Dealing with time

The problem

Sometimes code is time sensitive. Consider the following method:

Greeter.cs

public string GenerateGreetText()
{
    var dateTimeNow = DateTime.Now;
    return dateTimeNow.Hour switch
    {
        >= 5 and < 12 => "Good morning",
        >= 12 and < 18 => "Good afternoon",
        _ => "Good evening"
    };
}

This method uses the current time of your workstation to determine whether it is morning, afternoon or evening and produce a message.

Attempting to write unit tests for this method would be impossible. If you had 1 test per usecase then only one test out of 3 would pass at one time.

The solution

In order to solve this problem we need to make the date time provider mockable. To achieve that let's create an interface.

public interface IDateTimeProvider
{
    DateTime Now { get; }
}

The implementation will simply point to the same property invocation:

public class DateTimeProvider : IDateTimeProvider
{
    public DateTime Now => DateTime.Now;
}

We can now write a unit test that mocks the IDateTimeProvider that we just created and returns a given date per test.

Exercise

Implement 3 tests:

• GenerateGreetText_ShouldReturnGoodMorning_WhenItsMorning
• GenerateGreetText_ShouldReturnGoodAfternoon_WhenItsAfternoon
• GenerateGreetText_ShouldReturnGoodEvening_WhenItsEvening

Expand any of the code blocks below to see the solution to the exercise but you are highly encouraged to implement the tests on your own.

GenerateGreetText_ShouldReturnGoodMorning_WhenItsMorning

[Fact]
public void GenerateGreetText_ShouldReturnGoodMorning_WhenItsMorning()
{
    // Arrange
    _dateTimeProvider.Now.Returns(new DateTime(2022, 1, 1, 9, 0, 0));
    
    // Act
    var message = _greeter.GenerateGreetText();

    // Assert
    message.Should().Be("Good morning");
}

GenerateGreetText_ShouldReturnGoodAfternoon_WhenItsAfternoon

[Fact]
public void GenerateGreetText_ShouldReturnGoodAfternoon_WhenItsAfternoon()
{
    // Arrange
    _dateTimeProvider.Now.Returns(new DateTime(2022, 1, 1, 15, 0, 0));
    
    // Act
    var message = _greeter.GenerateGreetText();

    // Assert
    message.Should().Be("Good afternoon");
}

GenerateGreetText_ShouldReturnGoodEvening_WhenItsEvening

[Fact]
public void GenerateGreetText_ShouldReturnGoodEvening_WhenItsEvening()
{
    // Arrange
    _dateTimeProvider.Now.Returns(new DateTime(2022, 1, 1, 20, 0, 0));
    
    // Act
    var message = _greeter.GenerateGreetText();

    // Assert
    message.Should().Be("Good evening");
}