How Should I Access the Unity Container in My Application for Proper Dependency Injection?

How to Handle Unity Container Access in an Application

When using a dependency injection framework like Unity, it's crucial to consider how you handle access to the container within your application. This question explores three options and investigates the best approach according to dependency injection principles.

Singleton Container:

Creating a singleton container that you can access anywhere in the application may seem convenient. However, introducing a dependency on the container to use a dependency injection framework contradicts the principles of DI.

Passing the Container:

Passing the Unity container to child classes can become cumbersome and visually unappealing. It also introduces tight coupling and makes it harder to test your code independently.

Constructor Injection:

The correct approach to DI is to use constructor injection. This involves declaring the dependencies your class needs in its constructor. Unity will automatically wire these dependencies when creating an instance of the class.

Example:

Consider the example provided in the question:

public class TestSuiteParser
{
    private readonly TestSuite _testSuite;
    private readonly TestCase _testCase;

    public TestSuiteParser(TestSuite testSuite, TestCase testCase)
    {
        _testSuite = testSuite ?? throw new ArgumentNullException(nameof(testSuite));
        _testCase = testCase ?? throw new ArgumentNullException(nameof(testCase));
    }

    public TestSuite Parse(XPathNavigator someXml)
    {
        // Use the injected dependencies here
        foreach (XPathNavigator blah in aListOfNodes)
        {
            _testSuite.TestCase.Add(_testCase);
        }
    }
}

In your composition root, you would then configure Unity as follows:

container.RegisterType<TestSuite, ConcreteTestSuite>();
container.RegisterType<TestCase, ConcreteTestCase>();
container.RegisterType<TestSuiteParser>();

Using constructor injection, Unity will automatically inject the necessary dependencies into the TestSuiteParser class, ensuring proper DI and ease of testing.

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