Converting EPM operations to Tasks using the TPL


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The Event Programming Model (EPM from her on in) was introduced in .NET 2.0, it’s purpose was to serve as a simpler pattern for asynchronous operations than the Asynchronous Programming Model (APM / IAsyncResult, see my previous post on APM) where possible, mostly in UX code. Methods that use this pattern typically end in Async and have a Completion event.

The best known implementation of the EPM is the BackgroundWorker component, it’s got a distinct advantage in that it tries to use a synchronization context to fire the event on the thread from which it was called, the APM on the other hand offers no such guarantee.

Let’s see this in action (.net 4.0)


What you can see in the snippet above is a simple windows form (been a while my old friend) application. Let me paint you a picture, it’s early February 2012 and I’m stuck here at Brussels international airport, in the middle of a snow blizzard wondering if I’m going to have a flight home. The plane that will take me there is arriving in from Dublin so I’m looking at the live departures to see if it’s departed (already 15 mins late darn it.. ) anyway back to the post at hand; I’m downloading the page html with the call to DownloadStringAsync(), you can see in the completion event handler that I’m not doing any Invoking (dispatching to those of you that never had the pleasure of windows forms).

Now this is what it looks like after the event gets fired.


hey and looks like it’s running MS tech (notice that viewstate, incase the .aspx didn’t give it away!) nice! If you come from a web background this may not seem that odd to you, but if you started out desktop application development like me there was one golden rule you never forgot and that’s that always talk to the GUI in one thread and one thread only.

If the event handler wasn’t in the GUI thread above we would have received a cross thread exception like this:



Sadly TPL doesn’t handle the EPM as easily as the APM specifically in respect to the synchronization context, but lets see how we approach it, you may have to if you’re pre .net 4.0 as the DownloadStringAsync doesn’t exist!


With the code above we hit the cross-thread exception problem. We could do a Control.BeginInvoke (or Dispatcher.BeginInvoke in WPF), but lets imagine we were writing a library and we wanted it to be framework agnostic, how would we do this?

Actually it’s pretty simple, we just supply a context like this:


p.s. I got home at 4am Sad smile

Strategy pattern


So what is the strategy pattern? It’s one of the simplest object orientated design patterns, I find that it helps clean up day to day object orientated design. It’s purpose is to

  • Encapsulate a family of related algorithms such that they are callable through a common interface.
  • Independent evolution, algorithms can vary and evolve separately from classes using them.
  • Allow a class to serve a single purpose
  • Separates the calculation from the the delivery of it’s results. (separation of concerns)
How do we know when we should consider the strategy pattern?
  • Look for switch statements with possible common interface
  • Adding a new calculation to a class could break existing calculations (breaking the Open-Close principle, i.e. a class should be open for extension, but closed for modification.

UML – Strategy model



  • Strategies may not use class members from context
  • Tests may now be written for individual concrete strategies
  • Strategies may be mocked when testing the Context class
  • Adding a new Strategy does not modify the Context class

How to implement:

  • Class based
  • Functional programming approach with anonymous methods (Delegates and Funcs as opposed to new classes), I like this when the calculations are trivial
  • Property injection
  • Method strategy (passed to a method and not to the context class constructor)

Show me the code:

    Context class


    Here we see the strategy is getting passed to the context in the constructor, this class should be closed to modification, Trip is just an empty class for my demo and it’s not actually used in the calculation sample.

      Strategy interface

      Sample Strategy



      So, what is the strategy pattern again? It’s something you possibly do on a day to day basis and you don’t even realise it.

      e.g. If you write ASP.MVC code, you quite likely are passing interfaces to you controllers for dependency injection and testability ---> Strategy pattern.


    Converting APM operations to Tasks using the TPL


    Those of you have have already used .net 4.5 developer preview will know that tasks are becoming more common in the API, especially with the advent of the async await keywords.

    But many of you (including me) can’t really advocate .net 4.5 in the enterprise so what are our options should we like to use the TaskParallelLibrary?

    As you may be aware APM (Asynchronous Programming Model) was the original .NET mechanism for handling Async operations, it will be familiar to you as the IAsyncResult pattern.

    So lets take a common operation of reading from a stream, in .net 4.5 we already have a Stream.ReadAsync, but again what if we don’t have .net 4.5 at our disposal?

    The task parallel library helps bridge the gap with Task.Factory.FromAsync, here I place it in an extension method for ease of use.


    An invisible Azure Message


    When creating an Azure queue, you specify a lock duration, once a message is read from the queue it’s marked as invisible for other readers for a period of time, e.g. one minute.


