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

Object expressions

So as we saw in the previous post, implementing interfaces in F# is a bit more awkward than in C#. But F# has a trick up its sleeve, called “object expressions”.

With object expressions, you can implement an interface on-the-fly, without having to create a class.

Implementing interfaces with object expressions

Object expressions are most commonly used to implement interfaces. To do this, you use the syntax new MyInterface with ..., and the wrap the whole thing in curly braces (one of the few uses for them in F#!)

Here is some example code that creates a number of objects, each of which implements IDisposable.

// create a new object that implements IDisposable
let makeResource name =
   { new System.IDisposable
     with member this.Dispose() = printfn "%s disposed" name }

let useAndDisposeResources =
    use r1 = makeResource "first resource"
    printfn "using first resource"
    for i in [1..3] do
        let resourceName = sprintf "\tinner resource %d" i
        use temp = makeResource resourceName
        printfn "\tdo something with %s" resourceName
    use r2 = makeResource "second resource"
    printfn "using second resource"
    printfn "done."

If you execute this code, you will see the output below. You can see that Dispose() is indeed being called when the objects go out of scope.

using first resource
    do something with   inner resource 1
    inner resource 1 disposed
    do something with   inner resource 2
    inner resource 2 disposed
    do something with   inner resource 3
    inner resource 3 disposed
using second resource
second resource disposed
first resource disposed

We can take the same approach with the IAddingService and create one on the fly as well.

let makeAdder id =
   { new IAddingService with
     member this.Add x y =
         printfn "Adder%i is adding" id
         let result = x + y
         printfn "%i + %i = %i" x y result

let testAdders =
    for i in [1..3] do
        let adder = makeAdder i
        let result = adder.Add i i
        () //ignore result

Object expressions are extremely convenient, and can greatly reduce the number of classes you need to create if you are interacting with an interface heavy library.


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