Here is my anwer of this question. I combine the ‘Ambient Context’ pattern with IDisposable. So you can use the DateTimeProvider.Current in your normal program code and in the test you override the scope with a using statement.
using System;
using System.Collections.Immutable;
namespace ambientcontext {
public abstract class DateTimeProvider : IDisposable
{
private static ImmutableStack<DateTimeProvider> stack = ImmutableStack<DateTimeProvider>.Empty.Push(new DefaultDateTimeProvider());
protected DateTimeProvider()
{
if (this.GetType() != typeof(DefaultDateTimeProvider))
stack = stack.Push(this);
}
public static DateTimeProvider Current => stack.Peek();
public abstract DateTime Today { get; }
public abstract DateTime Now {get; }
public void Dispose()
{
if (this.GetType() != typeof(DefaultDateTimeProvider))
stack = stack.Pop();
}
// Not visible Default Implementation
private class DefaultDateTimeProvider : DateTimeProvider {
public override DateTime Today => DateTime.Today;
public override DateTime Now => DateTime.Now;
}
}
}
Here is how to use the above DateTimeProvider inside a Unit-Test
using System;
using Xunit;
namespace ambientcontext
{
public class TestDateTimeProvider
{
[Fact]
public void TestDateTime()
{
var actual = DateTimeProvider.Current.Today;
var expected = DateTime.Today;
Assert.Equal<DateTime>(expected, actual);
using (new MyDateTimeProvider(new DateTime(2012,12,21)))
{
Assert.Equal(2012, DateTimeProvider.Current.Today.Year);
using (new MyDateTimeProvider(new DateTime(1984,4,4)))
{
Assert.Equal(1984, DateTimeProvider.Current.Today.Year);
}
Assert.Equal(2012, DateTimeProvider.Current.Today.Year);
}
// Fall-Back to Default DateTimeProvider
Assert.Equal<int>(expected.Year, DateTimeProvider.Current.Today.Year);
}
private class MyDateTimeProvider : DateTimeProvider
{
private readonly DateTime dateTime;
public MyDateTimeProvider(DateTime dateTime):base()
{
this.dateTime = dateTime;
}
public override DateTime Today => this.dateTime.Date;
public override DateTime Now => this.dateTime;
}
}
}
Using ITimeProvider we were forced to take it into special shared common project that must be referenced from the rest of other projects. But this complicated the control of dependencies.
We searched for the ITimeProvider in the .NET framework. We searched for the NuGet package, and found one that can’t work with DateTimeOffset.
So we came up with our own solution, which depends only on the types of the standard library. We’re using an instance of Func<DateTimeOffset>.
How to use
public class ThingThatNeedsTimeProvider
{
private readonly Func<DateTimeOffset> now;
private int nextId;
public ThingThatNeedsTimeProvider(Func<DateTimeOffset> now)
{
this.now = now;
this.nextId = 1;
}
public (int Id, DateTimeOffset CreatedAt) MakeIllustratingTuple()
{
return (nextId++, now());
}
}
How to register
Autofac
builder.RegisterInstance<Func<DateTimeOffset>>(() => DateTimeOffset.Now);
(For future editors: append your cases here).
How to unit test
public void MakeIllustratingTuple_WhenCalled_FillsCreatedAt()
{
DateTimeOffset expected = CreateRandomDateTimeOffset();
DateTimeOffset StubNow() => expected;
var thing = new ThingThatNeedsTimeProvider(StubNow);
var (_, actual) = thing.MakeIllustratingTuple();
Assert.AreEqual(expected, actual);
}
I got same problem, but i was thinking we should not use the set datetime things on the same class. because it could lead to misuse one day. so i have used the provider like
public class DateTimeProvider
{
protected static DateTime? DateTimeNow;
protected static DateTime? DateTimeUtcNow;
public DateTime Now
{
get
{
return DateTimeNow ?? System.DateTime.Now;
}
}
public DateTime UtcNow
{
get
{
return DateTimeUtcNow ?? System.DateTime.UtcNow;
}
}
public static DateTimeProvider DateTime
{
get
{
return new DateTimeProvider();
}
}
protected DateTimeProvider()
{
}
}
For tests, at test project made a helper which will deal with set things,
public class MockDateTimeProvider : DateTimeProvider
{
public static void SetNow(DateTime now)
{
DateTimeNow = now;
}
public static void SetUtcNow(DateTime utc)
{
DateTimeUtcNow = utc;
}
public static void RestoreAsDefault()
{
DateTimeNow = null;
DateTimeUtcNow = null;
}
}
on code
var dateTimeNow = DateTimeProvider.DateTime.Now //not DateTime.Now
var dateTimeUtcNow = DateTimeProvider.DateTime.UtcNow //not DateTime.UtcNow
and on tests
[Test]
public void Mocked_Now()
{
DateTime now = DateTime.Now;
MockDateTimeProvider.SetNow(now); //set to mock
Assert.AreEqual(now, DateTimeProvider.DateTime.Now);
Assert.AreNotEqual(now, DateTimeProvider.DateTime.UtcNow);
}
[Test]
public void Mocked_UtcNow()
{
DateTime utcNow = DateTime.UtcNow;
MockDateTimeProvider.SetUtcNow(utcNow); //set to mock
Assert.AreEqual(utcNow, DateTimeProvider.DateTime.UtcNow);
Assert.AreNotEqual(utcNow, DateTimeProvider.DateTime.Now);
}
But need to remember one thing, sometime the real DateTime and provider’s DateTime doesn’t act same
[Test]
public void Now()
{
Assert.AreEqual(DateTime.Now.Kind, DateTimeProvider.DateTime.Now.Kind);
Assert.LessOrEqual(DateTime.Now, DateTimeProvider.DateTime.Now);
Assert.LessOrEqual(DateTimeProvider.DateTime.Now - DateTime.Now, TimeSpan.FromMilliseconds(1));
}
I assumed the deference would be maximum TimeSpan.FromMilliseconds(0.00002). But most of the time it’s even less
Find the sample at MockSamples
One clean way to do this is to inject VirtualTime. It allows you to control time.
