Unit testing methods with logical dependencies on other classes

Question

Suppose I have an interface declaring the following method signature:

SearchResults SearchProducts(string type, string filter, string anotherFilter);

Inside a concrete implementation of this, I instantiate a helper class (call it QueryBuilder), built to ease the construction of strongly-typed search queries for the specific search library I'm using. I want to unit test SearchProducts (ideally by mocking the query builder), which means I need to decouple the method from QueryBuilder.

How do I do this appropriately when the two things are logically coupled? That is to say:

• It's not appropriate to pass an IQueryBuilder into the method as the implementation of IQueryBuilder is tightly coupled to the implementer of ISearchProducts, i.e. the types of the return values of QueryBuilder's methods are specific to the library being used in the concrete SearchService.
• It's not appropriate to pass an instance of IQueryBuilder into the SearchService constructor (via DI or directly) as the logical scope of the query builder instance is local to the SearchProducts method call - it is instantiated in the method, its state is manipulated by adding sub-queries and a complete "query" is extracted for this SearchProducts call).

So, given the above, what's the appropriate way of handling this? It sounds like a case for a factory; however the implementation of this factory would be trivial and it seems a little contrived to define a factory interface and add a constructor parameter just for the sake of testability.

Is this indicative of a fundamental flaw in the design of my search interface? How do I decouple these things appropriately such that I can test both the search service and the query builder implementations in isolation?

Answer 1

This is the ideal use for Inversion of Control, or specifically, Dependency Injection. In your test context, your query builder is the mock; in your normal execution context, it's the real deal. No need for a factory, just a different context.