Dependency Injection With Unity Using Constructors With Runtime Parameters

Question

I have the basics down of dependency injection but where I'm struggling is trying to put it together properly in an MVC API C# application using Unity. The issue I have is that I will have a controller that has a method, and in that method it will have say two objects. One of these objects will have a dependency on a data access layer and the other doesn't. I'm not quite sure on exactly how to set this up.

Lets say I have a controller with the following method that gets users from a search object. It also uses a SearchParameters object to perform the search. If you search with no name you get all results, or if you put a name you get any users with that name.

public Users[] GetUsers(string name) {
    Company.SearchParameters searchParams = new Company.SearchParameters(name);
    Company.UserSearchService searchService = new Company.UserSearchService(searchParams);

    return searchService.Search();
}

This is of course a super simplified version but in this case UserSearchService in the Search method is doing an explicit database call. So I know that is a dependency I'd have to give it. SearchParameters really is just a class that holds data.

Here is where I'm not sure on exactly what to do next. The Controller itself doesn't have a dependency but since UserSearchService does I'm not sure how using unity I properly set that up and take into account runtime values for the constructor. I'm also not sure if SearchParameters is supposed to be considered a dependency or not.

Additionally SearchParameters and UserSearchService do not have any sort of backing interface if that matters and there are other methods on this controller that interact with other classes that have a similar need of a data access layer dependency such as perhaps Company.UserAccount.

Answer 1

I see here two tasks. The first is a refactoring where static dependencies have to be replaced with interfaces. The second task is to register your stuff in IoC container.

For the first task a minimum you need is to replace all references to Database in your UserSearchService with IDatabase interface (so that it can also be mocked) and allow it to be passed to constructor (constructor injection). To be able to provide an instance of IDatabase to the service you have to create the same dependency for the controller (again constructor injection). Then register the IDatabase implementation as shown in this post .

Update

I agree that dependency has to be removed from the controller. As you @Topojijo have suggested a factory for the UserSearchService can be used in this case. Guessing you have several cervices you need to make a factory for each and there may be an overhead if their count is large. In such case it's better to resolve the service directly from Unity container and move the searchParams to Search method:

public Users[] GetUsers(string name) {
    Company.SearchParameters searchParams = new Company.SearchParameters(name);
    Company.UserSearchService searchService = container.Resolve<Company.UserSearchService>();

    return searchService.Search(searchParams);
}

Answer 2

The search parameters shouldn't be part of constructor; it should be part of the "Search" method. The SearchParameter object should not even be known outside of the UserSearchService class (Encapsulation). There should be some refactoring as neleus suggested. At the very least, to get things going, it should be refactored to something similar this:

public Users[] GetUsers(string name) {
    // Or however you end up with your UserSearchService object.  
    // Ideally as an interface and an injected dependency...
    Company.UserSearchService searchService = new Company.UserSearchService();

    return searchService.Search(name);
}

Company.UserSearchService:

public Users[] Search(string name) {
    // A factory would be a better option.  This should also be an interface.
    Company.SearchParameters searchParams = new Company.SearchParameters(name);

    // Your logic here
}