Phillip Trelford's Array

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Exposing F# Dynamic Lookup to C#, WPF & Silverlight

Like C#, F# is primarily a statically typed programming language. That said both languages provide support for dynamic typing.

Sometimes dynamic typing can be a pragmatic way of solving a specific problem. For example say you’re a solutions provider with a core product and have a number of clients with bespoke requirements. One client asks for a product rating feature. This can be relatively easily achieved using dynamic properties:

  • a bunch of client specific properties are read from the database, including the rating value, which is then set as a dynamic property on the product object
  • zero changes are required to the core code
  • at the UI, on WPF, the dynamic property can simply be bound through XAML like any other object property
  • on Silverlight 4,0 direct binding is not currently possible, however there is a simple workaround - specifying the dynamic property as a parameter for a value converter (example later)

C# and F# use slightly different approaches for dynamic properties:

  • C# 4.0 provides a dynamic type, which tells the compiler that member lookup on the object should be deferred to run time
  • F# employs a dynamic lookup operator, which when overloaded defines the behaviour at runtime. This means that in F# dynamic lookup is explicit (the ? operator), and can be mixed with static lookup on the same object (the . operator)

Is it is still possible to implement dynamic properties in F# that can be consumed by other .Net languages like C#, VB.Net, IronRuby or IronPython; plus WPF and Silverlight. The trick is to inherit from .Net 4.0’s System.Dynamic.DynamicObject type and implement the System.ComponentModel.INotifyPropertyChanged interface:

open System.Dynamic

/// Dynamic Lookup type
type DynamicLookup () =
    inherit DynamicObject ()
    /// Synchronization object
    let sync = obj()
    /// Property Changed event
    let propertyChanged = Event<_,_>()    
    /// Properties
    let mutable properties = Map.empty
    /// Gets property value
    member private this.GetValue name = 
        Map.tryFind name properties
    /// Sets property value, creating a new property if none exists
    member private this.SetValue (name,value) =
        /// Atomically writes new properties reference
        let Write () = 
            properties <-
                properties 
                |> Map.remove name 
                |> Map.add name value
        // Synchronize property writes
        lock sync Write
        // Trigger property changed event
        (this,System.ComponentModel.PropertyChangedEventArgs(name))
        |> propertyChanged.Trigger    
    override this.TryGetMember(binder:GetMemberBinder,result:obj byref ) =     
        match this.GetValue binder.Name with
        | Some value -> result <- value; true
        | None -> false
    override this.TrySetMember(binder:SetMemberBinder, value:obj) =        
        this.SetValue(binder.Name,value)
        true
    override this.GetDynamicMemberNames() =
        properties |> Seq.map (fun pair -> pair.Key)
    [<CLIEvent>]
    member this.PropertyChanged = propertyChanged.Publish
    interface System.ComponentModel.INotifyPropertyChanged with
        [<CLIEvent>]
        member this.PropertyChanged = propertyChanged.Publish     
    static member (?) (lookup:#DynamicLookup,name:string) =
        match lookup.GetValue name with
        | Some(value) -> value
        | None -> raise (new System.MemberAccessException())        
    static member (?<-) (lookup:#DynamicLookup,name:string,value:'v) =
        lookup.SetValue (name,value)
    static member GetValue (lookup:DynamicLookup,name) =
        lookup.GetValue(name).Value

F# usage:

/// Product type inherits dynamic lookup
type Product (name,price) =
    inherit DynamicLookup ()
    member this.Name = name
    member this.Price = price
 
// Initiate product object with dynamic rating value
let p = Product("F# for Scientists",49.95M)
do p?Stars <- 5
// Access product's properties
let stars = System.Convert.ToInt32(p?Stars)
do printf "%s...%M %s" p.Name p.Price (System.String('*',stars))

C# usage:

// Create Product type with dynamic stars value
Product product = new Product("Expert F#",54.99M);
((dynamic)product).Stars = 5;       
// Read product properties
dynamic p = product;
string s = 
    string.Format("{0}...{1} {2}",
        p.Name, p.Price, new String('*', (int) p.Stars));

Use from Silverlight 4.0

<t:PropertyLookup x:Key="DynamicConverter"/>

 

<TextBox Text="{Binding Converter={StaticResource DynamicConverter},
                ConverterParameter=Stars}"/>

 

dynamic product = new Product("Real World FP", 35.99);
product.Stars = 5;
DataContext = product;

 

public class PropertyLookup : IValueConverter
{
    public object Convert(
        object value, 
        Type targetType, 
        object parameter, 
        CultureInfo culture)
    {        
        return DynamicLookup.GetValue(
            (DynamicLookup) value,
            (string) parameter);            
    }

    public object ConvertBack(object value, 
        Type targetType, 
        object parameter, 
        CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Note: Silverlight 4.0 requires Microsoft.CSharp.dll to use dynamic types.

