Delay's Blog


February, 2011

  • Delay's Blog

    Candy-eating lemming [Delay.Web.Helpers (and its easy-to-use Amazon S3 blob/bucket API) is now available via NuGet!]

    NuGet is all the rage right now, and it seemed like a good time to familiarize myself with it. Conveniently, it just so happened that I had something perfectly suited: my Delay.Web.Helpers assembly which offers ASP.NET web helper-style access to Amazon Simple Storage Service (S3) blobs and buckets. (For context, here's the introductory post for Delay.Web.Helpers and here's a follow-up discussing the addition of container support.)

    I'm pleased to say I've just published the following two packages to the public NuGet repository:


    Delay.Web.Helpers.SampleWebSite folder in Solution Explorer All you need to install to get going is the Delay.Web.Helpers package - that'll get you the binaries and full IntelliSense, too! If you also want some samples to check out, please install the Delay.Web.Helpers.SampleWebSite package.

    Because I don't like packages that add random files to my projects, I've configured the Delay.Web.Helpers.SampleWebSite package to install all its sample code under a same-named folder where it won't get mixed up with anyone's existing content. [You're welcome. :) ] I've tweaked the sample to run cleanly in this configuration, so all you need to do is right-click the Delay.Web.Helpers.SampleWebSite folder in Visual Studio's Solution Explorer and choose View in Browser to see it in action.

    The sample application lets you browse a previously-configured Amazon S3 account, add, view, and remove buckets, and add, view, and remove files within those buckets. It's not the most fully-featured S3 browser ever, but it does a pretty good job showing off the complete AmazonS3Storage API.

    Update at 1:50pm: The sample uses CSHTML (Razor) web pages, so you'll want to add it to a project that supports that file type. From Visual Studio, choose File, New, Web Site..., "ASP.NET Web Site (Razor)". If the resulting web site doesn't run by default due to a missing SQL dependency (i.e., before you add the samples), just comment-out the call to WebSecurity.InitializeDatabaseConnection in _AppStart.cshtml. And if the Razor web site type isn't available at all, please install ASP.NET MVC3 from here.



    • The Delay.Web.Helpers assembly included in the NuGet package is exactly the same DLL I previously released on my blog. If you're already using it successfully, there's no need to switch over to the NuGet version.

    • As I add more features in the future, I'll continue releasing bits via my blog and will also update these NuGet packages. So please pick the delivery mechanism you like best (ZIP or NuGet) and feel free to ignore the other one. :)

    • Creating your own NuGet package is quite simple and straightforward. The resources I used were the NuGet CodePlex site and the NuGet Gallery. The former has a link to download the NuGet.exe tool you'll need as well as some good documentation on the process of creating a package. The latter is where you go to upload your package - at which point it's automatically visible to the NuGet plug-in for Visual Studio, the ASP.NET MVC3 application admin interface, etc..


    Creating useful libraries and sharing them with others is a great way to contribute back to the community - but only if you can get the bits into the hands of people who want them! NuGet offers a lightweight packaging mechanism that's so simple, anyone can get started with a minimum investment of time or energy. The NuGet gallery has broad developer reach, is easily searchable, and is managed by someone else - so all you need to worry about is writing a good library!

    Less overhead means more development time - thanks, NuGet! :)

  • Delay's Blog

    Night of the Living WM_DEADCHAR [How to: Avoid problems with international keyboard layouts when hosting a WinForms TextBox in a WPF application]

    There was a minor refresh for the Microsoft Web Platform last week; you can find a list of fixes and instructions for upgrading in this forum post. The most interesting change for the WPF community is probably the fix for a WebMatrix problem brought to my attention by @radicalbyte and @jabroersen where Ctrl+C, Ctrl+X, and Ctrl+V would suddenly stop working when an international keyboard layout was in use. Specifically, once one of the prefix keys (ex: apostrophe, tilde, etc.) was used to type an international character in the editor, WebMatrix's copy/cut/paste shortcuts would stop working until the application was restarted. As it happens, the Ribbon buttons for copy/cut/paste continued to work correctly - but the loss of keyboard shortcuts was pretty annoying. :(

    Fortunately, the problem was fixed! This is the story of how...