First install VirtualTime
Install-Package VirtualTime
That allows to, for example, make time that moves 5 times faster on all calls to DateTime.Now or UtcNow
var DateTime = DateTime.Now.ToVirtualTime(5);
To make time move slower , eg 5 times slower do
var DateTime = DateTime.Now.ToVirtualTime(0.5);
To make time stand still do
var DateTime = DateTime.Now.ToVirtualTime(0);
Moving backwards in time is not tested yet
Here are a sample test:
[TestMethod]
public void it_should_make_time_move_faster()
{
int speedOfTimePerMs = 1000;
int timeToPassMs = 3000;
int expectedElapsedVirtualTime = speedOfTimePerMs * timeToPassMs;
DateTime whenTimeStarts = DateTime.Now;
ITime time = whenTimeStarts.ToVirtualTime(speedOfTimePerMs);
Thread.Sleep(timeToPassMs);
DateTime expectedTime = DateTime.Now.AddMilliseconds(expectedElapsedVirtualTime - timeToPassMs);
DateTime virtualTime = time.Now;
Assert.IsTrue(TestHelper.AreEqualWithinMarginOfError(expectedTime, virtualTime, MarginOfErrorMs));
}
You can check out more tests here:
What DateTime.Now.ToVirtualTime extension gives you is an instance of ITime which you pass to a method / class that depends on ITime. some DateTime.Now.ToVirtualTime is setup in the DI container of your choice
Here is another example injecting into a class contrustor
public class AlarmClock
{
private ITime DateTime;
public AlarmClock(ITime dateTime, int numberOfHours)
{
DateTime = dateTime;
SetTime = DateTime.UtcNow.AddHours(numberOfHours);
Task.Run(() =>
{
while (!IsAlarmOn)
{
IsAlarmOn = (SetTime - DateTime.UtcNow).TotalMilliseconds < 0;
}
});
}
public DateTime SetTime { get; set; }
public bool IsAlarmOn { get; set; }
}
[TestMethod]
public void it_can_be_injected_as_a_dependency()
{
//virtual time has to be 1000*3.75 faster to get to an hour
//in 1000 ms real time
var dateTime = DateTime.Now.ToVirtualTime(1000 * 3.75);
var numberOfHoursBeforeAlarmSounds = 1;
var alarmClock = new AlarmClock(dateTime, numberOfHoursBeforeAlarmSounds);
Assert.IsFalse(alarmClock.IsAlarmOn);
System.Threading.Thread.Sleep(1000);
Assert.IsTrue(alarmClock.IsAlarmOn);
}
An old question, but still valid.
