Dependency Injection provides a blueprint for structuring code to make it easier to understand by humans. It helps manage dependencies between different components of a system. Instead of each component creating and managing its dependencies, dependency injection allows dependencies to be provided from outside. This promotes loose coupling and flexibility in the management of the code.
Normally, dependencies are passed to a component through constructor parameters or setters. But with dependency injection, components become more modular and independent, making the code easier to maintain and extend. So, the idea is that everything is its own independent little black box that can be updated in isolation of everything else.
Note, that with dependency injection you are typically trading performance to easier maintainability. It's up to you to make the decision if it's worth the cost.
// Dependencies passed through constructor parameters
public void Square(int width, int height)
{
// Initializing resources
this.Width = width;
this.Height = height;
}When dependencies are passed through constructor parameters or setters, it means that a component explicitly receives the necessary dependencies from an external source, like a user or another component. This promotes easier testing because you can provide mock dependencies that simulate the desired behavior, allowing you to isolate and test the component's functionality without relying on real dependencies.
On the other hand, dependency injection (DI) is a specific approach to managing dependencies. It involves having an external entity (often called a "container") automatically provide the required dependencies to a component, without the component explicitly asking for them. DI further promotes easier testing by enabling you to easily swap real dependencies with mock ones, as the container can be configured to inject different dependencies based on the testing scenario.
Imagine you're building a car. The car has different parts like an engine, tires, and a steering wheel. The engine needs fuel to run, the tires need air, and the steering wheel needs to be connected to the car's frame.
In a simple scenario without dependency injection, each part of the car would be responsible for creating and managing its own dependencies. The engine would have to find and manage its fuel source, the tires would have to handle their own air supply, and the steering wheel would have to establish its connection to the car.
However, with dependency injection, the dependencies are provided from outside. Let's say there's a mechanic responsible for assembling the car. The mechanic can inject the fuel source, air supply, and steering mechanism into the respective parts of the car. This way, the parts don't need to worry about finding or managing their dependencies themselves.
By using dependency injection, the car's components become more independent, modular, and easier to work with. If any part needs to be replaced or updated, it can be done without affecting the other components. It also makes testing and reusing the parts easier because the dependencies can be easily mocked or swapped out.
Here's a simplified code example using C# and WinUI 3 that demonstrates this analogy. In the code, we have four different elements working together:
The CarViewModel class needs an IFuelProvider to get the fuel required to start the car's engine. The CarViewModel class is designed to work with any type of fuel provider that implements the IFuelProvider interface.
// CarViewModel.cs
public class CarViewModel : INotifyPropertyChanged
{
private IFuelProvider _fuelProvider;
public CarViewModel(IFuelProvider fuelProvider)
{
_fuelProvider = fuelProvider;
}
public void StartCar()
{
// Use the fuel provider to get fuel and start the car's engine.
Fuel fuel = _fuelProvider.GetFuel();
Car.StartEngine(fuel);
}
}When the CarViewModel is created, it receives an instance of an IFuelProvider (which could be a gas station, a fuel storage, or any other fuel source) through its constructor. This is done by someone else, like the MainWindow class. When the StartCar method is called, the CarViewModel uses the injected fuel provider to get the fuel and starts the car's engine.
By injecting the fuel provider, we make the CarViewModel more flexible and reusable. It can work with different types of fuel providers without needing to change its code.
Next, we have an interface called IFuelProvider. It defines a contract or set of rules for any class that wants to act as a fuel provider. The interface has a single method called GetFuel() which specifies that any class implementing this interface must have a method that returns a Fuel object.
// IFuelProvider.cs
public interface IFuelProvider
{
Fuel GetFuel();
}The purpose of this interface is to create a common language or agreement between different fuel provider implementations. By using this interface, we can have multiple classes that provide fuel, but they all adhere to the same rules defined in the interface. This makes it easier to swap or switch between different fuel providers without affecting the other parts of the code that rely on the IFuelProvider interface.
Next, the FuelProvider class. It's responsible for providing fuel to the car. When the GetFuel() method is called, it implements the logic to obtain fuel. For example, it might represent going to a gas station or retrieving fuel from a storage tank.
// FuelProvider.cs
public class FuelProvider : IFuelProvider
{
public Fuel GetFuel()
{
// Implement the logic to get fuel, e.g., from a gas station or storage.
return new Fuel();
}
}The purpose of this class is to abstract away the details of how fuel is obtained. By defining the IFuelProvider interface and implementing it in the FuelProvider class, we can easily swap different fuel providers (e.g., mechanic, gas station) without affecting the rest of the code. This flexibility allows us to test the car's behavior with different fuel scenarios and makes it easier to extend or modify the fuel acquisition process in the future.
Finally, in the code below, we have a MainWindow class representing a car control panel.
// MainWindow.xaml.cs
public sealed partial class MainWindow : Window
{
private CarViewModel _carViewModel;
public MainWindow()
{
InitializeComponent();
// Create an instance of the fuel provider (the mechanic).
IFuelProvider fuelProvider = new FuelProvider();
// Create the car view model (the car) and inject the fuel provider.
_carViewModel = new CarViewModel(fuelProvider);
}
private void StartCarButton_Click(object sender, RoutedEventArgs e)
{
// When the "Start Car" button is clicked, call the StartCar method on the car view model.
_carViewModel.StartCar();
}
}In the constructor of MainWindow, we create an instance of the FuelProvider class, which acts as the mechanic that provides fuel for the car.
Then, we create an instance of the CarViewModel class, which represents the car itself. We pass the FuelProvider instance (the fuel provider or mechanic) as a parameter to the CarViewModel constructor. This is called dependency injection, where the car view model is given its necessary dependency, the fuel provider, from outside.
Later, when the "Start Car" button is clicked, it triggers the StartCar method on the CarViewModel, which starts the car using the fuel provided by the injected FuelProvider.
By using the MVVM pattern and dependency injection, we can easily replace the fuel provider with a different implementation without modifying the CarViewModel or the MainWindow. This promotes modularity, testability, and flexibility in our code.
There is some similarity in terms of the fact that both C++ header files and .NET interfaces are used to declare methods and properties that need to be implemented somewhere else, but the similarity is superficial and the usage is quite different due to the differences in how the two languages handle types, interfaces, and implementations.
In C++, a header file (.h) is a file that contains forward declarations of identifiers, such as functions, variables, classes, or other types. The header file serves as an interface to the code that is implemented in corresponding source (.cpp) files. This is mainly because of the compile-time nature of C++.
In contrast, .NET interfaces are more than just a way to declare methods and properties. They are a contract which classes implementing them must fulfill. And with dependency injection, they allow for the loose coupling of components.
While interfaces in .NET can be used for similar purposes as .h files in C++, they have important differences:
So while there is some resemblance in terms of declaring methods and properties, the usage and behavior of .NET interfaces and C++ header files are quite different.