Understanding Method Injection in Dependency Injection - A Guide to Avoid Common Pitfalls

## Method Injection in DI: How to Shoot Yourself in the Foot! Dependency injection (DI) is a powerful technique that allows you to write flexible, scalable code. One of its key features is method injection - the ability to pass dependencies to methods directly rather than through constructor injection. But beware! This feature can be dangerous if used incorrectly. Here's what you need to know: ## So What Is The Problem with Method Injection? Method injection can make your code more complex and harder to test. If you're not careful, it can lead to tightly-coupled code that's difficult to maintain. One common mistake is to use method injection excessively, resulting in "hidden" dependencies that are hard to track down. Always remember: just because you can inject a dependency into a method doesn't mean you should. Second! It can lead to unnecessary object creation. When you inject a dependency into a method, you may end up creating a new instance of that object every time the method is called. This can be particularly problematic for expensive or resource-intensive objects. ## What Is A Better Way? ➡️ Constructor Injection Oh, you want to learn about Constructor Injection? How bold of you! Well, look no further - I have all the juicy details waiting for you on my blog. Ready or not, here comes the knowledge! ;) However, there are cases where method injection still makes sense. For example, if you have a single method that requires a specific dependency, it may be more appropriate to inject that dependency directly. ## Example Let's say we have a `UserService` class that requires a `UserRepository` dependency. Rather than injecting the repository through the constructor, we'll use method injection to inject it directly into our `getUserById` method: ```typescript class UserService { constructor(private logger: ILogger) {} public async getUserById(id: string, repo: UserRepository): Promise<User> { this.logger.info(`Getting user with ID ${id}`); return await repo.getUserById(id); } } ``` Notice that we also inject a logger dependency into the constructor using the more traditional constructor injection. P.S. when it comes to DI, method injection is like a gun - it can be quite dangerous if not handled carefully. ;-) cheeers. 🍺

1
2
3
4
5
6
7
8

class UserService {
  constructor(private logger: ILogger) {}

  public async getUserById(id: string, repo: UserRepository): Promise<User> {
    this.logger.info(`Getting user with ID ${id}`);
    return await repo.getUserById(id);
  }
}