Unit Testing DateTime Now

Question

I have some unit tests that expects the  current time  to be different than DateTime Now and I don t want to change the computer s time  obviously    What s the best strategy to achieve this

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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 lt DateTimeOffset gt    How to use  public class ThingThatNeedsTimeProvider       private readonly Func lt DateTimeOffset gt  now      private int nextId       public ThingThatNeedsTimeProvider Func lt DateTimeOffset gt  now                this now   now          this nextId   1             public  int Id  DateTimeOffset CreatedAt  MakeIllustratingTuple                 return  nextId    now               How to register  Autofac  builder RegisterInstance lt Func lt DateTimeOffset gt  gt       gt  DateTimeOffset Now      For future editors  append your cases here    How to unit test  public void MakeIllustratingTuple WhenCalled FillsCreatedAt         DateTimeOffset expected   CreateRandomDateTimeOffset        DateTimeOffset StubNow     gt  expected      var thing   new ThingThatNeedsTimeProvider StubNow        var     actual    thing MakeIllustratingTuple         Assert AreEqual expected  actual

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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 lt INow gt   nowMock       private AddTimeSpanToCurrentDateAndTime  systemUnderTest        SetUp      public void Initialize                 this  nowMock   new Moq Mock lt INow gt  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   gt  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

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Mock Objects   A mock DateTime that returns a Now that s appropriate for your test

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A thread safe SystemClock using ThreadLocal lt T gt  works great for me   ThreadLocal lt T gt  is available in the  Net Framework v4 0 and higher        lt summary gt      Provides access to system time while allowing it to be set to a fixed  lt see cref  DateTime   gt  value       lt  summary gt       lt remarks gt      This class is thread safe       lt  remarks gt  public static class SystemClock       private static readonly ThreadLocal lt Func lt DateTime gt  gt   getTime           new ThreadLocal lt Func lt DateTime gt  gt       gt       gt  DateTime Now             lt inheritdoc cref  DateTime Today   gt      public static DateTime Today               get   return  getTime Value   Date                    lt inheritdoc cref  DateTime Now   gt      public static DateTime Now               get   return  getTime Value                      lt inheritdoc cref  DateTime UtcNow   gt      public static DateTime UtcNow               get   return  getTime Value   ToUniversalTime                      lt summary gt          Sets a fixed  deterministic  time for the current thread to return by  lt see cref  SystemClock   gt            lt  summary gt      public static void Set DateTime time                if  time Kind    DateTimeKind Local              time   time ToLocalTime              getTime Value        gt  time                  lt summary gt          Resets  lt see cref  SystemClock   gt  to return the current  lt see cref  DateTime Now   gt            lt  summary gt      public static void Reset                  getTime Value        gt  DateTime Now            Usage example    TestMethod  public void Today         SystemClock Set new DateTime 2015  4  3         DateTime expectedDay   new DateTime 2015  4  2       DateTime yesterday   SystemClock Today AddDays -1D       Assert AreEqual expectedDay  yesterday        SystemClock Reset

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Moles    Test    public void TestOfDateTime               var firstValue   DateTime Now        MDateTime NowGet        gt  new DateTime 2000 1 1         var secondValue   DateTime Now        Assert firstValue  gt  secondValue      would be false if  moleing  failed     Disclaimer - I work on Moles

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These are all good answers  this is what I did on a different project   Usage   Get Today s REAL date Time  var today   SystemTime Now   Date    Instead of using DateTime Now  you need to use SystemTime Now      It s not hard change but this solution might not be ideal for all projects   Time Traveling  Lets go 5 years in the future   SystemTime SetDateTime today AddYears 5      Get Our Fake  today   will be 5 years from  today    var fakeToday   SystemTime Now   Date    Reset the date  SystemTime ResetDateTime             lt summary gt      Used for getting DateTime Now    time is changeable for unit testing      lt  summary gt  public static class SystemTime            lt summary gt  Normally this is a pass-through to DateTime Now  but it can be overridden with SetDateTime       for testing or debugging           lt  summary gt      public static Func lt DateTime gt  Now        gt  DateTime Now            lt summary gt  Set time to return when SystemTime Now   is called           lt  summary gt      public static void SetDateTime DateTime dateTimeNow                Now        gt   dateTimeNow                  lt summary gt  Resets SystemTime Now   to return DateTime Now           lt  summary gt      public static void ResetDateTime                 Now        gt  DateTime Now

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Add a fake assembly for System  right click on System reference  Add fake assembly    And write into your test method   using  ShimsContext Create         System Fakes ShimDateTime NowGet        gt  new DateTime 2014  3  10      MethodThatUsesDateTimeNow

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Regarding to  crabcrusherclamcollector answer there is issue when using that approach in EF queries  System NotSupportedException  The LINQ expression node type  Invoke  is not supported in LINQ to Entities   I modified implementation to that   public static class SystemTime               private static Func lt DateTime gt  UtcNowFunc        gt  DateTime UtcNow           public static void SetDateTime DateTime dateTimeNow                        UtcNowFunc        gt  dateTimeNow                     public static void ResetDateTime                         UtcNowFunc        gt  DateTime UtcNow                     public static DateTime UtcNow                       get                               DateTime now   UtcNowFunc Invoke                    return now

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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 lt DateTimeProvider gt  stack   ImmutableStack lt DateTimeProvider gt  Empty Push new DefaultDateTimeProvider          protected DateTimeProvider                 if  this GetType      typeof DefaultDateTimeProvider               stack   stack Push this              public static DateTimeProvider Current   gt  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   gt  DateTime Today           public override DateTime Now   gt  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 lt DateTime gt  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 lt int gt  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   gt  this dateTime Date               public override DateTime Now   gt  this dateTime

