Need Help understanding Interfaces

Question

Hi

I am still having trouble understanding what interfaces are good for. I read a few tutorials and I still don't know what they really are for other then "they make your classes keep promises" and "they help with multiple inheritance".

Thats about it. I still don't know when I would even use an interface in a real work example or even when to identify when to use it.

From my limited knowledge of interfaces they can help because if something implements it then you can just pass the interface in allowing to pass in like different classes without worrying about it not being the right parameter.

But I never know what the real point of this since they usually stop short at this point from showing what the code would do after it passes the interface and if they sort of do it it seems like they don't do anything useful that I could look at and go "wow they would help in a real world example".

So what I guess I am saying is I am trying to find a real world example where I can see interfaces in action.

I also don't understand that you can do like a reference to an object like this:

ICalculator myInterface = new JustSomeClass();

So now if I would go myInterface dot and intellisense would pull up I would only see the interface methods and not the other methods in JustSomeClass. So I don't see a point to this yet.

Also I started to do unit testing where they seem to love to use interfaces but I still don't understand why.

Like for instance this example:

public AuthenticationController(IFormsAuthentication formsAuth)
{
    FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();
}

public class FormsAuthenticationWrapper : IFormsAuthentication
{
    public void SetAuthCookie(string userName, bool createPersistentCookie)
    {
        FormsAuthentication.SetAuthCookie(userName, createPersistentCookie);
    }
    public void SignOut()
    {
        FormsAuthentication.SignOut();
    }

}

public IFormsAuthentication FormsAuth
{
    get;
    set;
}

Like why bother making this interface? Why not just make FormsAuthenticationWrapper with the methods in it and call it a day? Why First make an interface then have the Wrapper implement the interface and then finally write the methods?

Then I don't get what the statement is really saying.

Like I know know that the statement is saying this

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

if formsAuth is null then make a new FormsAuthenticationWrapper and then assign it to the property that is an Interface.

I guess it goes back to the whole point of why the reference thing. Especially in this case since all the methods are exactly the same. The Wrapper does not have any new methods that the interface does not have and I am not sure but when you do this the methods are filled right(ie they have a body) they don't get converted to stubs because that would really seem pointless to me(it it would be converted back to an interface).

Then in the testing file they have:

var formsAuthenticationMock = new Mock<AuthenticationController.IFormsAuthentication>();

So they just pass in the FormsAuthentication what I am guessing makes all the fake stubs. I am guessing the wrapper class is used when the program is actually running since it has real methods that do something(like sign a person out).

But looking at new Mock(from moq) it accepts a class or an interface. Why not just again made the wrapper class put those methods in and then in the new Mock call that?

Would that not just make the stubs for you?

Thanks

Answer 1

Perhaps the best way to get a good understanding of interfaces is to use an example from the .NET framework.

Consider the following function:

void printValues(IEnumerable sequence)
{
    foreach (var value in sequence)
     Console.WriteLine(value);
}

Now I could have written this function to accept a List<T>, object[], or any other type of concrete sequence. But since I have written this function to accept a parameter of type IEnumerable that means that I can pass any concrete type into this function that implements the IEnumerable interface.

The reason this is desirable is that by using the interface type your function is more flexible than it would otherwise be. Also you are increasing the utility of this function as many different callers will be able to make use of it without requiring modification.

By using an interface type you are able to declare the type of your parameter as a contract of what you need from whatever concrete type is passed in. In my example I don't care what type you pass me, I just care that I can iterate it.

Answer 2

Interfaces define contracts.

In the example you provide, the ?? operator just provides a default value if you pass null to the constructor and doesn't really have anything to do with interfaces.

What is more relevant is that you might use an actual FormsAuthenticationWrapper object, but you can also implement your own IFormsAuthentication type that has nothing to do with the wrapper class at all. The interface tells you what methods and properties you need to implement to fulfill the contract, and allows the compiler to verify that your object really does honor that contract (to some extent - it's simple to honor a contract in name, but not in spirit), and so you don't have to use the pre-built FormsAuthenticationWrapper if you don't want to. You can build a different class that works completely differently but still honors the required contract.

In this respect interfaces are much like normal inheritance, with one important difference. In C# a class can only inherit from one type but can implement many interfaces. So interfaces allow you to fulfill multiple contracts in one class. An object can be an IFormsAuthentication object and also be something else, like IEnumerable.

