How to instantiate objects of classes that have dependencies injected?

Question

Let's say I have some class with dependency injected:

public class SomeBusinessCaller {
   ILogger logger;
   public SomeBusinessCaller(ILogger logger) {
       this.logger = logger;
   }
}

My question is, how do I instantiate an object of that class? Let's say I have an implementation for this, called AppLogger. After I say

ObjectFactory.For<ILogger>().Use<AppLogger>();

how do I call constructor of SomeBusinessCaller? Am I calling

SomeBusinessCaller caller = ObjectFactory.GetInstance<SomeBusinessCaller>();

or there is a different strategy for that?

Answer 1

The code which uses caller does not live in a vacuum. Instead of assuming you need to create an instance of SomeBusinessCaller yourself, simply declare a dependency on it:

public class SomeOtherClass
{
    public SomeOtherClass(SomeBusinessCaller caller)
    {
        // ...
    }
}

The container will figure out that SomeBusinessCaller requires an ILogger, and it will automatically provide an instance of AppLogger.

And, when something needs an instance of that class:

public class YetAnotherClass
{
    public YetAnotherClass(SomeOtherClass other)
    {
        // ...
    }
}

If you follow that logic for all the objects you write, eventually you will have exactly 1 object for which you actually request an instance:

public static void Main()
{
    // ...Initialize ObjectFactory...

    var compositionRootObject = ObjectFactory.GetInstance<YetAnotherClass>();

    compositionRootObject.DoYourThing();
}

Answer 2

Depending on which DI Container you use, then yes: ask the container to provide you with an instance of SomeBusinessCaller.

The container should use auto-wiring to automatically figure out that the SomeBusinessCaller requires an instance of ILogger, and since AppLogger is registered, it can satisfy that requirement.

However, don't use the DI Container as a static Service Locator.

Instead, you should let the DI Container compose your entire dependency graph in one go in the application's Composition Root.