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Maybe less Professional but simpler solution could be make a DateTime parameter at consumer method For example instead of make method like SampleMethod   make SampleMethod1 with parameter Testing of SampleMethod1 is easier  public void SampleMethod                 DateTime anotherDateTime   DateTime Today AddDays -10           if   DateTime Now-anotherDateTime  TotalDays gt 10                                 public void SampleMethod1 DateTime dateTimeNow                DateTime anotherDateTime   DateTime Today AddDays -10           if   dateTimeNow - anotherDateTime  TotalDays  gt  10

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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        gt  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

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To test a code that depends on the System DateTime  the system dll must be mocked   There are two framework that I know of that does this  Microsoft fakes and Smocks   Microsoft fakes require visual studio 2012 ultimatum and works straight out of the compton   Smocks is an open source and very easy to use  It can be downloaded using NuGet   The following shows a mock of System DateTime   Smock Run context   gt      context Setup      gt  DateTime Now  Returns new DateTime 2000  1  1           Outputs  2000     Console WriteLine DateTime Now Year

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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    gt  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

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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 lt DateTime gt   utcNow        gt  DateTime UtcNow       static AsyncLocal lt Func lt DateTime gt  gt   override   new AsyncLocal lt Func lt DateTime gt  gt          public static DateTime UtcNow   gt    override Value     utcNow          public static void Set Func lt DateTime gt  func                 override Value   func             public static void Reset                  override Value   null            Sourced from https   gist github com CraftyFella 42f459f7687b0b8b268fc311e6b4af08

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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

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The best strategy is to wrap the current time in an abstraction and inject that abstraction into the consumer     Alternatively  you can also define a time abstraction as an Ambient Context   public abstract class TimeProvider       private static TimeProvider current           DefaultTimeProvider Instance       public static TimeProvider Current              get   return TimeProvider current           set                      if  value    null                              throw new ArgumentNullException  value                            TimeProvider current   value                     public abstract DateTime UtcNow   get        public static void ResetToDefault                   TimeProvider current   DefaultTimeProvider Instance                       This will enable you to consume it like this   var now   TimeProvider Current UtcNow    In a unit test  you can replace TimeProvider Current with a Test Double Mock object  Example using Moq   var timeMock   new Mock lt TimeProvider gt     timeMock SetupGet tp   gt  tp UtcNow  Returns new DateTime 2010  3  11    TimeProvider Current   timeMock Object    However  when unit testing with static state  always remember to tear down your fixture by calling TimeProvider ResetToDefault

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You have some options for doing it    Use mocking framework and use a DateTimeService  Implement a small wrapper class and inject it to production code   The wrapper implementation will access DateTime and in the tests you ll be able to mock the wrapper class  Use Typemock Isolator  it can fake DateTime Now and won t require you to change the code under test  Use Moles  it can also fake DateTime Now and won t require change in production code    Some examples   Wrapper class using Moq    Test  public void TestOfDateTime          var mock   new Mock lt IDateTime gt          mock Setup fake   gt  fake Now            Returns new DateTime 2000  1  1          var result   new UnderTest mock Object  CalculateSomethingBasedOnDate       public class DateTimeWrapper   IDateTime         public DateTime Now   get   return DateTime Now          Faking DateTime directly using Isolator    Test  public void TestOfDateTime          Isolate WhenCalled      gt  DateTime Now  WillReturn new DateTime 2000  1  1          var result   new UnderTest   CalculateSomethingBasedOnDate        Disclaimer - I work at Typemock

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You can change the class you are testing to use a Func lt DateTime gt  which will be passed through it s constructor parameters  so when you create instance of the class in real code  you can pass      gt  DateTime UtcNow to the Func lt DateTime gt  parameter  and on the test  you can pass the time you wish to test   For example        TestMethod      public void MyTestMethod                 var instance   new MyClass      gt  DateTime MinValue           Assert AreEqual instance MyMethod    DateTime MinValue               public void RealWorldInitialization                 new MyClass      gt  DateTime UtcNow              class MyClass               private readonly Func lt DateTime gt   utcTimeNow           public MyClass Func lt DateTime gt  UtcTimeNow                         utcTimeNow   UtcTimeNow                     public DateTime MyMethod                         return  utcTimeNow

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Good practice is  when DateTimeProvider implements IDisposable   public class DateTimeProvider   IDisposable          ThreadStatic       private static DateTime   injectedDateTime        private DateTimeProvider                            lt summary gt           Gets DateTime now            lt  summary gt            lt value gt           The DateTime now            lt  value gt       public static DateTime Now                 get                         return  injectedDateTime    DateTime Now                               lt summary gt           Injects the actual date time            lt  summary gt            lt param name  actualDateTime  gt The actual date time  lt  param gt       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

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One special note on mocking DateTime Now with TypeMock     The value of DateTime Now must be placed into a variable for this to be mocked properly  For example   This does not work   if   DateTime Now - message TimeOpened Value   gt  new TimeSpan 1  0  0     However  this does   var currentDateTime   DateTime Now  if   currentDateTime - message TimeOpened Value   gt  new TimeSpan 1  0  0

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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   https   github com VirtualTime VirtualTime blob master VirtualTimeLib Tests when virtual time is used cs   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      gt                        while   IsAlarmOn                                IsAlarmOn    SetTime - DateTime UtcNow  TotalMilliseconds  lt  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

[c#] Unit Testing: DateTime.Now

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