How to configure Dependency Injection in my own class

Question

I wish to use settings from appsettings.json in my own class.

I have this working well in a controller and in razor. I tried to use the same code as in a controller in my own class:

public class Email
{
    private readonly IConfiguration _config;

    public Email(IConfiguration config)
    {
        _config = config;
    }

but when I try to call this

Email sendEmail = new Email();

it requires that I provide config as a parameter. Shouldn't the DI system provide (inject) this? In ConfigureServices I have this:

services.AddSingleton(Configuration);

Do I need to register Email class somewhere too? Do I need to call it some different way?

Answer 1

When you use the following code:

Email sendEmail = new Email();

The DI system isn't involved at all - You've taken things into your own hands. Instead, you should add Email to the DI system and then have it injected. eg:

services.AddSingleton<Email>(); // You might prefer AddScoped, here, for example.

Then, as an example, if you're accessing Email in a controller, you can have it injected too:

public class SomeController : Controller
{
    private readonly Email _email;

    public SomeController(Email email)
    {
        _email = email;
    }

    public IActionResult SomeAction()
    {
         // Use _email here.
         ...
    }
}

Essentially, this just means you need to use DI all the way. If you want to provide more details about where you're currently creating your Email class, I can tailor the examples more to that.

---

It's a bit of an side, but you can also inject dependencies using the [FromServices] attribute inside of an action. Using this means you can skip the constructor and private field approach. eg:

public class SomeController : Controller
{
    public IActionResult SomeAction([FromServices] Email email)
    {
         // Use email here.
         ...
    }
}

Answer 2

As you mentioned, you defined a constructor which requires the parameter.

Please check out the concept of Class Constructors .

Injection is design pattern, when we use class and interfaces to implement it, it should still follow the basic Class methodology and concept. Hope it helps.