If you’re diving into the world of MVC APIs in C#, chances are you’ve heard the term “Dependency Injection” (also known as DI). It might sound like another one of those overly technical buzzwords, but trust me, C# dependency injection is a powerful tool that can make your life as a developer much easier. In this article, we’re going to break it down in an informal, easy-to-understand way, and best of all, I’ll show you plenty of examples along the way.
By the time you finish reading this, you’ll not only understand how to use C# dependency injection in an MVC API, but you’ll also see why it’s such an important concept in software development.
Table Of Contents
What Is Dependency Injection?
Let’s kick off with the basics. Imagine you have a car. To make that car go, you need an engine. But instead of building that engine inside the car, what if you could just “inject” the engine into the car whenever you needed it? This is the essence of C# dependency injection It’s about giving your objects (like a car) the things they need (like an engine) without them having to build or know how to build them.
In C#, this translates to injecting dependencies—such as services, repositories, or other objects—into your controllers or classes. These dependencies could be anything your class needs to function properly.
Without C# dependency injection, your classes might look something like this:
public class Car
{
private readonly Engine _engine;
public Car()
{
_engine = new Engine(); // The class is responsible for creating the engine
}
public void Drive()
{
_engine.Run();
}
}
Notice here that the Car class is responsible for creating an Engine. This tight coupling can cause problems. What if you wanted to use a different type of engine? You’d have to modify the Car class. That’s where dependency injection comes to the rescue.
Why Should You Care About C# Dependency Injection?
Before we jump into the code, let’s talk about why C# dependency injection is worth your time. Here are a few reasons why it’s awesome:
- Loose Coupling: Your classes don’t need to worry about how to create dependencies. This makes your code more flexible and easier to maintain.
- Easier Testing: If a class doesn’t create its dependencies, you can easily replace them with mock objects during unit testing.
- Better Code Organization: By using C# dependency injection, your code will naturally evolve into smaller, more focused classes that are easier to read and maintain.
- Single Responsibility Principle (SRP): dependency injection encourages you to follow SRP since it moves the responsibility of managing dependencies out of your classes.
Setting Up C# Dependency Injection in an MVC API
Alright, enough theory. Let’s see C# dependency injection in action in an MVC API. The first thing we need to do is set up a simple API. If you’re starting from scratch, create a new MVC API project in Visual Studio.
Once you’ve got your project up and running, it’s time to introduce some dependencies.
Step 1: Create an Interface
We’ll start by creating an interface for a service that will perform some basic operations. This will make it easy to swap out the actual implementation later, which is a key feature of C# dependency injection.
public interface IOperationService
{
string GetOperation();
}
Step 2: Implement the Interface
Now, let’s create a class that implements the IOperationService interface. This will be the dependency we inject into our controller later.
public class OperationService : IOperationService
{
public string GetOperation()
{
return "This is a sample operation!";
}
}
Step 3: Register the Service in the Startup.cs
The magic happens in the Startup.cs file. Here’s where we tell our application how to handle C# dependency injection. In the ConfigureServices method, register your OperationService with the C# dependency injection container.
public void ConfigureServices(IServiceCollection services)
{
services.AddControllers();
// Register the OperationService as a dependency
services.AddScoped<IOperationService, OperationService>();
}
AddScoped means the service will be created once per request. Other options include AddTransient (creates a new instance every time it’s requested) and AddSingleton (creates a single instance for the entire application lifetime).
Injecting the Service into a Controller
Now that we’ve set up our service, it’s time to inject it into a controller. Here’s a simple controller that uses C# dependency injection to get an instance of IOperationService.
[ApiController]
[Route("[controller]")]
public class OperationController : ControllerBase
{
private readonly IOperationService _operationService;
// Constructor injection
public OperationController(IOperationService operationService)
{
_operationService = operationService;
}
[HttpGet]
public string Get()
{
return _operationService.GetOperation();
}
}
Notice how we inject IOperationService into the controller via the constructor. This is known as constructor injection, the most common type of C# dependency injection in C#. The controller doesn’t care about the implementation of IOperationService; it just uses whatever implementation is provided by the dependency injection container.
Different Lifetimes of Dependencies
One of the powerful aspects of dependency injection in ASP.NET Core is the ability to control the lifetime of your services. Let’s take a closer look at the different options:
Transient (AddTransient)
A new instance is created each time the service is requested. Use this for lightweight, stateless services.
services.AddTransient<IOperationService, OperationService>();
Scoped (AddScoped)
A new instance is created once per request, which makes it great for services that need to maintain state during a single operation.
services.AddScoped<IOperationService, OperationService>();
Singleton (AddSingleton)
A single instance is created and shared throughout the application’s lifetime. Be careful with this as it can cause issues with state management in multi-threaded environments.
services.AddSingleton<IOperationService, OperationService>();
Here’s a fun fact: you can mix and match lifetimes. For instance, if you register two different services—one as Singleton and the other as Scoped—the C# dependency injection container will handle them appropriately.
Practical Example: Using C# Dependency Injection with a Repository Pattern
Let’s expand our example. Suppose you have a database, and you want to use the repository pattern for data access. First, define an IRepository interface:
public interface IRepository<T>
{
IEnumerable<T> GetAll();
T GetById(int id);
}
Then, implement it with a concrete class:
public class UserRepository : IRepository<User>
{
public IEnumerable<User> GetAll()
{
// Imagine this gets data from a database
return new List<User> { new User { Id = 1, Name = "John Doe" } };
}
public User GetById(int id)
{
// Fake getting a user by id
return new User { Id = id, Name = "Jane Doe" };
}
}
Finally, register this repository in the Startup.cs file:
public void ConfigureServices(IServiceCollection services)
{
services.AddControllers();
services.AddScoped<IRepository<User>, UserRepository>();
}
And now you can inject IRepository into your controller:
public class UserController : ControllerBase
{
private readonly IRepository<User> _repository;
public UserController(IRepository<User> repository)
{
_repository = repository;
}
[HttpGet]
public IEnumerable<User> GetUsers()
{
return _repository.GetAll();
}
}
Wrapping Up
C# dependency injection is a core concept in modern C# development, especially when building APIs with MVC. By leveraging dependency injection, you can write cleaner, more modular code that’s easier to test and maintain. We’ve only scratched the surface here, but you now have a solid foundation to start using DI in your projects.
Remember, the key to C# dependency injection is understanding that your classes shouldn’t be responsible for creating the objects they depend on. Instead, those dependencies should be injected, either via the constructor or other methods like property or method injection (though constructor injection is by far the most common).
So, go ahead and refactor some of your tightly coupled classes, and enjoy the magic of C# dependency injection!