    To begin with, it's helpful to reproduce the problem on your own machine. To do that, you'll need to have one of the international keyboard layouts for Windows installed. If you spend a lot of time outside the United States, you probably already do, but if you're an uncultured American like me, you'll need to add one manually. Fortunately, there's a Microsoft Support article that covers this very topic: How to use the United States-International keyboard layout in Windows 7, in Windows Vista, and in Windows XP! Not only does it outline the steps to enable an international keyboard layout, it also explains how to use it to type some of the international characters it is meant to support.

    Aside: Adding a new keyboard layout is simple and unobtrusive. The directions in the support article explain what to do, though I'd suggest skipping the part about changing the "Default input language": if you stop at adding the new layout, then your default experience will remain the same and you'll be able to selectively opt-in to the new layout on a per-application basis.

    With the appropriate keyboard layout installed, the other thing you need to reproduce the problem scenario is a simple, standalone sample application, so...

    [Click here to download the InternationalKeyboardBugWithWpfWinForms sample application.]


    Compiling and running the sample produces a .NET 4 WPF application with three boxes for text. The first is a standard WPF TextBox and it works just like you'd expect. The second is a standard Windows Forms TextBox (wrapped in a WindowsFormsHost control) and it demonstrates the problem at hand. The third is a trivial subclass of that standard Windows Forms TextBox that includes a simple tweak to avoid the problem and behave as you'd expect. Here's how it looks with an international keyboard layout selected:

    InternationalKeyboardBugWithWpfWinForms sample

    Although the sample application doesn't reproduce the original problem exactly, it demonstrates a related issue caused by the same underlying behavior. To see the problem in the sample application, do the following:

    1. Make the InternationalKeyboardBugWithWpfWinForms window active.
    2. Switch to the United States-International keyboard layout (or the corresponding international keyboard layout for your country).
    3. Press Ctrl+O and observe the simple message box that confirms the system-provided ApplicationCommands.Open command was received.
    4. Type one of the accented characters into the WinForms TextBox (the middle one).
    5. Press Ctrl+O again and notice that the message box is no longer displayed (and the Tab key has stopped working).


    As you can see, the InternationalTextBox subclass avoids the problem its WinForms TextBox base class exposes. But how? Well, rather simply:

    /// <summary>
    /// Trivial subclass of the Windows Forms TextBox to work around problems
    /// that occur when hosted (by WindowsFormsHost) in a WPF application.
    /// </summary>
    public class InternationalTextBox : System.Windows.Forms.TextBox
        /// <summary>
        /// WM_DEADCHAR message value.
        /// </summary>
        private const int WM_DEADCHAR = 0x103;
        /// <summary>
        /// Preprocesses keyboard or input messages within the message loop
        /// before they are dispatched.
        /// </summary>
        /// <param name="msg">Message to pre-process.</param>
        /// <returns>True if the message was processed.</returns>
        public override bool PreProcessMessage(ref Message msg)
            if (WM_DEADCHAR == msg.Msg)
                // Mark message as handled; do not pass it on
                return true;
            // Call base class implementation
            return base.PreProcessMessage(ref msg);

    The underlying problem occurs when the WM_DEADCHAR message is received and processed by both WPF and WinForms, so the InternationalTextBox subclass shown above prevents that by intercepting the message in PreProcessMessage, marking it handled (so nothing else will process it), and skipping further processing. Because the underlying input-handling logic that actually maps WM_DEADCHAR+X to an accented character still runs, input of accented characters isn't affected - but because WPF doesn't get stuck in its WM_DEADCHAR mode, keyboard accelerators like Ctrl+O continue to be processed correctly!

    Aside: The change above is not exactly what went into WebMatrix... However, this change appears to work just as well and is a bit simpler, so I've chosen to show it here. The actual change is similar, but instead of returning true, maps the WM_DEADCHAR message to a WM_CHAR message (ex: msg.MSG = WM_CHAR;) and allows the normal base class processing to handle it. I'm not sure the difference matters in most scenarios, but it's what appeared to be necessary at the time, it's what the QA team signed off on, and it's what the WPF team reviewed. :) I'm optimistic the simplified version I show here will work well everywhere, but if you find somewhere it doesn't, please try the WM_CHAR translation instead. (And let me know how it works out for you!)


