What's a good way to overwrite DateTime.Now during testing?

Question

I\'ve got some (C#) code that relies on today\'s date to correctly calculate things in the future. If I use today\'s date in the testing, I have to repeat the calculation in

Answer 1

Ayende Rahien uses a static method that is rather simple...

public static class SystemTime
{
    public static Func<DateTime> Now = () => DateTime.Now;
}

Answer 2

I'd suggest using IDisposable pattern:

[Test] 
public void CreateName_AddsCurrentTimeAtEnd() 
{
    using (Clock.NowIs(new DateTime(2010, 12, 31, 23, 59, 00)))
    {
        string name = new ReportNameService().CreateName(...);
        Assert.AreEqual("name 2010-12-31 23:59:00", name);
    } 
}

In detail described here: http://www.lesnikowski.com/blog/index.php/testing-datetime-now/

Answer 3

Have you considered using conditional compilation to control what happens during debug/deployment?

e.g.

DateTime date;
#if DEBUG
  date = new DateTime(2008, 09, 04);
#else
  date = DateTime.Now;
#endif

Failing that, you want to expose the property so you can manipulate it, this is all part of the challenge of writing testable code, which is something I am currently wrestling myself :D

Edit

A big part of me would preference Blair's approach. This allows you to "hot plug" parts of the code to aid in testing. It all follows the design principle encapsulate what varies test code is no different to production code, its just no one ever sees it externally.

Creating and interface may seem like a lot of work for this example though (which is why I opted for conditional compilation).

Answer 4

My preference is to have classes that use time actually rely on an interface, such as

interface IClock
{
    DateTime Now { get; } 
}

With a concrete implementation

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

Then if you want, you can provide any other kind of clock you want for testing, such as

class StaticClock: IClock
{
     DateTime Now { get { return new DateTime(2008, 09, 3, 9, 6, 13); } }
}

There may be some overhead in providing the clock to the class that relies on it, but that could be handled by any number of dependency injection solutions (using an Inversion of Control container, plain old constructor/setter injection, or even a Static Gateway Pattern).

Other mechanisms of delivering an object or method that provides desired times also work, but I think the key thing is to avoid resetting the system clock, as that's just going to introduce pain on other levels.

Also, using DateTime.Now and including it in your calculations doesn't just not feel right - it robs you of the ability to test particular times, for example if you discover a bug that only happens near a midnight boundary, or on Tuesdays. Using the current time won't allow you to test those scenarios. Or at least not whenever you want.

Answer 5

The key to successful unit testing is decoupling. You have to separate your interesting code from its external dependencies, so it can be tested in isolation. (Luckily, Test-Driven Development produces decoupled code.)

In this case, your external is the current DateTime.

My advice here is to extract the logic that deals with the DateTime to a new method or class or whatever makes sense in your case, and pass the DateTime in. Now, your unit test can pass an arbitrary DateTime in, to produce predictable results.

Answer 6

I faced this situation so often, that I created simple nuget which exposes Now property through interface.

public interface IDateTimeTools
{
    DateTime Now { get; }
}

Implementation is of course very straightforward

public class DateTimeTools : IDateTimeTools
{
    public DateTime Now => DateTime.Now;
}

So after adding nuget to my project I can use it in the unit tests

You can install module right from the GUI Nuget Package Manager or by using the command:

Install-Package -Id DateTimePT -ProjectName Project

And the code for the Nuget is here.

The example of usage with the Autofac can be found here.