Interfaces are even more useful when you look at it from the other direction: they allow you to treat many different types as if they were all the same. A good example of this is with the various collections classes. Take this code sample:

void OutputValues(string[] values)
{
   foreach (string value in values)
   {
       Console.Writeline(value);
   }
}

This accepts an array and outputs it to the console. Now apply this simple change to use an interface:

void OutputValues(IEnumerable<string> values)
{
   foreach (string value in values)
   {
       Console.Writeline(value);
   }
}

This code still takes an array and outputs it to the console. But it also takes a List<string> or anything else you care to give it that implements IEnumerable<string>. So we've taken an interface and used it to make a simple block of code much more powerful.

Another good example is the ASP.Net membership provider. You tell ASP.Net that you honor the membership contract by implementing the required interfaces. Now you can easily customize the built-in ASP.Net authentication to use any source, and all thanks to interfaces. The data providers in the System.Data namespace work in a similar fashion.

One final note: when I see an interface with a "default" wrapper implementation like that, I consider it a bit of an anit-pattern, or at least a code smell. It indicates to me that maybe the interface is too complicated, and you either need to split it apart or consider using some combination of composition + events + delegates rather than derivation to accomplish the same thing.

Answer 3

We use interfaces (or abstract base classes) to allow polymorphism, which is a very central concept in object-oriented programming. It allows us to compose behavior in very flexible ways. If you haven't already, you should read Design Patterns - it contains numerous examples of using interfaces.

In relation to Test Doubles (such as Mock objects), we use interfaces to be able to remove functionality that we currently don't want to test, or that can't work from within a unit testing framework.

Particularly when working with web development, a lot of the services that we rely on (such as the HTTP Context) isn't available when the code executes outside of the web context, but if we hide that functionality behind an interface, we can replace it with something else during testing.

Answer 4

Ok, I had a hard time understanding too at first, so don't worry about it.

Think about this, if you have a class, that lets say is a video game character.

public class Character
{
}

Now say I want to have the Character have a weapon. I could use an interface to determin the methods required by a weapon:

interface IWeapon
{
    public Use();
}

So lets give the Character a weapon:

public class Character
{
    IWeapon weapon;

    public void GiveWeapon(IWeapon weapon)
    {
        this.weapon = weapon;
    }

    public void UseWeapon()
    {
        weapon.Use();
    }
}

Now we can create weapons that use the IWeapon interface and we can give them to any character class and that class can use the item.

public class Gun : IWeapon
{
    public void Use()
    {
        Console.Writeline("Weapon Fired");
    }
}

Then you can stick it together:

Character bob = new character();
Gun pistol = new Gun();
bob.GiveWeapon(pistol);
bob.UseWeapon();

Now this is a general example, but it gives a lot of power. You can read about this more if you look up the Strategy Pattern.

Answer 5

The way I understood it was:

Derivation is 'is-a' relationship e.g., A Dog is an Animal, A Cow is an Animal but an interface is never derived, it is implemented.

So, interface is a 'can-be' relationship e.g., A Dog can be a Spy Dog, A Dog can be a Circus Dog etc. But to achieve this, a dog has to learn some specific things. Which in OO terminology means that your class has to able to do some specific things (contract as they call it) if it implements an interface. e.g., if your class implements IEnumerable, it clearly means that your class has (must have) such a functionality that it's objects can be Enumerated.

So, in essence, through Interface Implementation a Class exposes a functionality to its users that it can do something and it is NOT inheritance.

Answer 6

ICalculator myInterface = new JustSomeClass();
JustSomeClass myObject = (JustSomeClass) myInterface;

Now you have both "interfaces" to work with on the object.

I am pretty new to this too, but I like to think of interfaces as buttons on a remote control. When using the ICalculator interface, you only have access to the buttons (functionality) intended by the interface designer. When using the JustSomeClass object reference, you have another set of buttons. But they both point to the same object.

There are many reasons to do this. The one that has been most useful to me is communication between co-workers. If they can agree on an interface (buttons which will be pushed), then one developer can work on implementing the button's functionality and another can write code that uses the buttons.

Hope this helps.

Answer 7

With almost everything written about interfaces, let me have a shot.

In simple terms, interface is something which will relate two or more , otherwise, non related classes.

Interfaces define contract which ensures that any two or more classes, even if not completely related, happens to implement a common interface, will contain a common set of operations.

Combined with the support of polymorphism , one can use interfaces to write cleaner and dynamic code.

eg.

Interface livingBeings

-- speak() // says anybody who IS a livingBeing need to define how they speak

class dog implements livingBeings

--speak(){bark;} // implementation of speak as a dog

class bird implements livingBeings

--speak(){chirp;}// implementation of speak as a bird

Answer 8

I'll give you an example below but let me start with one of your statements. "I don't know how to identify when to use one". to put it on edge. You don't need to identify when to use it but when not to use it. Any parameter (at least to public methods), any (public) property (and personally I would actually extend the list to and anything else) should be declared as something of an interface not a specific class. The only time I would ever declare something of a specific type would be when there was no suitable interface.