    Hosting Windows Forms controls in a WPF application isn't the most common thing to do, but sometimes it's necessary (or convenient!), so it's good to ensure everything works well together. If your application exposes any TextBox-like WinForms classes, you might want to double-check that things work well with an international keyboard layout. And if they don't, a simple change like the one I show here may be all it takes to resolve the problem! :)

    PS - I mention above that the WPF team had a look at this workaround for us. They did more than that - they fixed the problem for the next release of WPF/WinForms so the workaround will no longer be necessary! Then QA checked to make sure an application with the workaround wouldn't suddenly break when run on a version of .NET/WPF/WinForms with the actual fix - and it doesn't! So apply the workaround today if you need it - or wait for the next release of the framework to get it for free. Either way, you're covered. :)
  • Delay's Blog

    sudo localize --crossplatform [Free PseudoLocalizer class makes it easy to identify localization issues in WPF, Silverlight, and Windows Phone 7 applications!]


    Two posts ago, I explained the benefits of pseudo-localization and showed an easy way to implement it for WPF - then said I'd outline how to do the same for Silverlight and Windows Phone 7. In my previous post, I went off on the seeming diversion of implementing a PNG encoder for Silverlight. With this post, I'll fulfill my original promise and unify the previous two posts! As you'll see, the basic principles of my approach to WPF localization translate fairly directly to Silverlight - though some limitations in the latter platform make achieving the same result more difficult. Even though more code and manual intervention are required for Silverlight and Windows Phone 7, the benefits are the same and pseudo-localization remains a great way to identify potential problems early in the development cycle.

    For completeness I'll show examples and techniques for all platforms below...


    Normal WPF application PseudoLocalizer in a WPF application

    Please see the original post for an explanation of the changes shown above.


    Adding pseudo-localization of RESX resources to a WPF application

    1. Add the PseudoLocalizer.cs file from the sample download to the project.

    2. Add PSEUDOLOCALIZER_ENABLED to the (semi-colon-delimited) list of conditional compilation symbols for the project (via the Project menu, Properties item, Build tab in Visual Studio).

    3. Add the following code somewhere it will be run soon after the application starts (for example, add a constructor for the App class in App.xaml.cs):

    4. If necessary: Add a project reference to System.Drawing (via Project menu, Add Reference, .NET tab) if building the project now results in the error "The type or namespace name 'Drawing' does not exist in the namespace 'System' (are you missing an assembly reference?)".

    5. If necessary: Right-click Resources.resx and choose Run Custom Tool if running the application under the debugger (F5) throws the exception "No matching constructor found on type 'ProjectName.Properties.Resources'. You can use the Arguments or FactoryMethod directives to construct this type.".


    Adding pseudo-localization of RESX resources to a Silverlight or Windows Phone 7 application

    1. Add the PseudoLocalizer.cs and PngEncoder.cs files from the sample download to the project.


    3. Make the following update to the auto-generated resource wrapper class by editing Resources.Designer.cs directly (the highlighted portion is the primary change):

          global::System.Resources.ResourceManager temp =
              new Delay.PseudoLocalizerResourceManager("PseudoLocalizerSL.Resources", typeof(Resources).Assembly);
          global::System.Resources.ResourceManager temp =
              new global::System.Resources.ResourceManager("PseudoLocalizerSL.Resources", typeof(Resources).Assembly);

      In case it's not clear, this change simply duplicates the existing line of code that creates an instance of ResourceManager, modifies it to create an instance of Delay.PseudoLocalizerResourceManager instead, and wraps the two versions in an appropriate #if/#else/#endif so pseudo-localization can be completely controlled by whether or not PSEUDOLOCALIZER_ENABLED is #defined.

      Important: This change will be silently overwritten the next time (and every time!) you make a change to Resources.resx with the Visual Studio designer. Please see my notes below for more information on this Silverlight-/Windows Phone-specific gotcha.


    PseudoLocalizer in a Silverlight application


    Adding a pseudo-localizable string (all platforms)

    1. Double-click Resources.resx to open the resource editor.

    2. Add the string by name and value.

    3. Reference it from code/XAML.

    4. Silverlight/Windows Phone 7: Re-apply the Delay.PseudoLocalizerResourceManager change to Resources.Designer.cs which was silently undone when the new resource was added.


