One of the principles of a unit test is that it must have full control of the system under test. This is problematic when production code includes
calls to static methods, which cannot be changed or controlled. Date/time functions are usually provided by system libraries as static methods.
This can be improved by wrapping the system calls in an object or service that can be controlled inside the unit test.
Noncompliant code example
Public Class Foo
Public Function HelloTime() As String
Return $"Hello at {DateTime.UtcNow}"
End Function
End Class
Compliant solution
There are different approaches to solve this problem. One of them is suggested below. There are also open source libraries (such as NodaTime) which
already implement an IClock interface and a FakeClock testing class.
Public Interface IClock
Function UtcNow() As Date
End Interface
Public Class Foo
Public Function HelloTime(clock As IClock) As String
Return $"Hello at {clock.UtcNow()}"
End Function
End Class
Public Class FooTest
Public Class TestClock
Implements IClock
' implement
End Class
<Fact>
Public Sub HelloTime_Gives_CorrectTime()
Dim dateTime = New DateTime(2017, 06, 11)
Assert.Equal((New Foo()).HelloTime(New TestClock(dateTime)), $"Hello at {dateTime}")
End Sub
End Class
Another possible solution is using an adaptable module, ideally supports an IDisposable method, that not only adjusts the time behaviour for the
current thread only, but also for scope of the using.
Public Module Clock
Public Function UtcNow() As Date
End Function
Public Function SetTimeForCurrentThread(time As Func(Of Date)) As IDisposable
End Function
End Module
Public Class Foo
Public Function HelloTime() As String
Return $"Hello at {Clock.UtcNow()}"
End Function
End Class
Public Class FooTest
<Fact>
Public Sub HelloTime_Gives_CorrectTime()
Dim dateTime = New DateTime(2017, 06, 11)
Using SetTimeForCurrentThread(Function() dateTime)
Assert.Equal((New Foo()).HelloTime(), $"Hello at {dateTime}")
End Using
End Sub
End Class
