Manual Dependency Injection in MVC - Trickcode

how to perform dependency injection manually

Part 1: Introduction to Dependency Injection

Back in part one, I discussed what dependency injection was and why you should use it. In this part, I’m going to cover how to perform dependency injection manually. I’ll use the example of a controller that uses a service class that depends on a repository.

First up is the repository. Here we’ll create an interface IRepo and class DbRepo. IRepo will get used later. DbRepo will have two constructors. One that accepts a connection string and one that’s parameterless which calls the other one passing a default connection string. This allows us to change the database connection for testing.

public interface IRepo
{
    // Required methods
}
 
public class DbRepo : IRepo
{
    public DbRepo(string dsn)
    {
        // Connect here
    }
 
    public DbRepo()
        : self("Default Connection String")
    {
    }
}

Now let’s create an interface for our business logic that we’ll use later and a class that implements it. Similar to the repository it will have a constructor that takes one parameter, this time an instance of IRepo, and one that’s parameterless which calls the other constructor passing an instance of DbRepo.

public IBusinessLogic
{
    // Declare methods here
}
 
public AppBusinessLogic
{
    private IRepo _repo;
 
    public AppBusinessLogic(IRepo repo)
    {
        _repo = repo;
    }
 
    public AppBusinessLogic
        : self(new DbRepo())
    {
    }
}

public class AppController
{
    private IBusinessLogiv _service;
 
    public AppController(IBusinessLogic service)
    {
        _service = service;
    }
 
    public AppController()
        : self(new AppBusinessLogic())
    {
    }
}