Neat Samples: Extend your F# program with MEF - Sharp Things - Site Home

Jomo Fisher—The Managed Extensibility Framework is an interesting new technology in .NET 4.0. It lets you set up a plug in system for your application so that your program can acquire new functionality just by, for example, adding a .dll into a particular directory.

This is a simple example in F#. This code sets up MEF hosting and asks for all extensions in the c:\extensions folder.

Code Snippet
 // Host.fs
// A MEF host that eats tasty treats
open System.Reflection
open System.IO
open System.ComponentModel.Composition
open System.ComponentModel.Composition.Hosting
 
// Expose an interface
type ITastyTreat =
    abstract Description : string
 
// Set up MEF
let catalog = new AggregateCatalog()
let directoryCatalog = new DirectoryCatalog(@"c:\Extensions","*.dll")
let container = new CompositionContainer(catalog)
catalog.Catalogs.Add(directoryCatalog)
 
// Jar that will contain tasty treats
type TreatJar() = 
    [<ImportMany(typeof<ITastyTreat>)>] 
    let cookies : seq<ITastyTreat> = null
 
    member __.EatTreats() =
        cookies |> Seq.iter(fun tt->printfn "Yum, it was a %s" tt.Description)
 
let jar = TreatJar()
container.ComposeParts(jar)
jar.EatTreats()
 

This is how you implement an extension. Just build this into a .dll and drop it in the c:\extensions directory.

Code Snippet
 namespace ChocolateCookie
open Host
open System.ComponentModel.Composition
 
[<Export(typeof<ITastyTreat>)>]
type ChocolateCookie() = 
    interface ITastyTreat with 
        member __.Description = "a delicious chocolate cookie"
 