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Part of the "Object-oriented programming in F#" series (more)

College Watkins 2 Orange Jersey Sammy Stitched Limited Interfaces are available and fully supported in F#, but there are number of important ways in which their usage differs from what you might be used to in C#.

Defining interfaces

Defining an interface is similar to defining an abstract class. So similar, in fact, that you might easily get them confused.

Here’s an interface definition:

type MyInterface =
   // abstract method
   abstract member Add: int ->Cavaliers Jersey Guy Kyle College Stitched Basketball 5 White int -> int

   // abstract immutable property
   abstract member Pi : float 

   // abstract read/write property
   abstract member Area : float with getCollege Watkins 2 Orange Jersey Sammy Stitched Limited ,set

And here’s the definition for the equivalent abstract base class:

[<AbstractClass>]
type AbstractBaseClass() College Watkins 2 Orange Jersey Sammy Stitched Limited =
   College Watkins 2 Orange Jersey Sammy Stitched Limited // abstract method
   abstract member Add: int -> int -> int

   // abstract immutable property
   abstractArchie Stitched Jersey Limited Buckeyes College 45 White Griffin member Pi : floatCollege White Crimson Reggie Ragland Jersey Tide Stitched 19 

   // abstract read/write property
   abstract member Area : float with get,set

So what’s the difference? As usual, all abstract members are defined by signatures only. The only difference seems to be the lack of the [Jersey Horned Frogs Trevone 2 Stitched White Boykin College

When it comes time to implement an interface in a class, F# is quite different from C#. In C#, you can add a list of interfaces to the class definition and implement the interfaces implicitly.

Not so in F#. In F#, all interfaces must be explicitly implemented.

In an explicit interface implementation, the interface members can only be accessed through an interface instance (e.g. by casting the class to the interface type). The interface members are not visible as part of the class itself.

C# has support for both explicit and implicit interface implementations, but almost always, the implicit approach is used, and many programmers are not even aware of explicit interfaces in C#.

Implementing interfaces in F#

So, how do you implement an interface in F#? You cannot just “inherit” from it, as you would an abstract base class. You have to provide an explicit implementation for each interface member using the syntax interface XXX with, as shown below:

type IAddingService =
    abstract memberBasketball North Johnson Carolina College Brice White Stitched 11 Jersey Add: int -> int College Watkins 2 Orange Jersey Sammy Stitched Limited -> int

type MyAddingService() =
    
    interface IAddingService with 
        member this.Add x y = 
            x College Watkins 2 Orange Jersey Sammy Stitched Limited + y

    interface System.IDisposable with 
        member this.Dispose() = 
            printfn "disposed"

The above code shows how the class MyAddingService explicitly implements the College Watkins 2 Orange Jersey Sammy Stitched Limited IAddingService and the College Watkins 2 Orange Jersey Sammy Stitched Limited IDisposable interfaces. After the required interface XXX with section, the members are implemented in the normal way.

(As an aside, note again that MyAddingService() has a constructor, while IAddingService does not.)

Using interfaces

So now let’s try to use the adding service interface:

let mas = new MyAddingService()
mas.Add 1 2    // error 

Immediately, we run into an error. It appears that the instance does not implement the Add method at all. Of course, what this really means is that we must cast it to the interface first using the :> operator:

// cast to the interface
let mas = new MyAddingService()
College Watkins 2 Orange Jersey Sammy Stitched Limited let adder = mas :> IAddingService
adder.Add 1 2  // ok

College Watkins 2 Orange Jersey Sammy Stitched Limited This might seem incredibly awkward, but in practice it is not a problem as in most cases the casting is done implicitly for you.

For example, you will typically be passing an instance to a function that specifies an interface parameter. In this case, the casting is done automatically:

// function that requires an interface
let testAddingService (adder:IAddingService) = 
    printfn "1+2=%i" <| adderCollege Watkins 2 Orange Jersey Sammy Stitched Limited .Add 1 2  // ok

let College Watkins 2 Orange Jersey Sammy Stitched Limited masCase Stitched C-usa College Keenum White Patch Jersey Cougars 7 = new MyAddingService()
testAddingService mas // cast automatically

And in the special case of IDisposable, the use keyword will also automatically cast the instance as needed:

let College Watkins 2 Orange Jersey Sammy Stitched Limited testDispose = 
    use mas = new MyAddingService()
    printfn "testing"
    // Dispose() is called here

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