How do you inject your dependencies when they need differents parameters?

Question

For instance I have this bit of code

public class ProductService{
private IProductDataSource _dataSource = DependencyManager.Get<IProductDataSource>();
public Product Get(int id){
return _dataSource.Select(id);
}
}

I have 2 different data source:

• XML file which contains the informations only in 1 language,
• a SQL data base which contains the informations in many languages.

So I created 2 implementation for IProductDataSource, for for each kind of datasource. But how do I send the required language to the SQL data source ?

• I add the parameter "language" to the method "IProductDataSource.Select" even if I won't use it in the case of the XML implementation.
• Inside the SQL implementation I get the language from a global state ?
• I add the language to the constructor of my SQL implementation, but then I won't use my DependencyManager and handle my self the dependency injection.

Maybe my first solution is not good.

Answer 1

The third option is the way to go. Inject the language configuration to your SQL implementation. Also get rid of your DependencyManager ServiceLocator and use constructor injection instead.

Answer 2

If your application needs to work with multiple languages in a single instance I think point one is a sensible approach. If the underlying data does not provide translations for a request language then return null. There is another solution in this scenario. I'm assuming that what you have is a list of products and language translations for each product. Can you refactor your model so that you do not need to specify or asertain the langauge until you reference language specific text? The point being a product is a product regardless of the language you choose to describe it. i.e. one product instance per product, only the product id on the Datasource.Select(..) method and some other abstraction mechanism to deal with accessing the correct text translation.

If however each instance of your application is only concerned with one language set I second Mr Gloor.

Answer 3

First of all I need to point out that you are NOT injecting any dependencies with your example - you are depending on a service locator (DependencyManager) to get them for you. Dependency injection, simply put, is when your classes are unaware of who provides the dependencies, e.g. using a constructor, a setter, a method. As it was already mentioned in the other answers, Service locator is an anti-pattern and should be avoided. The reasons are described in this great article.

Another thing is that the settings you are mentioning, such as language or currency, seem to be localization related and would probably be better dealt with using the built-in mechanisms of your language of choice (e.g. resource files, etc).

Now, having said that, depending on how the rest of your code is structured you have several options to solve this while still using Service locator:

• You could have SqlDataSource depend on some ILanguageProvider which pulls the current language from somewhere. However, with more settings like these (or if it is difficult to get current language in an isolated way) this can get messy very fast.
• You could depend on IProductDataSourceFactory instead (or, if you are using C#, Func<IProductDataSource>) which would return the concrete implementation with the correct settings. Again, you need to be able to get the current language in an isolated way in order to use this.
• You could go with option 1 in your question. This would be a leaky abstraction but would be the simplest to implement.

However, if you decide to get rid of service locator and start using some DI container, the best solution would be using option 3 (as it was already stated) and configuring container accordingly to provide the correct value. Some good ideas of how to do this in an elegant way can be found in the answer to this question