    Adding a pseudo-localizable image (WPF only)

    1. Double-click Resources.resx to open the resource editor.

    2. Click the "expand" arrow for Add Resource and choose Add Existing File....

    3. Open the desired image file.

    4. Reference it from code/XAML (possibly via BitmapToImageSourceConverter.cs from the sample ZIP).


    Adding a pseudo-localizable image (all platforms)

    1. Rename the image file from Picture.jpg to Picture.jpg-bin.

    2. Double-click Resources.resx to open the resource editor.


    4. Open the desired image file.

    5. Reference it from code/XAML (probably via ByteArrayToImageSourceConverter.cs from the sample ZIP).


    7. Optionally: Restore the image's original file name in the Resources folder of the project and manually update its file name in Resources.resx using a text editor like Notepad. (I've done this for the sample project; it makes things a little clearer and it's easier to edit the image resource without having to rename it each time.)


    PseudoLocalizer in a Windows Phone 7 application


    There you have it - simple text and image pseudo-localization for WPF, Silverlight, and Windows Phone 7 applications is within your grasp! :) The basic concept is straightforward, though limitations make it a bit more challenging for Silverlight-based platforms. Nevertheless, the time you're likely to save by running PseudoLocalizer early (and often) should far outweigh any inconvenience along the way. By finding (and fixing) localization issues early, your application will be more friendly to customers - no matter what language they speak!


    [Click here to download the complete source code for PseudoLocalizer, various helper classes, and the WPF, Silverlight, and Windows Phone 7 sample applications shown above.]



    • For a brief overview of using RESX resources in a WPF, Silverlight, or Windows Phone 7 application, please see the "Notes" section of my original PseudoLocalizer post. You'll want to be sure the basic stuff is all hooked up and working correctly before adding PseudoLocalizer into the mix.

    • The act of using RESX-style resources in a Silverlight application is more difficult than it is in a WPF application (independent of pseudo-localization). WPF allows you to directly reference the generated resources class directly from XAML:

      <TextBlock Text="{Binding Path=Message, Source={StaticResource Resources}}"/>

      However, that approach doesn't work on Silverlight (or Windows Phone 7) because the generated constructor is internal and Silverlight's XAML parser refuses to create instances of such classes. Therefore, most people create a wrapper class (as Tim Heuer explains here):

      /// <summary>
      /// Class that wraps the generated Resources class (for Resources.resx) in order to provide access from XAML on Silverlight.
      /// </summary>
      public class ResourcesWrapper
          private Resources _resources = new Resources();
          public Resources Resources
              get { return _resources; }

      And reference that instead:

          <local:ResourcesWrapper x:Key="Resources" xmlns:local="clr-namespace:PseudoLocalizerSL"/>
      <TextBlock Text="{Binding Path=Resources.Message, Source={StaticResource Resources}}"/>

      Obviously, the extra level of indirection adds overhead to every place resources are used in XAML - but that's a small price to pay for dodging the platform issue. :)

    • WPF supports private reflection and PseudoLocalizer takes advantage of that to enable a simple, seamless, "set it and forget it" hook-up (via the call to Enable above). Unfortunately, private reflection isn't allowed on Silverlight, so the same trick doesn't work there. I considered a variety of different ways around this, and ultimately settled on editing the generated wrapper class code because it applies exactly the same customization as on WPF. And while it's pretty annoying to have this tweak silently overwritten every time the RESX file is edited, it's simple enough to re-apply and it's easy to spot when reviewing changes before check-in.

    • I explained what's wrong with the default behavior of adding an image to a RESX file in my PngEncoder post:

      [...] the technique I used for [WPF] (reading the System.Drawing.Bitmap instance from the resources class and manipulating its pixels before handing it off to the application) won't work on Silverlight. You see, the System.Drawing namespace/assembly doesn't exist for Silverlight! So although the RESX designer in Visual Studio will happily let you add an image to a Silverlight RESX file, actually doing so results in an immediate compile error [...].

      Fortunately, the renaming trick I use above works well for Silverlight and Windows Phone - and WPF, too. So if you're looking to standardize on a single technique, this is the one. :)

      Even if you're devoted to WPF and don't care about Silverlight, you should still consider the byte[] approach: although System.Drawing.Bitmap is easier to deal with, it's not the right format. (Recall from the original PseudoLocalizer post that I wrote an IValueConverter to convert from it to System.Windows.Media.ImageSource.) Instead of loading images as System.Drawing.Bitmap and converting them with BitmapToImageSourceConverter, why not load them as byte[] and convert them with ByteArrayToImageSourceConverter.cs - and save a few CPU cycles by not bouncing through an unnecessary format?

