Autofac and Lazy Dependency Injection: 2025 edition

 Thank you C#!  I couldn't believe it's been two years since I last posted about my lazy dependency implementation. Since that time there have been a few updates to the C# language, in particular around auto-properties that have greatly simplified the use of lazy dependencies and their property overrides to simplify unit testing. I also make use of primary constructors, which are ideally suited to the pattern since unlike regular constructor injection, the assertions happen in the property accessors, not a constructor.

The primary goal of this pattern is still to leverage lazy dependency injection while making it easier to swap in testing mocks. Classes like controllers can have a number of dependencies that they use, but depending on the action and state passed in, many of those dependencies don't actually get used in all situations. Lazy loading dependencies sees dependencies initialized/provided only if they are needed, however this adds a layer of abstraction to access the dependency when it's needed, and complicates mocking the dependency out for unit tests a tad more ugly.

The solution which I call lazy dependencies +property mitigates these two issues. The property accessor handles the unwrapping of the lazy proxy to expose the dependency for the class to consume. It also allows for a proxy to be injected. Each lazy dependency in the constructor is optional. If the IoC Container doesn't provide a new dependency or a test does not mock a referenced dependency, the property accessor throws a for-purpose DependencyMissingException to note when a dependency was not provided.

Updated pattern:

 public class SomeClass(Lazy<ISomeDependency>? _lazySomeDependency = null)
 {
     [field: MaybeNull]
     public ISomeDependency SomeDependency
     {
         protected get => field ??= _lazySomeDependency?.Value ?? throw new DependencyMissingException(nameof(SomeDependency));
         init;
     }
 }

This is considerably simpler than the original implementation. We can use the primary constructor syntax since we do not need to assert whether a dependency was injected or not. Under normal circumstances all lazy dependencies will be injected by the container, but asserting them falls on the property accessor. No code, save the accessor property, should attempt to access dependencies through the lazy dependency. The auto property syntax, new to C# gives us access to the "field" keyword. We also leverage a public init setter so that our tests can inject mocks for any dependencies they will use, while the getter remains protected (or private) for accessing the dependency within the class. The dependency property will look for an initialized instance, then check the lazy injected source, before raising an exception if the dependency has not been provided.

Unit tests provide a mock through the init setter rather than trying to mock a lazy dependency:

Mock<ISomeDependency> mockDependency = new();
mockDependency.Setup(x => /* set up mocked scenario */);

var classUnderTest = new SomeClass
{
    SomeDependency = mockDependency.Object
};

// ... Test behaviour, assert mocks.

In this simple example it will not look particularly effective, but in controllers that have several, maybe a dozen dependencies, this can significantly simplify test initialization. If a test scenario is expected to touch 3 out of 10 dependencies then you only need to provide mocks for the 3 dependencies rather than always mocking all 10 for every test.  If internal code is updated to touch a 4th dependency then the test(s) will break until they are updated with suitable mocks for the extra dependency. This allows you to mock only what you need to mock, and avoid silent or confusing failure scenarios when catch-all defaulted mocks try responding to scenarios they weren't intended to be called upon.