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

Blue North 5 Stitched Carolina Basketball College Marcus Jersey 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 ->Golden College Jersey Aaron Bears Stitched Blue Rodgers Premier 8 Navy int -> int

   // abstract immutable property
   abstract member Pi : float 

   // abstract read/write property
   abstract member Area : float with getBlue North 5 Stitched Carolina Basketball College Marcus Jersey ,set

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

[<AbstractClass>]
type AbstractBaseClass() Blue North 5 Stitched Carolina Basketball College Marcus Jersey =
   Blue North 5 Stitched Carolina Basketball College Marcus Jersey // abstract method
   abstract member Add: int -> int -> int

   // abstract immutable property
   abstractPeyton Jersey Basketball White Cardinals Stitched College 3 Siva member Pi : floatBlue College Ian 12 Book Navy Jersey Limited Irish Stitched Fighting 

   // 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 member20 College Basketball Markelle Fultz Jersey White Huskies Stitched Add: int -> int Blue North 5 Stitched Carolina Basketball College Marcus Jersey -> int

type MyAddingService() =
    
    interface IAddingService with 
        member this.Add x y = 
            x Blue North 5 Stitched Carolina Basketball College Marcus Jersey + y

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

The above code shows how the class MyAddingService explicitly implements the Blue North 5 Stitched Carolina Basketball College Marcus Jersey IAddingService and the Blue North 5 Stitched Carolina Basketball College Marcus Jersey 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()
Blue North 5 Stitched Carolina Basketball College Marcus Jersey let adder = mas :> IAddingService
adder.Add 1 2  // ok

Blue North 5 Stitched Carolina Basketball College Marcus Jersey 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" <| adderBlue North 5 Stitched Carolina Basketball College Marcus Jersey .Add 1 2  // ok

let Blue North 5 Stitched Carolina Basketball College Marcus Jersey masCollege Limited Tigers Purple Stitched Lawrence Trevor Jersey 16 = 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 Blue North 5 Stitched Carolina Basketball College Marcus Jersey testDispose = 
    use mas = new MyAddingService()
    printfn "testing"
    // Dispose() is called here

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