Question

I stumbled across some code and I was curious as to possible reasons why it would be this way.

There are two constructors, the main one being called in code and the second one being used in testing for dependency injection. The second constructor accepted a function that returns the object rather than an instance of the object itself.

MainConstructor()  
:this(() => Factory.Current.GetInstance<IQueryService>()){ 
}

SecondConstructor(Func<IQueryService> getQueryService){
}

I was curious as to advantages you would get from passing in a function instead of an instance.

Answer 1

There are several possible benefits for doing something like this:

1. Decoupling: By allowing the caller to pass in a delegate rather than an instance, both the caller and the callee (in your case the class being constructed) can be decoupled from the object being passed and where it comes from.
2. Lazy Acquisition: By passing in a delegate, the instance can be acquired in a lazy manner. In other words, the constructor could in principle cache the delegate and call it at the last possible moment, only when an instance is actually needed. If the instance is never needed, it may never get created.
3. Recreatability: By passing in a delegate rather than an instance, the class can cache the mechanism used to create a query service on demand. In this way, the delegate acts as a factory, allowing instances of the type needed to be created on demand.