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Isolating Code Under Test with Microsoft Fakes

Microsoft Fakes help you isolate the code you are testing by replacing other parts of the application with stubs or shims. These are small pieces of code that are under the control of your tests. By isolating your code for testing, you know that if the test fails, the cause is there and not somewhere else. Stubs and shims also let you test your code even if other parts of your application are not working yet.

Fakes come in two flavors:

  • A stub replaces a class with a small substitute that implements the same interface. To use stubs, you have to design your application so that each component depends only on interfaces, and not on other components. (By "component" we mean a class or group of classes that are designed and updated together and typically contained in an assembly.)

  • A shim modifies the compiled code of your application at run time so that instead of making a specified method call, it runs the shim code that your test provides. Shims can be used to replace calls to assemblies that you cannot modify, such .NET assemblies.

Fakes replace other components

Requirements

  • Visual Studio Ultimate or Premium

Typically, you would consider a Visual Studio project to be a component, because you develop and update those classes at the same time. You would consider using stubs and shims for calls that the project makes to other projects in your solution, or to other assemblies that the project references.

As a general guide, use stubs for calls within your Visual Studio solution, and shims for calls to other referenced assemblies. This is because within your own solution it is good practice to decouple the components by defining interfaces in the way that stubbing requires. But external assemblies such as System.dll typically are not provided with separate interface definitions, so you must use shims instead.

Other considerations are:

Performance. Shims run slower because they rewrite your code at run time. Stubs do not have this performance overhead and are as fast as virtual methods can go.

Static methods, sealed types. You can only use stubs to implement interfaces. Therefore, stub types cannot be used for static methods, non-virtual methods, sealed virtual methods, methods in sealed types, and so on.

Internal types. Both stubs and shims can be used with internal types that are made accessible by using the assembly attribute InternalsVisibleToAttribute.

Private methods. Shims can replace calls to private methods if all the types on the method signature are visible. Stubs can only replace visible methods.

Interfaces and abstract methods. Stubs provide implementations of interfaces and abstract methods that can be used in testing. Shims can’t instrument interfaces and abstract methods, because they don’t have method bodies.

In general, we recommend that you use stub types to isolate from dependencies within your codebase. You can do this by hiding the components behind interfaces. Shim types can be used to isolate from third-party components that do not provide a testable API.

(For a more detailed description, see Using stubs to isolate parts of your application from each other for unit testing.)

  1. Inject interfaces

    To use stubs, you have to write the code you want to test in such a way that it does not explicitly mention classes in another component of your application. By "component" we mean a class or classes that are developed and updated together, and typically contained in one Visual Studio project. Variables and parameters should be declared by using interfaces and instances of other components should be passed in or created by using a factory. For example, if StockFeed is a class in another component of the application, then this would be considered bad:

    return (new StockFeed()).GetSharePrice("COOO"); // Bad

    Instead, define an interface that can be implemented by the other component, and which can also be implemented by a stub for test purposes:

    Public Function GetContosoPrice(feed As IStockFeed) As Integer
     Return feed.GetSharePrice("COOO")
    End Function
    
  2. Add Fakes Assembly

    1. In Solution Explorer, expand the test project’s reference list. If you are working in Visual Basic, you must choose Show All Files in order to see the reference list.

    2. Select the reference to the assembly in which the interface (for example IStockFeed) is defined. On the shortcut menu of this reference, choose Add Fakes Assembly.

    3. Rebuild the solution.

  3. In your tests, construct instances of the stub and provide code for its methods:

    <TestClass()> _
    Class TestStockAnalyzer
    
        <TestMethod()> _
        Public Sub TestContosoStockPrice()
            ' Arrange:
            ' Create the fake stockFeed:
            Dim stockFeed As New StockAnalysis.Fakes.StubIStockFeed
            With stockFeed
                .GetSharePriceString = Function(company)
                                           Return 1234
                                       End Function
            End With
            ' In the completed application, stockFeed would be a real one:
            Dim componentUnderTest As New StockAnalyzer(stockFeed)
            ' Act:
            Dim actualValue As Integer = componentUnderTest.GetContosoPrice
            ' Assert:
            Assert.AreEqual(1234, actualValue)
        End Sub
    End Class
    

    The special piece of magic here is the class StubIStockFeed. For every interface in the referenced assembly, the Microsoft Fakes mechanism generates a stub class. The name of the stub class is the derived from the name of the interface, with "Fakes.Stub" as a prefix, and the parameter type names appended.

    Stubs are also generated for the getters and setters of properties, for events, and for generic methods. For more information, see Using stubs to isolate parts of your application from each other for unit testing.

(For a more detailed description, see Using shims to isolate your application from other assemblies for unit testing.)

Suppose your component contains calls to DateTime.Now:

// Code under test:
    public int GetTheCurrentYear()
    {
       return DateTime.Now.Year;
    }

During testing, you would like to shim the Now property, because the real version inconveniently returns a different value at every call.

To use shims, you don’t have to modify the application code or write it a particular way.

  1. Add Fakes Assembly

    In Solution Explorer, open your unit test project’s references and select the reference to the assembly that contains the method you want to fake. In this example, the DateTime class is in System.dll. To see the references in a Visual Basic project, choose Show All Files.

    Choose Add Fakes Assembly.

  2. Insert a shim in a ShimsContext

    <TestClass()> _
    Public Class TestClass1
        <TestMethod()> _
        Public Sub TestCurrentYear()
            Using s = Microsoft.QualityTools.Testing.Fakes.ShimsContext.Create()
                Dim fixedYear As Integer = 2000
                ' Arrange:
                ' Detour DateTime.Now to return a fixed date:
                System.Fakes.ShimDateTime.NowGet = _
                    Function() As DateTime
                        Return New DateTime(fixedYear, 1, 1)
                    End Function
    
                ' Instantiate the component under test:
                Dim componentUnderTest = New MyComponent()
                ' Act:
                Dim year As Integer = componentUnderTest.GetTheCurrentYear
                ' Assert: 
                ' This will always be true if the component is working:
                Assert.AreEqual(fixedYear, year)
            End Using
        End Sub
    End Class
    

    Shim class names are made up by prefixing Fakes.Shim to the original type name. Parameter names are appended to the method name. (You don’t have to add any project reference to System.Fakes)

The previous example uses a shim for a static method. To use a shim for an instance method, write AllInstances between the type name and the method name:

System.IO.Fakes.ShimFile.AllInstances.ReadToEnd = ...

You don’t have to insert any reference to System.IO.Fakes: it’s created by the shim generation process.

You can also create shims for specific instances, for constructors, and for properties. For more information, see Using shims to isolate your application from other assemblies for unit testing.

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