A very common challenge to which you will be writing code for is problems revolving around the current date and time.

We had addressed this in a prior post, “Writing Maintainable, Testable Code That Depends On The Current Time”.

The example was as follows:

public string Greet()
{
  return DateTime.Now.Hour switch
  {
    >= 0 and < 13 => "Good Morning",
    >= 13 and <= 16 => "Good Afternoon",
    > 16 and <= 20 => "Good Evening",
    _ => "Good Night"
  };
}

Testing this as it is is very problematic, as the method would return different results depending on the time you run it.

To address this, we had created a custom interface, IClock, to which we had provided two implementations - the actual time SystemClock and a customizable time, FakeClock.

There is a solution to this problem in .NET - the FakeTimeProvider in the Microsoft.Extensions.Time.Testing namespace coupled with the TimeProvider class.

Any code that depends on the current date will now get the date from an injected TimeProvider.

We can thus rewrite our Greeter class as follows:

public sealed class Greeter
{
  private readonly TimeProvider _timeProvider;

  public Greeter(TimeProvider timeProvider)
  {
    _timeProvider = timeProvider;
  }

  public string Greet()
  {
  	return _timeProvider.GetLocalNow().Hour switch
          {
              >= 0 and < 13 => "Good Morning",
              >= 13 and <= 16 => "Good Afternoon",
              > 16 and <= 20 => "Good Evening",
              _ => "Good Night"
          };
  }
}

The Greeter now expects a TimeProvider to be injected.

In production use, you would inject TimeProvider.System, to indicate the current (system) time. This would typically be set up in your DI container, or manually, depending on your use case.

In tests, you would create a FakeTimeProvider, which allows you to set the date and time that you want.

Thus, you would specify your test cases as follows:

public class GreeterTests
{
  [Theory]
  [InlineData(1, "Good Morning")]
  [InlineData(12, "Good Morning")]
  [InlineData(13, "Good Afternoon")]
  [InlineData(16, "Good Afternoon")]
  [InlineData(17, "Good Evening")]
  [InlineData(20, "Good Evening")]
  [InlineData(21, "Good Night")]
  [InlineData(23, "Good Night")]
  public void Greeter_Returns_Correct_Greeting(int hour, string greeting)
  {
    // Create a fake time provider
    var provider = new FakeTimeProvider();
    // Set the time based on passed values (hour)
    provider.SetUtcNow(new DateTime(2024, 1, 1, hour, 1, 0));
    // Create instance of system under test
    var sut = new Greeter(provider);
    // Assert result
    sut.Greet().Should().Be(greeting);
  }
}

Here we are passing the hour we want to test against, as well as the expected returned greeting.

The tests should pass successfully:

TImeProviderTests

TLDR

The TimeProvider class allows you to architect and abstract your types to avoid dependencies on DateTime.Now or DateTime.UtcNow offers a flexible setup for testing purposes using the FakeDateTimeProvider.

The code is in my GitHub.

Happy hacking!