    Choosing the invisibility time is a trade-off between expected processing time and application recovery time.

    When a message is dequeued, the application specifies the amount of time for which the message is invisible to workers dequeueing messages from the same queue. This time should be large enough to complete the operation specified by the queue message.

    If the timeout is too large, the time it takes to finish processing the message is affected when there are failures. For example, if the invisibility time is set at 30 minutes , and the application crashes after 10 minutes, the message will not have a chance of being started again for another 20 minutes.

    If the invisibility time is too small, the message may become visible when someone is still processing it. Thus, multiple workers could end up processing the same message, and one may not be able to delete the message from the queue (see the next section).

    The application could address this as follows

    1. If the amount of time to process a message is predictable, set the invisibility timeout large enough so that a message can be completed within that time.

    2. Sometimes the processing time for different types of messages may vary significantly. In that case, one can use separate queues for different types of messages, where messages in each queue take a similar amount of time to be processed. Appropriate invisibility timeout value can then be set to each queue.

    3. Furthermore, ensure that the operations performed on the messages are idempotent and resume-able. The following can be done to improve efficiency

    a. The processing should be stopped before the invisibility time is reached to avoid redundant work.

    b. The work for a message can be done in small chunks, where a small invisibility time may be sufficient. In this way, the next time the work is picked up from the queue after it becomes visible again, the work can be resumed from where it is left off.

    4. Finally, if the message invisibility time is too short and too many dequeued messages are becoming visible before they can be deleted, applications may want to dynamically change the invisibility time that is being set for new messages put onto the queues. This could be detected by counting at the worker roles how many times message deletes are failing due to messages becoming visible. Then based on a threshold communicate that back to the front-end web roles, so they can increase the invisibility time for new messages put into the queue if the invisibility time needs to be tuned.

    Manage the invisibility on the fly

    The “Update Message” REST API is used to extend the lease period (aka visibility timeout) and/or update the message content. A worker that is processing a message can now determine the extra processing time it needs based on the content of a message. The lease period, specified in seconds, must be >= 0 and is relative to the current time. 0 makes the message visible at that time in the queue as a candidate for processing. The maximum value for lease period is 7 days. Note, when updating the visibility timeout it can go beyond the expiry time (or time to live) that is defined when the message was added to the queue. But the expiry time will take precedence and the message will be deleted at that time from the queue.

    Azure Service Bus


    When communicating between roles in an Azure application we’ve a few options; to name a few:

    • Http
    • Tcp
    • Queues

    While Http and Tcp are tried and trusted they do come with some limitations that queues help overcome.

    In the last few months Microsoft have released pub/sub service bus to the world. This is similar to a basic queue, in the basic queue, each message is consumed by an individual consumer, but with subscription topics, multiple clients can consume the same message, each subscription logically maintains its own queue of messages.





    The diagram above shows a typical communication between worker roles and web roles on the Azure platform.

    As previously stated, this decoupling has several advantages over direct messaging.

    Load Leveling

    In the system the load can vary over time, where the amount of effort in processing the mid-tier business logic remains somewhat constant, with the queue in place it’s only necessary to have enough servers to handle the average load irrelevant of peak load. This can save money in terms of infrastructure required to handle peak load.

    Temporal Decoupling

    With queues decoupling the messaging effectively making the messaging async, publishers and subscriber need not be online at the same time, the service bus reliably stored the messages in the queue until the subscriber pulls them off and processes them. This allows different roles to be taken offline for maintenance etc.

    Load Balancing

    As load increases more worker roles can be added to service the queue (e.g. an online toy shop around the Christmas period). The system ensures that only one worker role will process the message, also in given that the worker roles are pulling the messages off the queue, they don’t have to be running on the same infrastructure, (Azure favours multiple low power roles in comparison the fewer higher powered roles).


    Migrate SqlServer DB to Azure Sql


    Here’s one way to migrate your SqlServer Database to the Azure platform.

    1) Get the SQL Azure Migration Wizard


    2) Start the wizard and select SQL Database Migrate option


    3) Select your source database


    4) Choose the objects you wish to migrate (all in my case)



    5) See the results and review the SQL Script if necessary.


    6) Now we need Sql Azure in the cloud for the next part, log into your account (get a 3 month free trial if you don’t have one)

    Select your Azure Server and create a new database.


    7) You’ll be prompted to select where you want your server located if you don’t already have one.




    8) Add some rules to your database, you’ll need to do this to allow access for MS Services and Visual Studio



    9) So now that you have a database in the cloud you’ll need to continue with your migration wizard by selecting this database.






