Faking Future Instances with Typemock: Simplifying .NET Mocking for Robust Unit Tests

By Eli Lopian

November 17, 2024

0

.NET, HowTo, Legacy Code, Unit Test

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

[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&lt;Dependency>()
    : This line tells Typemock to fake the next instance of the 
    Dependency
    class that will be created.
  • Isolate.WhenCalled(() => handle.Check()).IgnoreCall()
    : This line suppresses the 
    Check
    method call on the mocked instance, preventing the exception from being thrown. You can also configure other behaviors if needed.
  • In the 
    AddSecurely
    method, a new 
    Dependency
    object is instantiated, and its 
    Check
    method is called. Without faking, this would throw an exception. By faking the next instance of 
    Dependency
    and 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

// 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&lt;T>()
    can be called multiple times to fake multiple instances of .NET objects.
  • Each call to 
    Isolate.Fake.NextInstance&lt;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&lt;T&gt;()
. This approach is useful when you need a consistent behavior across any instance created during the test.

Example

// 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&lt;T>()
    ensures that every instance of 
    T
    created 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

// 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&lt;T&gt;()
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&lt;T&gt;()
and 
Isolate.Fake.AllInstances&lt;T&gt;()
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.

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