DateTime.Now, one of the automated test killer

If you have read a little bit about unit testing and stuff, you probably have heard few golden rules: new keyworld is bad, static and singleton are dangerous, and so on.
But there is another classical “test killer” in the nature: DateTime.Now!

Let’s take an example:
You are running a physical store and a website, and you want your customers to be able to list your products only during your work hours (which will give you time to update your list everyday, because your shop is not yet equiped with a system which synchronize your real stocks and your website :))

You have then a method GetProductsIfStoreIsOpen which can list your products. You call DateTime.Now to know the current time and return your products only if the current hour is between 8:00AM and 19:00PM.
So far so good, everything is working. Now let’s see how to test that… You call the GetProductsIfStoreIsOpen method in your test, and… Oh wait! How to tell the method that the store is opened or closed since your method is using DateTime.Now to know that ???
Well, good job, what you have is called an “untestable code” (or a least hard to test) :)

Want so more official reading? Have a look at the offical Microsoft documentation Unit testing best practices with .NET Core and .NET Standard which gives another example about this subject.

Mock or Fake DateTime in .NET by using an interface abstraction

Who never wished to be able to control Time?
Well fortunately in .NET, being able to control the implementation of something in an automated testing context is fairly simple. Basically the easiest way to achieve that is by using some abstraction: Let’s use an interface which can return the current time!

Chronos.Net nuget package to the rescue

As I find myself almost all the time recreating these time abstracting libraries, I decided to package them into some NuGet packages called Chronos.Net, Chronos.Abstractions and Chronos.AspNetCore.

Basically, the first one Chronos.Abstraction contains a simple IDateTimeProvider interface which expose two properties: Now and UtcNow.

public interface IDateTimeProvider
    DateTime UtcNow { get; }
    DateTime Now { get; }

The second one Chronos.Net is the implementation of this interface, and obviously return either DateTime.Now or DateTime.UtcNow.

public class DateTimeProvider : IDateTimeProvider
    public DateTime UtcNow => DateTime.UtcNow;

    public DateTime Now => DateTime.Now;

The last one Chronos.AspNetCore is a little bonus to be help configuring the dependency injection of this system by simply calling UseDateTimeProvider() with your aspnetcore IWebHostBuilder.

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>

Testing and choosing what time it is

Using Chronos.Net, you are now able to abstract time using your favorite mocking librarye (FakeItEasy in the following example)

public void GetProductsIfStoreIsOpen_ShouldReturnAnErrorMessage_WhenCurrentTimeIsOutsideStoreOpenedHours()
    // Arrange
    var dateTimeInStoreOpenedHours = new DateTime(2019, 3, 6, 23, 0, 0);
    var expected = new string[] { "store is not opened" };

    var dateTimeProvider = A.Fake<IDateTimeProvider>();
    A.CallTo(() => dateTimeProvider.Now).Returns(dateTimeInStoreOpenedHours);

    var controller = new DateTimeDependentController(dateTimeProvider);

    // Act
    var result = controller.GetProductsIfStoreIsOpen();

    // Assert
    CollectionAssert.AreEquivalent(expected, result.Value);

You can also see a sample aspnetcore application using Chronos.Net to abstract time in its tests


I will continue to improve these packages as I will use them in the future.
If you want to give it a try, you can have a look to the NuGet packages as well as the source code on GitHub.

NuGet Version github

Any feedback is of course warmly welcomed :)

May the code be with you!