In unit testing, developers often encounter scenarios where the code under test instantiates objects internally, making it challenging to inject mocks or fakes. Typemock Isolator addresses this challenge with its Faking Future Instances feature, allowing you to intercept and fake objects that are created within the method under test. This capability makes Typemock one of the most powerful .NET mocking frameworks available.
When to Use Faking Future Instances
This technique is particularly useful when:
- The object to be faked is instantiated inside the method under test, and there’s no straightforward way to pass a mock or fake into the method.
- You need to control or verify the behavior of these internally created objects during testing.
How to Implement Faking Future Instances
Typemock provides the Isolate.Fake.NextInstance<T>() method to fake the next instance of a specified type T that will be created. Here’s how you can use it:
Example Code
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[TestMethod, Isolated] public void Fake_FutureInstance() { // Arrange: Fake the next instance of the Dependency class var handle = Isolate.Fake.NextInstance<Dependency>(); // Configure behavior when Check() is called on the faked instance Isolate.WhenCalled(() => handle.Check()).IgnoreCall(); // Act: Call the method under test var result = ClassUnderTest.AddSecurely(1, 2); // Assert: Verify the expected outcome Assert.AreEqual(3, result); } public static int AddSecurely(int x, int y) { var dependency = new Dependency(); dependency.Check(); return x + y; } public class Dependency { public void Check() { throw new Exception("No Entry"); } } |
Explanation of the Code
Isolate.Fake.NextInstance<Dependency>(): This line tells Typemock to fake the next instance of theDependencyclass that will be created.Isolate.WhenCalled(() => handle.Check()).IgnoreCall(): This line suppresses theCheckmethod call on the mocked instance, preventing the exception from being thrown. You can also configure other behaviors if needed.- In the
AddSecurelymethod, a newDependencyobject is instantiated, and itsCheckmethod is called. Without faking, this would throw an exception. By faking the next instance ofDependencyand controlling its behavior, the test can proceed without errors.
Handling Multiple Instances
If your method creates multiple instances of the same type, you can fake these instances sequentially using Typemock:
Example
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// Fake the first two instances of the Dependency class var firstHandle = Isolate.Fake.NextInstance<Dependency>(); var secondHandle = Isolate.Fake.NextInstance<Dependency>(); // Configure behaviors as needed Isolate.WhenCalled(() => firstHandle.Check()).IgnoreCall(); Isolate.WhenCalled(() => secondHandle.Check()).WillReturn("Handled"); |
Isolate.Fake.NextInstance<T>()can be called multiple times to fake multiple instances of .NET objects.- Each call to
Isolate.Fake.NextInstance<T>()returns a handle to the next mock instance.
Handling All Instances
To fake all future instances of a specific type, use Isolate.Fake.AllInstances<T>(). This approach is useful when you need a consistent behavior across any instance created during the test.
Example
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// Fake all future instances of the Dependency class var allHandle = Isolate.Fake.AllInstances<Dependency>(); // Configure behavior for all instances Isolate.WhenCalled(() => allHandle.Check()).IgnoreCall(); |
Isolate.Fake.AllInstances<T>()ensures that every instance ofTcreated within the scope of the test will be mocked and can be controlled as needed.- This method is ideal for scenarios where the codebase frequently instantiates new objects, making mocking essential for maintaining test stability and performance.
Note
Using this API will also cover instances that have already been created (past instances)
Example
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// Arrange: Create an instance of the Dependency class before calling // Fake.AllInstances, this can also happen in the test setup var existingInstance = new Dependency(); // Fake all instances of the Dependency class, including those already created var allHandle = Isolate.Fake.AllInstances<Dependency>(); // Configure behavior for all instances Isolate.WhenCalled(() => allHandle.Check()).IgnoreCall(); // Act: Call a method on the existing instance existingInstance.Check(); // This will now be faked and will not throw an exception // Create a new instance and use it var newInstance = new Dependency(); newInstance.Check(); // This will also be fake |
Isolate.Fake.AllInstances<T>() ensures that all instances of T—both newly created and existing instances—will be faked. This makes it an ideal approach for scenarios where you need to control the behavior of objects across the entire test context, providing consistent and reliable test outcomes.
Conclusion
By leveraging Typemock’s future mocking capabilities, you can easily mock internal objects created during your tests. The flexibility provided by Typemock’s features such as Isolate.Fake.NextInstance<T>() and Isolate.Fake.AllInstances<T>() ensures that your unit tests remain robust and adaptable to changes in your .NET codebase.
Whether you need to mock static methods, constructor behavior, or internal dependencies, Typemock provides the comprehensive solution you need to keep your tests future-proof.
For more detailed information and advanced usage, refer to the Typemock Documentation on Faking Future Instances.
