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Last modified on August 15, 2012 17:20
Add system utility classes like SystemTime and SystemGuid to your application so you can test component behaviors that are otherwise sensitive to unpredictable values coming from the system environment (e.g., the current time or a new Guid).
The problem
.NET environment functions typically return values that come from the operating system such as DateTime.Now and Guid.NewGuid(). These values are different everytime which complicates repeatable testing.
You can control these values if you replace calls to the .NET functions with calls to similar functions in system utility classes that you wrote. These utility classes behave like the .NET functions under normal conditions but you can change their behavior during testing.
SystemTime
DateTime is especially problematic for testers who want to validate logic that is time sensitive. Suppose you had business logic that expected something to happen three days from now. You certainly don't want a three-day test.
You can write a SystemTime class such as the one included below. SystemTime has Now() and Today() methods that return a DateTime. If you do nothing, they behave exactly like DateTime.Now and DateTime.Today.
Now everywhere in your application you always call SystemTime.Now() and SystemTime.Today() wherever you would have called DateTime.Now and DateTime.Today.
If you do nothing, the behavior of your application is unchanged.
During a test, however, you can replace the SystemTime.NowFn with a test-time function that returns any DateTime value you need.
Remember when you are done to restore NowFn to the proper default behavior. A "ResetNowFn" method makes that easier for testers.
| C# | /// <summary> /// Replacement for <see cref="DateTime"/> /// ** ALWAYS USE THIS INSTEAD OF CALLING DATETIME DIRECTLY /// </summary> /// <remarks> /// Covers the <see cref="DateTime"/> static methods so that /// test and development scenarios can reset the "Current" time /// to whatever they like. See <see cref="NowFn"/>. /// TODO: Extend to support UtcNow /// </remarks> public static class SystemTime { /// <summary> /// Gets a <see cref="DateTime"/> that is set to the "current" date and time /// in the manner of <see cref="DateTime.Now"/>. /// See <see cref="NowFn"/>. /// </summary> public static DateTime Now() { return NowFn(); } /// <summary> /// Gets "current" date /// in the manner of <see cref="DateTime.Today"/>. /// See <see cref="NowFn"/>. /// </summary> public static DateTime Today() { var now = Now(); return now-now.TimeOfDay; } /// <summary> /// Function for getting the "current" date and time. /// Defaults to <see cref="DateTime.Now"/> but /// could be replaced in a test or development scenario. /// </summary> public static Func<DateTime> NowFn = () => DateTime.Now; /// <summary> /// Reset <see cref="NowFn"/> to application default /// </summary> public static void ResetNowFn() { NowFn = () => DateTime.Now; } } |
| VB | ''' <summary> ''' Replacement for <see cref="DateTime"/> ''' ** ALWAYS USE THIS INSTEAD OF CALLING DATETIME DIRECTLY ''' </summary> ''' <remarks> ''' Covers the <see cref="DateTime"/> static methods so that ''' test and development scenarios can reset the "Current" time ''' to whatever they like. See <see cref="NowFn"/>. ''' TODO: Extend to support UtcNow ''' </remarks> Public NotInheritable Class SystemTime ''' <summary> ''' Gets a <see cref="DateTime"/> that is set to the "current" date and time ''' in the manner of <see cref="DateTime.Now"/>. ''' See <see cref="NowFn"/>. ''' </summary> Private Sub New() End Sub Public Shared Function Now() As Date Return NowFn() End Function ''' <summary> ''' Gets "current" date ''' in the manner of <see cref="DateTime.Today"/>. ''' See <see cref="NowFn"/>. ''' </summary> Public Shared Function Today() As Date Dim now = SystemTime.Now() Return now-now.TimeOfDay End Function ''' <summary> ''' Function for getting the "current" date and time. ''' Defaults to <see cref="DateTime.Now"/> but ''' could be replaced in a test or development scenario. ''' </summary> Public Shared NowFn As Func(Of Date) = Function() Date.Now ''' <summary> ''' Reset <see cref="NowFn"/> to application default ''' </summary> Public Shared Sub ResetNowFn() NowFn = Function() Date.Now End Sub End Class |
SystemGuid
Guid.NewGuid() returns a new and deliberately unpredictable value every time. You may find yourself writing a test where you would like to control the value of that new Guid.
You can write a SystemGuid class such as the one included below. SystemGuid has a NewGuid() method that return a Guid. If you do nothing, it behaves exactly like Guid.NewGuid().
Now everywhere in your application you always call SystemGuid.NewGuid() wherever you would have called Guid.NewGuid.
If you do nothing, the behavior of your application is unchanged.
During a test, however, you can replace the SystemGuid.NewGuidFn with a test-time function that returns any Guid value you need.
Remember when you are done to restore NewGuidFn to the proper default behavior. A "ResetNewGuidFn" method makes that easier for testers.
| C# | public static class SystemGuid { /// <summary> /// Gets a new <see cref="Guid"/> /// in the manner of <see cref="Guid.NewGuid"/>. /// See <see cref="NewGuidFn"/>. /// </summary> public static Guid NewGuid() { return NewGuidFn(); } /// <summary> /// Function for getting a new <see cref="Guid"/>. /// Defaults to <see cref="Guid.NewGuid"/> but /// could be replaced in a test or development scenario. /// </summary> public static Func<Guid> NewGuidFn = Guid.NewGuid; /// <summary> /// Reset <see cref="NewGuidFn"/> to application default /// </summary> public static void ResetNewGuidFn() { NewGuidFn = Guid.NewGuid; } } |
| VB | Public NotInheritable Class SystemGuid ''' <summary> ''' Gets a new <see cref="Guid"/> ''' in the manner of <see cref="Guid.NewGuid"/>. ''' See <see cref="NewGuidFn"/>. ''' </summary> Private Sub New() End Sub Public Shared Function NewGuid() As Guid Return NewGuidFn() End Function ''' <summary> ''' Function for getting a new <see cref="Guid"/>. ''' Defaults to <see cref="Guid.NewGuid"/> but ''' could be replaced in a test or development scenario. ''' </summary> Public Shared NewGuidFn As Func(Of Guid) = Guid.NewGuid ''' <summary> ''' Reset <see cref="NewGuidFn"/> to application default ''' </summary> Public Shared Sub ResetNewGuidFn() NewGuidFn = Guid.NewGuid End Sub End Class |
Make your own system utility classes
Look for other places in your application where you call a .NET static function or property and consider following the pattern you see here.