My approach is to create a new interface and class to wrap the System.DateTime.Now call
public interface INow
{
DateTime Execute();
}
public sealed class Now : INow
{
public DateTime Execute()
{
return DateTime.Now
}
}
This interface can be injected into any class that needs to get the current date and time. In this example, I have a class that adds a timespan to the current date and time (a unit testable System.DateTime.Now.Add(TimeSpan))
public interface IAddTimeSpanToCurrentDateAndTime
{
DateTime Execute(TimeSpan input);
}
public class AddTimeSpanToCurrentDateAndTime : IAddTimeSpanToCurrentDateAndTime
{
private readonly INow _now;
public AddTimeSpanToCurrentDateAndTime(INow now)
{
this._now = now;
}
public DateTime Execute(TimeSpan input)
{
var currentDateAndTime = this._now.Execute();
return currentDateAndTime.Add(input);
}
}
And tests can be written to ensure that it functions correctly. I’m using NUnit and Moq but any test framework will do
public class Execute
{
private Moq.Mock<INow> _nowMock;
private AddTimeSpanToCurrentDateAndTime _systemUnderTest;
[SetUp]
public void Initialize()
{
this._nowMock = new Moq.Mock<INow>(Moq.MockBehavior.Strict);
this._systemUnderTest = AddTimeSpanToCurrentDateAndTime(
this._nowMock.Object);
}
[Test]
public void AddTimeSpanToCurrentDateAndTimeExecute0001()
{
// arrange
var input = new TimeSpan(911252);
// arrange : mocks
this._nowMock
.Setup(a => a.Execute())
.Returns(new DateTime(348756););
// arrange : expected
var expected = new DateTime(911252 + 348756);
// act
var actual = this._systemUnderTest.Execute(input).Result;
// assert
Assert.Equals(actual, expected);
}
}
This pattern will work for any functions which depend on external factors, like System.Random.Next(), System.DateTime.Now.UtcNow, System.Guid.NewGuid() etc.
See https://loadlimited.visualstudio.com/Stamina/_git/Stamina.Core for further examples or get the https://www.nuget.org/packages/Stamina.Core nuget package.
Good practice is, when DateTimeProvider implements IDisposable.
public class DateTimeProvider : IDisposable
{
[ThreadStatic]
private static DateTime? _injectedDateTime;
private DateTimeProvider()
{
}
/// <summary>
/// Gets DateTime now.
/// </summary>
/// <value>
/// The DateTime now.
/// </value>
public static DateTime Now
{
get
{
return _injectedDateTime ?? DateTime.Now;
}
}
/// <summary>
/// Injects the actual date time.
/// </summary>
/// <param name="actualDateTime">The actual date time.</param>
public static IDisposable InjectActualDateTime(DateTime actualDateTime)
{
_injectedDateTime = actualDateTime;
return new DateTimeProvider();
}
public void Dispose()
{
_injectedDateTime = null;
}
}
Next, you can inject your fake DateTime for unit tests
using (var date = DateTimeProvider.InjectActualDateTime(expectedDateTime))
{
var bankAccount = new BankAccount();
bankAccount.DepositMoney(600);
var lastTransaction = bankAccount.Transactions.Last();
Assert.IsTrue(expectedDateTime.Equals(bankAccount.Transactions[0].TransactionDate));
}
See example Example of DateTimeProvider
I’m surprised no one has suggested one of the most obvious ways to go:
public class TimeDependentClass
{
public void TimeDependentMethod(DateTime someTime)
{
if (GetCurrentTime() > someTime) DoSomething();
}
protected virtual DateTime GetCurrentTime()
{
return DateTime.Now; // or UtcNow
}
}
Then you can simply override this method in your test double.
I also kind of like injecting a TimeProvider class in some cases, but for others, this is more than enough. I’d probably favor the TimeProvider version if you need to reuse this in several classes though.
EDIT: For anyone interested, this is called adding a “seam” to your class, a point where you can hook in to it’s behavior to modify it (for testing purposes or otherwise) without actually having to change the code in the class.
We were using a static SystemTime object, but ran into problems running parallel unit tests. I attempted to use Henk van Boeijen’s solution but had problems across spawned asynchronous threads, ended up using using AsyncLocal in a manner similar to this below:
public static class Clock
{
private static Func<DateTime> _utcNow = () => DateTime.UtcNow;
static AsyncLocal<Func<DateTime>> _override = new AsyncLocal<Func<DateTime>>();
public static DateTime UtcNow => (_override.Value ?? _utcNow)();
public static void Set(Func<DateTime> func)
{
_override.Value = func;
}
public static void Reset()
{
_override.Value = null;
}
}
Sourced from https://gist.github.com/CraftyFella/42f459f7687b0b8b268fc311e6b4af08
I ran into this same issue but found a research project from Microsoft that solves this issue.
http://research.microsoft.com/en-us/projects/moles/
Moles is a lightweight framework for test stubs and detours in .NET that is based on delegates. Moles may be used to detour any .NET method, including non-virtual/static methods in sealed types
// Let's detour DateTime.Now
MDateTime.NowGet = () => new DateTime(2000,1, 1);
if (DateTime.Now == new DateTime(2000, 1, 1);
{
throw new Exception("Wahoo we did it!");
}
The sample code was modified from the original.
I had done what other suggested and abstracted the DateTime into a provider. It just felt wrong and I felt like it was too much just for testing. I’m going to implement this into my personal project this evening.