    • In addition to the renaming technique for accessing RESX images from Silverlight, there's a similar approach (courtesy of Justin Van Patten) that renames to .wav instead and exposes the resource as a System.IO.Stream. For the purposes of pseudo-localization, the two renaming approaches should be basically equivalent - which led me to go as far as hooking everything up and writing StreamToImageSourceConverter.cs before I realized why the Stream approach isn't viable...

      What it comes down to is an unfortunate API definition - the thing that's exposed by the wrapper class isn't a Stream, it's an UnmanagedMemoryStream! And while that would be perfectly fine as an implementation detail, it's not: the type of the auto-generated property is UnmanagedMemoryStream and the type returned by ResourceManager.GetStream is also UnmanagedMemoryStream. But UnmanagedMemoryStream can't be created by user code in Silverlight (and requires unsafe code in WPF), so this breaks PseudoLocalizer's approach of decoding/pseudo-localizing/re-encoding the image because it means the altered bytes can't be wrapped back up in a UnmanagedMemoryStream to maintain the necessary pass-through behavior!

      If only the corresponding RESX interfaces had used the Stream type (a base class of UnmanagedMemoryStream), it would have been possible to wrap the altered image in a MemoryStream and return that - a technique supported by all three platforms. Without digging into this too much more, it seems to me that the Stream type could have been used with no loss of generality - though perhaps there's a subtlety I'm missing.

      Aside: As a general API design guideline, always seek to expose the most general type that makes sense for a particular scenario. That does not mean everything should expose the Object type and cast everywhere - but it does mean that (for example) APIs exposing a stream should use the Stream type and thus automatically work with MemoryStream, UnmanagedMemoryStream, NetworkStream, etc.. Only when an API needs something from a specific subclass should it use the more specific subclass.

      Be that as it may, I didn't see a nice way of wrapping images in a UnmanagedMemoryStream, and therefore recommend using the byte[] approach instead!

  • Delay's Blog

    What it lacks in efficiency, it makes up for in efficiency! [Silverlight-ready PNG encoder implementation shows one way to use .NET IEnumerables effectively]


    At the end of my previous post about easily pseudo-localizing WPF applications, I said this post would show how to apply those concepts to a Silverlight application. Unfortunately, I seem to have made an off-by-one error: while this post is related to that topic, it is not the post I advertised. But it seems interesting in its own right, so I hope you enjoy it. :)

    Okay, so what does a PNG (Portable Network Graphics) image encoder have to do with pseudo-localizing on the Silverlight platform? Almost nothing - except for the fact that I went above and beyond with my last post and showed how to pseudo-localize not just text, but images as well. It turns out the technique I used for that (reading the System.Drawing.Image instance from the resources class and manipulating its pixels before handing it off to the application) won't work on Silverlight. You see, the System.Drawing namespace/assembly doesn't exist for Silverlight! So although the RESX designer in Visual Studio will happily let you add an image to a Silverlight RESX file, actually doing so results in an immediate compile error: The type or namespace name 'Drawing' does not exist in the namespace 'System' (are you missing an assembly reference?)...


    But all is not lost - there are other ways of adding an image to a RESX file! True, the process is a little wonky and cumbersome, but at least it works. However, the resulting resource is exposed as either a byte[] or a Stream instance - both of which are basically just a sequence of bytes. And because there's no SetPixel method for byte arrays, this is a classic "Houston, we have a problem" [sic; deliberately misquoting] moment for my original approach of pseudo-localizing the image by manipulating its pixels... Hum, what's a girl to do?

    Well, those bytes do correspond to an encoded image, so it ought to be possible to decode the image - at which point we could wrap it in a WriteableBitmap and do the pixel manipulation via its Pixels property. After that, we could re-encode the altered pixels back to an image file (remember that the resource data is expected to be the encoded bytes of an image) and things should work as seamlessly as they did for the original scenario. And they actually do! Well, on WPF, at least...


    PngEncoder sample image

    Sample PNG file encoded by PngEncoder


    Not on Silverlight, though. Silverlight will get you all the way to the last step, but that's the end of the line: the platform doesn't expose a way to do the re-encoding. :( As you might guess, this is hardly the first time this issue has come up - a quick web search turns up plenty of examples of people wanting to encode images under Silverlight. The typical recommendation is to find (or write) your own image encoder, and so there are a number of examples of Silverlight-compatible image encoders to be found!