I'd go

IEnumerable<T> sequence;

when ever I can and hardly ever (the only case I can think off is if I really needed the ForEach method)

List<T> sequence;

and now an example. Let's say you are building a sytem that can compare prices on cars and computers. Each is displayed in a list.

The car prices are datamined from a set of websites and the computer prices from a set of services.

a solution could be: create one web page, say with a datagrid and Dependency Injection of a IDataRetriever (where IDataRetriver is some interface making data fetching available with out you having to know where the data came from (DB,XML,web services or ...) or how they were fetched (data mined, SQL Quering in house data or read from file).

Since the two scenarios we have have nothing but the usage in common a super class will make little sense. but the page using our two classes (one for cars and one for computers) needs to perform the exact same operations in both cases to make that possible we need to tell the page (compiler) which operations are possible. We do that by means of an interface and then the two classes implement that interfcae.

using dependency injection has nothing to do with when or how to use interfaces but the reason why I included it is another common scenario where interfaces makes you life easier. Testing. if you use injection and interfaces you can easily substitute a production class for a testing class when testing. (This again could be to switch data stores or to enforce an error that might be very hard to produce in release code, say a race condition)

Answer 9

All of the answers here have been helpful and I doubt I can add anything new to the mix but in reading the answers here, two of the concepts mentioned in two different answers really meshed well in my head so I will compose my understanding here in the hopes that it might help you.

A class has methods and properties and each of the methods and properties of a class has a signature and a body

public int Add(int x, int y)
{
return x + y;
}

The signature of the Add method is everything before the first curly brace character

public int Add(int x, int y)

The purpose of the method signature is to assign a name to a method and also to describe it's protection level (public, protected, internal, private and / or virtual) which defines where a method can be accessed from in code

The signature also defines the type of the value returned by the method, the Add method above returns an int, and the arguments a method expects to have passed to it by callers

Methods are generally considered to be something an object can do, the example above implies that the class the method is defined in works with numbers

The method body describes precisly (in code) how it is that an object performs the action described by the method name. In the example above the add method works by applying the addition operator to it's parameters and returing the result.

One of the primary differences between an interface and a class in terms of language syntax is that an interface can only define the signature of a methd, never the method body.

Put another way, an interface can describe in a the actions (methods) of a class, but it must never describe how an action is to be performed.

Now that you hopefully have a better understanding of what an interface is, we can move on to the second and third parts of your question when, and why would we use an interface in a real program.

One of the main times interfaces are used in a program is when one wants to perform an action, without wanting to know, or be tied to the specifics of how those actions are performed.

That is a very abstract concept to grapsp so perhaps an example might help to firm things up in your mind

Imagine you are the author of a very popular web browser that millions of people use every day and you have thousands of feature requests from people, some big, some little, some good and some like "bring back <maquee> and <blink> support".

Because you only have a relitivly small number of developers, and an even smaller number of hours in the day, you can't possibly implement every requested feature yourself, but you still want to satisfy your customers

So you decide to allow users to develop their own plugins, so they can <blink 'till the cows come home.

To implement this you might come up with a plugin class that looks like:

public class Plugin
{
public void Run (PluginHost browser)
{
//do stuff here....
}
}

But how could you reasonably implement that method? You can't possibly know precisly how every poossible future plugin is going to work

One possible way around this is to define Plugin as an interface and have the browser refer to each plugin using that, like this:

public interface IPlugin
{
void Run(PluginHost browser);
}

public class PluginHost
{
public void RunPlugins (IPlugin[] plugins)
{
foreach plugin in plugins
{
plugin.Run(this);
}
}
}

Note that as discussed earlier the IPlugin interface describes the Run method but does not specify how Run does it's job because this is specific to each plugin, we don't want the plugin host concerned with the specifics of each individual plugin.

To demonstrate the "can-be-a" aspect of the relationship between a class and an interface I will write a plugin for the plugin host below that implements the <blink> tag.

public class BlinkPlugin: IPlugin
{
private void MakeTextBlink(string text)
{
//code to make text blink.
}
public void Run(PluginHost browser)
{
MakeTextBlink(browser.CurrentPage.ParsedHtml);
}
}

From this perspective you can see that the plugin is defined in a class named BlinkPlugin but because it also implements the IPlugin interface it can also be refered to as an IPlugin object,as the PluginHost class above does, because it doesn't know or care what type the class actually is, just that it can be an IPlugin

I hope this has helped, I really didnt intend it to be quite this long.