My book contains a section on the Ambient Context pattern that uses a TimeProvider as an example. It’s used like this:

this.closedAt = TimeProvider.Current.UtcNow;

Yesterday I was TDDing a state machine that consumes TimeProvider and needed to freeze and advance time at different places in the test. Always on the lookout for making unit tests more readable, I decided to have a little fun with literal extensions and TimeProvider. I ended up with this test:

// Fixture setup

var fixture = new WcfFixture();

DateTime.Now.Freeze();

fixture.Register(1.Minutes());

var sut = fixture.CreateAnonymous<CircuitBreaker>();

sut.PutInOpenState();

2.Minutes().Pass();

// Exercise system

sut.Guard();

// Verify outcome

Assert.IsInstanceOfType(sut.State,

    typeof(HalfOpenCircuitState));

// Teardown

There are several items of note. Imagine that we can freeze time!

DateTime.Now.Freeze();

With the TimeProvider and an extension method, we can:

internal static void Freeze(this DateTime dt)

{

    var timeProviderStub = new Mock<TimeProvider>();

    timeProviderStub.SetupGet(tp => tp.UtcNow).Returns(dt);

    TimeProvider.Current = timeProviderStub.Object;

}

This effectively sets up the TimeProvider to always return the same time.

Later in the test I state that 2 minutes pass:

2.Minutes().Pass();

I particularly like the grammatically correct English. This is accomplished with a combination of a literal extension and changing the state of TimeProvider.

First, the literal extension:

internal static TimeSpan Minutes(this int m)

{

    return TimeSpan.FromMinutes(m);

}

Given the TimeSpan returned from the Minutes method, I can now invoke the Pass extension method:

internal static void Pass(this TimeSpan ts)

{

    var previousTime = TimeProvider.Current.UtcNow;

    (previousTime + ts).Freeze();

}

Note that I just add the TimeSpan to the current time and invoke the Freeze extension method with the new value.

Last, but not least, I should point out that the PutInOpenState method isn’t some smelly test-specific method on the SUT, but rather yet another extension method.