One thing that comes to mind is a more advanced property system. WPF introduced a notion of DependencyProperty which has proved its success. But.. can C# support something similar natively – properties which could store their states and dynamic meta data in addition to plain values? Why not?

Consider the following code sample:

// A product type in this sample has a property Name of type String
var product = new Product();

// This will print "Product.Name.IsDirty: False"
Console.WriteLine("Product.Name.IsDirty: {0}", product.Name.IsDirty);

// Subscribe to an event
product.Name.BeforeChange += (s, e) => {
		if (e.NewValue == "C++") { e.Cancel = true; }
	};

// Now let's try to update product's Name property
product.Name = "C# 4.0 in a Nutshell";

// And now this will print "Product.Name.IsDirty: True"
Console.WriteLine("Product.Name.IsDirty: {0}", product.Name.IsDirty);

// Then when you try to save this product in database, it will be easier
// for your Store Repository to decide which fields must be saved
// and which can be skipped
storeRepository.Add(product);
storeRepository.SubmitChanges();

So far so good? But.. how this Product class is implemented? Actually not a magic.

Here is one of possible implementations – Codename “Properties inside properties” or “Nested Properties” in short:

public interface IProduct
{
	string Name { get; set; bool IsDirty { get; } }
}

public class Product : IProduct
{
	private string name;
	private bool nameIsDirty;

	public string Name
	{
		get { return name; }
		set { if (name != value) { name = value; nameIsDirty = true; } }
		bool IsDirty { get { return nameIsDirty; } }
	}
}

Happy coding!

P.S.: To tell you the truth, this is not the brightest example of what to expect in the next version of C#. But if this blog post motivated you to creative thinking – then my goal is completed. Don’t be afraid to innovate; be different!

Teaching Programming Language Concepts with F#, Part 2
In this second part, Peter finishes the first “demo” lecture of the F#-based programming language course.

Pixel Fonts for Silverlight and WPF at CodePlex

Objectives

• It should work asynchronously
  The executing thread shouldn’t be blocked during download so we could run this operation even on a UI thread without locking it
• Stock quotes should be downloaded in parallel
  But no more than 5 download streams at any particular moment in time
• Downloaded quotes should be parsed in parallel
  In order to utilize the available hardware resources efficiently
• Log actions & exceptions
  In this example we will log events by using printfn

Implementation

open System
open System.Net
open System.Web
open Microsoft.FSharp.Collections

/// Stock price quote record
type Quote = {
    Symbol : string
    Date   : DateTime
    Open   : float
    High   : float
    Low    : float
    Close  : float
    Volume : int64
}

/// Stock prices download manager
type QuotesDownloader() =

    // en-US CultureInfo used for data parsing
    let ci = System.Globalization.CultureInfo.GetCultureInfo("en-US")

    /// Download quotes for a given symbol
    member this.downloadQuotesAsync (symbol : string) = async {
        try
            let tickerUri = Uri(@"http://www.google.com/finance/historical?q=" + HttpUtility.UrlEncode(symbol) + "&output=csv")
            use webClient = new WebClient()
            let! csv = webClient.AsyncDownloadString(tickerUri)
            try
                let quotes =
                    csv.Split([|'\n'|], StringSplitOptions.RemoveEmptyEntries)
                    |> Seq.skip 1   // skip header
                    |> PSeq.map (fun line   -> line.Split [|','|])
                    |> PSeq.map (fun values ->
                    {
                        Symbol  = symbol
                        Date    = DateTime.Parse (values.[0], ci)
                        Open    = Double.Parse   (values.[1], ci)
                        High    = Double.Parse   (values.[2], ci)
                        Low     = Double.Parse   (values.[3], ci)
                        Close   = Double.Parse   (values.[4], ci)
                        Volume  = Int64.Parse    (values.[5], ci)
                    })
                printfn "Downloaded a list of quotes for %s symbol." symbol
                return Some(quotes)
            with
                ex ->
                    printfn "Failed to parse quotes for %s symbol. %s" symbol ex.Message
                    return None
        with
            ex ->
                printfn "Failed to download quotes for %s symbol. %s" symbol ex.Message
                return None }

    member this.worker n f =
        MailboxProcessor<string>.Start(fun inbox ->
            let workers =
                Array.init n (fun i -> MailboxProcessor<string>.Start(f))
            let rec loop i = async {
                let! msg = inbox.Receive()
                workers.[i].Post(msg)
                return! loop ((i + 1) % n)
            }
            loop 0
        )

    member this.agent =
        this.worker 5 (fun inbox ->
            let rec loop() = async {
                let! msg = inbox.Receive()
                let! quote = this.downloadQuotesAsync msg
                return! loop()
            }
            loop()
        )

    /// Download quotes for a list of simbos
    member this.downloadQuotes (symbols : string list) =
        for symbol in symbols do
            this.agent.Post symbol

// Let's execute it
let quotesDownloader = QuotesDownloader()
quotesDownloader.downloadQuotes [ "MSFT"; "GOOG";
    "AAPL"; "T"; "CVX"; "XOM"; "GE"; "HBC"; "IBM"; "JNJ"; "JPM";
    "PG"; "WMT"; "MMM"; "ABT"; "ACN"; "MO"; "AMZN"; "AXP"; "APA";
    "BLK"; "CAT"; "CVX"; "CSCO"; "CVS"; "DELL"; "DVN"; "FDX" ]

See also

• Records (F#) at MSDN
• Computation Expressions (F#) at MSDN
• Asynchronous Workflows (F#) at MSDN
• Async and Parallel Design Patterns in F#: Agents by Don Syme
• The F# Power Pack at CodePlex

Download Source Codes