Does Dependency Injection (DI) rely on Interfaces?

Question

This may seem obvious to most people, but I'm just trying to confirm that Dependency Injection (DI) relies on the use of Interfaces.

More specifically, in the case of a class which has a certain Interface as a parameter in its constructor or a certain Interface defined as a property (aka. Setter), the DI framework can hand over an instance of a concrete class to satisfy the needs of that Interface in that class. (Apologies if this description is not clear. I'm having trouble describing this properly because the terminology/concepts are still somewhat new to me.)

The reason I ask is that I currently have a class that has a dependency of sorts. Not so much an object dependency, but a URL. The class looks like this [C#]:

using System.Web.Services.Protocols;
public partial class SomeLibraryService : SoapHttpClientProtocol 
{
        public SomeLibraryService() 
        {
            this.Url = "http://MyDomainName.com:8080/library-service/jse";
        }
}

The SoapHttpClientProtocol class has a Public property called Url (which is a plain old "string") and the constructor here initializes it to a hard-coded value.

Could I possibly use a DI framework to inject a different value at construction? I'm thinking not since this.Url isn't any sort of Interface; it's a String.

[Incidentally, the code above was "auto-generated by wsdl", according to the comments in the code I'm working with. So I don't particularly want to change this code, although I don't see myself re-generating it either. So maybe changing this code is fine.]

I could see myself making an alternate constructor that takes a string as a parameter and initializes this.Url that way, but I'm not sure that's the correct approach regarding keeping loosely coupled separation of concerns. (SoC)

Any advice for this situation?

Answer 1

It's not required to use interfaces -- you could use concrete types or abstract base classes. But many of the advantages of DI (such as being able to change an implementation of a dependancy) come when using interfaces.

Castle Windsor (the DI framework I know best), allows you to map objects in the IoC container to Interfaces, or to just names, which would work in your case.