What are some advantages to using an interface in C#?

Question

I was forced into a software project at work a few years ago, and was forced to learn C# quickly. My programming background is weak (Classic ASP).

I\'ve learned qui

Answer 1

The basic case is the "IWriter" case.

Suppose you are making a class that can write to the console, and it has all kinds of useful functions like write() and peek().

Then you would like to write a class that can write to the printer, so instead of reinventing a new class, you use the IWriter interface.

Now the cool thing about interfaces is you can write all your writing code, without knowing what is your writing target beforehand, and then can when the user decides (at runtime) weather he wants to write to the console or the printer, you just define the object as a console/printer writer and you don't need to change anything in your writing code, because they both use the same front end (interface).

Answer 2

An interface says how something should work. Think of it as a contract or a template. It is key to things such as Inverson of Control or Dependancy Injection.

I use Structure Map as my IoC container. This allows me to define an interface for all of my classes. Where you might say

Widget w = new Widget();

I would say

IWidget w = ObjectFactory.GetInstance<IWidget>();

This is very powerful in that my code isn't saying necessarily what a Widget truely is. It just knows what a Widget can do based on the interface of IWidget.

This has some great power to it in that now that I am using an IoC container I can do a couple more nifty things. In my unit tests where I need to use a Widget I can create a mock for Widget. So say that my Widget does something very powerful by way of connecting to a database or a web service, my mock can simulate connecting to these resources and return to me stubbed data. This makes my test run faster and behave in a way that is more reliable. Because I am using StructureMap I can tell StructureMap to load the real implementation of my Widget during production use of my code and the mocked version of the Widget during testing either programatically or by configuration.

Also, because I am using an IoC container I can provide cool new features to my application such as writing three different ways to cache data. I can have a local developer box cache using a tool such as Lucene.NET for a local cache. I can have a development server use the .NET cache which runs great on one box. And then I can have a third option for my production servers use a cache layer such as MemCache Win32 or Velocity. As long as all three caching implementations conform to the same interface, their actual implementation doesn't concern me (or my code) at all. I simply ask StructureMap to go get the current environments implementation and then go to work.

If you follow Dependency Injection at all then interfaces come in handy here also with an IoC container such as StructureMap in that I can declare the usage of a class by way of an Interface in the constructor of my class.

public class Widget(IWidgetRepository repository, IWidgetService service) : IWidget
{
    //do something here using my repository and service
}

And then when I new up an instance of Widget by way of StructureMap such as this

IWidget widget = ObjectFactory.GetInstance<IWidget>();

Notice that I am not specifying the repository or service in the constructor. StructureMap knows by way of the interfaces specified in the constructor how to go get the appropriate instances and pass them in too. This makes very powerful and clean code!

All from the simple definition of Interfaces and some clever usage of them!

Answer 3

good article.

An interface is a contract that guarantees to a client how a class or struct will behave.

http://www.codeguru.com/csharp/csharp/cs_syntax/interfaces/article.php/c7563

This might be the clearest easiest way of explaining that I have come across:

"The answer is that they provide a fairly type-safe means of building routines that accept objects when you don't know the specific type of object that will be passed ahead of time. The only thing you know about the objects that will be passed to your routine are that they have specific members that must be present for your routine to be able to work with that object. The best example I can give of the need for interfaces is in a team environment. Interfaces help define how different components talk to each other. By using an interface, you eliminate the possibility that a developer will misinterpret what members they must add to a type or how they will call another type that defines an interface. Without an interface, errors creep into the system and don't show up until runtime, when they are hard to find. With interfaces, errors in defining a type are caught immediately at compile time, where the cost is much less."

Answer 4

Couple of things, when you inherit from an interface it forces you to implement all the methods defined in the interface. For another, this is also a good way to bring in multiple inheritance which is not supported for regular classes. http://msdn.microsoft.com/en-us/library/ms173156.aspx