    So why did I write my own??

    Well, because none of the examples I found was quite what I wanted. The most obvious candidate just flat out didn't work; it crashed on every input I gave it. The runner-up (deliberately) took shortcuts for speed and didn't produce valid output. The third option was released under a license I'm not able to work with. And the fourth was part of a much larger image encoding library I didn't feel like pulling apart. Besides, I release full source code on my blog, and I don't want to be in the business of distributing other peoples' code with my samples. A lot of times it's just easier and safer to code something myself - and I've had a lot of great learning experiences as a result! :)


    When choosing an image format for re-encoding on the fly, Silverlight makes the decision fairly easy because it supports only two image formats: JPEG and PNG. Because JPEG is a lossy format, it's pretty much a non-starter (we don't want to degrade image quality) and therefore lossless PNG is the obvious choice. Conveniently, the PNG image format is fairly simple - especially if you're willing to punt on (losslessly) compressing the image! All you need to encode a PNG file is a format prefix (8 bytes), a header chunk (5 bytes), an image chunk with the properly encoded pixels, and an end chunk (0 bytes). There's a decent amount of bookkeeping to be done along the way (scanline filtering, compression block creation, two different hash algorithms, etc.), but it's all fairly straightforward. And the PNG specification is well-written and fairly easy to follow - what more could you ask for?

    Aside: Giving up on compression may seem like a big deal, but I don't think it is for the scenario at hand. While small file size is important for making the best use of long-term storage, the pseudo-localization scenario creates its PNG file on the fly, loads it, and immediately discards it. The encoded image simply isn't around for very long and so its large size shouldn't matter.

    Okay, so PNG encoding isn't rocket science - but I still feel that if I'm going to reinvent the wheel, then at least I should try to contribute something new or interesting to the mix! :)


    What I've done here is to use IEnumerable everywhere:

    namespace Delay
        /// <summary>
        /// Class that encodes a sequence of pixels to a sequence of bytes representing a PNG file (uncompressed).
        /// </summary>
        /// <remarks>
        /// Reference:
        /// </remarks>
        static class PngEncoder
            /// <summary>
            /// Encodes the specified pixels to a sequence of bytes representing a PNG file.
            /// </summary>
            /// <param name="width">Width of the image.</param>
            /// <param name="height">Height of the image.</param>
            /// <param name="pixels">Pixels of the image in ARGB format.</param>
            /// <returns>Sequence of bytes representing a PNG file.</returns>
            public static IEnumerable<byte> Encode(int width, int height, IEnumerable<int> pixels) { /* ... */ }

    And IEnumerable isn't just for the public API, it's used throughout the code, too! This is the meaning behind the title of this post: giving up on compression is inefficient from a storage space perspective, but operating exclusively on enumerations is very efficient from a memory consumption and a computational efficiency perspective! Because of that, this class can encode a 20-megabyte PNG file without allocating any more memory than it takes to encode a 1-byte PNG file. What's more, no work is done before it needs to be: if you're streaming a PNG file across a slow transport, the file will be read and encoded only as quickly as the receiver consumes the bytes - and if encoding is aborted mid-way for some reason, no unnecessary effort has been wasted!

    This efficiency is possible thanks to the way IEnumerable works and the convenience of C#'s yield return which makes it easy to write code that returns an IEnumerable without explicitly implementing IEnumerator. As a result, the code for PngEncoder is clear and linear with no hint (or visible complexity) of the allocation savings or deferred processing that occur under the covers. Instead, everything communicates in terms of byte sequences - translating one sequence to another inline as necessary. For example, every horizontal line of an encoded PNG image is prefixed with a byte indicating which filtering algorithm was used. These filter bytes aren't part of the original pixels that are passed into the Encode call, so they need to be added. What's cool is that it's easy to write a method to do so - and because it takes an IEnumerable input parameter and returns an IEnumerable result, such a method can be trivially "injected" into any data flow!


