MOQ or other testing library to moq the interface. Check out SystemWrapper ( https://github.com/jozefizso/SystemWrapper ) project for an example.