    10) That’s pretty much it. Hope these screenshots helps someone out.

    Azure Tools


    This evening I decided I’d install the new Azure tools after watching the latest vids that have appeared.

    I right click on my MVC3 app and choose to: Add Windows Azure Deployment Project



    Then I hit F5 to run the project and I get an error

    Microsoft Visual Studio Unable to find file DFUI.exe  Baring teeth smile



    In the 1.5 SDK there used to be a registry key that pointed to the emulator, with 1.6 this no longer exists and Visual Studio is looking for the dfui.exe in a different location (use Process Monitor from to tell you where)


    Once you find where Visual is looking for it, it’s a matter of copying the files in
    C:\Program Files\Windows Azure Emulator\emulator\ to this location.

    Try run you app now and it should work.


    Synchronize you controllers when necessary

    Earlier today I happened to lend a hand to a friend of mine that was experiencing a race condition in an ASP.MVC application, like a rag to a bull is multithreading to me.

    Here’s the scenario; my friend was calling two web services using methods like BeginXXX/EndXXX. Because her website was IO bound she was correctly using an AsyncController.

    She called method to increment the outstanding operations by 2, then proceeded to call

    service1.BeginGetValuations(v, ar => {
        AsyncManager.Parameters["valuations"] = service1.EndGetValuations();
    }, null);
    service2.BeginGetValuations(v, ar => {
        AsyncManager.Parameters["valuationsActual"] = service2.EndGetValuations();


    Looked pretty much ok, except once in a while when load tested the valuationsActual parameter was null.
    So what could be the cause… Well basically it turned out that there was a race condition accessing the dictionary from two threads.

    The solution:

    synchronize access to the Parameters, i first thought of doing this with a plain old lock but I was worried about other access on the parameters from the framework itself so I had a quick read of the documentation and turns out that the AsyncManager has a sync method.

    service1.BeginGetValuations(v, ar => {
        AsyncManager.Sync(() => {
            AsyncManager.Parameters["valuations"] = service1.EndGetValuations();
    }, null);

    Do the same for service2.

    jQuery and a little bit of javascript


    I’m still working on the LiveResume website, just something I’m playing with in my free time, no I’m not looking for a job Smile just have an idea that I’ve not seen anywhere else (no I’m not telling you what it is until the site is live Winking smile )

    This little post is to demonstrate how effective the jQuery library is.

    Firstly lets have a look at the html to see what we are trying to achieve.


    Now, I’m not using jQuery.ui tabs (yet…); so I want to handle the styling of the active tab.

    I’ve got a style called highlight that does this.

    The code below shows how effective jQuery is at removing the style from all anchor elements and adding the style to the clicked anchor.


    Let’s break it down.

    The first two commented lines are to enable intellisense in visual studio.

    After this we create a variable menuHandler, I’m using the revealing module pattern for this.

    image the return function exposed the init function publically. I’ll be adding more functions to this menuHandler as I go along.


    image what I’m saying here is select all anchor elements that are children of the element with id=”topmenu” and add an event handler to the click event.

    image this line removes the highlight class from all the anchors in the same topmenu element.

    image this line adds the style to the clicked element.

    image this is somewhat equivalent to an onload event handler for the page (except it doesn’t wait for images… see jQuery docs); what I’m doing in the load handler is creating an instance of the menuHandler function and calling the init method on it.


    I can recommend the book: jQuery in action if you wish to get started or improve your jQuery.

    Full height divs in html5


    Hi all,


    This has probably been beaten to death elsewhere, but somehow it’s the first time I’ve ever come across it.

    I’m working on the LiveResume website this evening (hey it was this or watch XFactor.

    Anyway, I wanted to ensure that all different views of my web app filled the screen, for example if the personal details only occupied 30% of the height, I wanted to containing div to go the whole way to the bottom of the screen given it is a different color.

    This secret to this is ensuring that the parent has an explicit height or the div reverts to auto.

    So lets look at the simplified sample I’ve put together.


    <!DOCTYPE html>
    BODY {
         BACKGROUND:#ff0000; height:100%; 
    #one {
         BACKGROUND:#00ff00; height:100%; 
    #two {
         BACKGROUND:#0000ff; height:100%; 
        <div id="one">    
            <div id="two">

    This is what the rendered webpage looks like in IE9


    I was expecting the green section to be full height, I mean this always worked fine before.


    So what has happened

    html5 <!DOCTYPE html>

    requires a height on the html element too…and for whatever reason I never noticed this before.


    Add html to the first style