MVMMC – MVVM grows a Controller

Ray Booysen does it again! The first time I saw Ray do a talk was at NxtGen in Cambridge, when he gave me a timely introduction to MVVM just as I was starting to play with WPF. MVVM (Model-View-View Model) is an architectural pattern, that improves on code behind, making it possible to unit test your UI code (View Model), a good thing. Over a year later and I’m just starting to play with Silverlight, Ray delivers another timely talk on Silverlight gotchas at the Edge UG in London, based on his real-world development experience (developing Financial applications). This time he introduces among other good things MVVMC (MVVM plus Controller). Here the controller takes responsibility for communication between the Model and View Model, leaving the View Model with the single responsibility of providing data to the View.

Somewhat shell shocked I was left with a few questions on implementation details, again Ray to the rescue via Twitter:

ptrelford Excellent @raybooysen Silverlight gotchas talk - MVVMC (C for controller), Network is on UI thread, browser connection limits #edgeug

ptrelford @raybooysen really liked Fat Controller slide, of Thomas the Tank Engine; controller takes responsiblity for marshalling threads etc #edgeug

raybooysen @ptrelford fat controller should be in all slide decks. :)

raybooysen Talk finished at @edge_ug. Think it went ok. Feel like I rambled a little.

ptrelford So with MVVMC the Model is observed by a Controller, which constructs & updates a View Model, which the (XAML) View binds to

raybooysen @ptrelford thanks for coming tonight. :)

ptrelford @raybooysen using MVVMC the Controller observes the Model & updates View Model, which observes user input and passes messages to the Model?

raybooysen @ptrelford viewmodel observes and notifies the the controller.

ptrelford @raybooysen thanks! So View Model takes a reference to the parent controller?

raybooysen @ptrelford no. Can surface the ui interaction via events. Or use rx if you're in for some fun.

ptrelford @raybooysen Cool thanks again! That makes sense, View Model exposes IObservable<T> properties. BTW your talk blew me away (hence questions)!

raybooysen @ptrelford no worries. Iobservable feels quite right in these scenarios. Since controller essentially owns the viewmodel

Finally, this was the same talk that Ray delivered at DDD Scotland, so the slides are already available here

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F# Agents and Retlang: quick comparison

Erlang style message passing is a great way of simplifying concurrency. The open source Retlang library aims to bring some of the benefits to .Net languages like C# and VB.Net. Whereas F# has built-in message passing support with the MailboxProcessor also referred to as Agents. To compare Retlang against F# Agents I have coded up the Retlang Summation example (171 lines) as an equivalent F# implementation (41 lines):

type Agent<'a> = MailboxProcessor<'a>

type Summation =
    | Add of int
    | Total of AsyncReplyChannel<int>    
and SummationAgent () =
    let agent = Agent.Start ( fun inbox ->    
        let rec loop total =
            async {
            let! message = inbox.Receive()
            match message with 
            | Add n -> do! loop (n + total)
            | Total reply -> reply.Reply(total)
            }
        loop 0
    )    
    /// Adds value to total
    member this.Add n = Add(n) |> agent.Post
    /// Returns total and ends computation
    member this.Total () = (fun reply -> Total(reply)) |> agent.PostAndReply

/// Invokes specified function with numbers from 1 to limit
let numberSource f limit =
    async {
        for i = 1 to limit do
            f i            
            if i % 10 = 0 then System.Console.WriteLine("{0}\t({1})",i,limit)        
    }

do  /// Summation agent instance
    let agent = SummationAgent ()    
    // Post series of numbers to summation agent in parallel
    [100;50;200]
    |> Seq.map (numberSource agent.Add)
    |> Async.Parallel
    |> Async.RunSynchronously
    |> ignore    
    // Get total
    let value = agent.Total ()
    System.Diagnostics.Debug.Assert(26425 = value);
    value |> System.Console.WriteLine

 

If you compare this to the Retlang example implementation, it should be clear that C# is missing the easy message definition (discriminated unions) and pattern matching part of message passing. You may also notice that in the F# version, the functionality is accomplished, not only without locks, but also without any low-level threading primitives (like AutoResetEvent). However, if you find yourself constrained to C# development then Retlang is definitely worth a look.