[RESOLVED] Can somebody explain to me why I should use interfaces?

[gonzalioz]

Hi all,

I have read some articles and tutorials about interfaces but I don't understand why you shoul use them. As far as I know they don't add any functionality? They can just be used to make sure you don't forget to add some property's or methods when creating a class.

Can somebody explain or provide me a link with a good example about when to use interfaces?

[Pino]

Hi all,

I have read some articles and tutorials about interfaces but I don't understand why you shoul use them. As far as I know they don't add any functionality? They can just be used to make sure you don't forget to add some property's or methods when creating a class.

Can somebody explain or provide me a link with a good example about when to use interfaces?

You have covered some of the key points about interfaces. Have a look at the link below for some more info.

http://www.daniweb.com/forums/thread114364.html#

[NickThissen]

The true power of interfaces is being able to do this for example:

Public Sub DisplayAllElements(ByVal col As IEnumerable)
        For Each obj As Object In col
            MessageBox.Show(obj.ToString)
        Next
    End Sub

The method accepts an argument of type IEnumerable. Actually, what this means is that it will accept any type that implements IEnumerable. As such, it will accept arrays, ArrayLists, generic Lists, collections, etc.
This makes interfaces very useful sometimes, because you don't have to convert a List to an array (for example), as both are accepted. As long as the method does nothing more than iterate through the list, it does not need to know whether the argument is an array, list, collection, etc.


Basically, an interface can be used to tell 'the compiler' (and you, really) that some unknown instance of some unknown class must at least implement a few methods or properties.


I was in the situation just yesterday where I needed an interface. I am building a 'database engine' that works with 'entity classes'. An entity class is just a class with a property for every database attribute for some table.
The idea is that I can add a new entity class at a later stage, so that my engine can work with it. This is obviously not going to work by itself, because the engine (written now) cannot know anything about the entity class (not written yet). So, I have my entity class implement an IEntity interface which dictates it to have at least an Id property.
Now, the engine still doesn't know which class it is dealing with, but it does know that, whatever class it is, it has a property Id, so I can use that property.

[gonzalioz]

Thanks guys! That's brilliant.