    One particularly handy realization of this technique is demonstrated by a class I called WrappedEnumerable - here's what it looks like to the developer:

    /// <summary>
    /// Class that wraps an IEnumerable(T) or IEnumerator(T) in order to do something with each byte as it is enumerated.
    /// </summary>
    /// <typeparam name="T">Type of element being enumerated.</typeparam>
    abstract class WrappedEnumerable<T> : IEnumerable<T>, IEnumerator<T>
        /// <summary>
        /// Method called to initialize for a new (or reset) enumeration.
        /// </summary>
        protected virtual void Initialize() { }
        /// <summary>
        /// Method called for each byte output by the underlying enumerator.
        /// </summary>
        /// <param name="value">Next value.</param>
        protected abstract void OnValueEnumerated(T value);

    WrappedEnumerable is useful for PngEncoder because the encoding process makes use of two different hash algorithms: CRC-32 and Adler-32. Long-time readers of this blog know I'm no stranger to hash functions - in fact, I've previously shared code to implement CRC-32 based on the very same PNG specification! But as much as I love .NET's HashAlgorithm, using it here seemed like it might be overkill. HashAlgorithm is ideal for processing large chunks of data at a time, but the sequence-oriented nature of PngEncoder deals with a single byte at a time. So what I did was to create WrappedEnumerable subclasses Crc32WrappedEnumerable and Adler32WrappedEnumerable which override the two methods above to calculate their hash values as the bytes flow through the system! These are both simple classes and work quite well for "injecting" hash math into a data flow. The PNG format puts stores its hash values after the corresponding data, so by the time that value is needed, the relevant data has already flowed through the WrappedEnumerable and the final hash is ready to be retrieved.


    However, the same cannot be said of the length fields in the PNG specification... Lengths are stored before the corresponding data, and that poses a distinct problem when you're trying to avoid looking ahead: without knowing what data is coming, it's hard to know how much there is! But I'm able to cheat here: because PngEncoder doesn't compress, it turns out that all the internal lengths can be calculated from the original width and height values passed to the call to Encode! So while this is a bit algorithmically impure, it completely sidesteps the length issue and avoids the need to buffer up arbitrarily long sequences of data just to know how long they are.

    Aside: After spending so much time celebrating the IEnumerable way of life, this is probably a good time to highlight one of its subtle risks: multiple enumeration due to deferred execution. Multiple enumeration is something that comes up a lot in the context of LINQ - an IEnumerable<byte> is not the same thing as a byte[]. Whereas it's perfectly reasonable to pass a byte[] off to two different functions to deal with, doing the same thing with an IEnumerable<byte> usually results in that sequence being created and enumerated twice! Depending on where the sequence comes from, this can range from inefficient (a duplication of effort) to catastrophic (it may not be possible to generate the sequence a second time). The topic of multiple enumeration is rich enough to merit its own blog post (here's one and here's another and here's another), and I won't go into it further here. But be on the look-out, because it can be tricky!
    Further aside: To help avoid this problem in PngEncoder, I created the TrackingEnumerable and TrackingEnumerator classes in the sample project. These are simple IEnumerable/IEnumerator implementations except that they output a string with Debug.WriteLine whenever a new enumeration is begun. For the purposes of PngEncoder, seeing any more than one of these outputs represents a bug!


    In an attempt to ensure that my PngEncoder implementation behaves well and produces valid PNG files, I've written a small collection of automated tests (included with the download). Most of them are concerned with parameter validation and correct behavior with regard to the IEnumerable idiosyncrasies I mentioned earlier, but the one called "RandomImages" is all about output verification. That test creates 25 PNG files of random size and contents, encodes them with PngEncoder, then decodes them with two different decoder implementations (System.Drawing.Bitmap and System.Windows.Media.Imaging.PngBitmapDecoder) and verifies the output is identical to the input. I've also verified that PngEncoder's PNG files can be opened in a variety of different image viewing/editing applications. (Interesting tidbit: Internet Explorer seems to be the strictest about requiring valid PNG files!) While none of this is a guarantee that all images will encode successfully, I'm optimistic typical scenarios will work well for people. :)


    [Click here to download the source code for the complete PngEncoder.cs implementation, a simple test application, and the automated test suite.]


    At the end of the day, the big question is whether my focus (obsession?) with an IEnumerable-centric implementation was justified. From a practical point of view, you could argue this either way - I happen to think things worked out pretty elegantly, but others might argue the code would be clearer with explicit allocations. I'll claim the memory/computational benefits I describe here are pretty compelling (especially in resource-constrained scenarios like Windows Phone), but others could reasonably argue that naïve consumers of PngEncoder are likely to write their code in a way that negates those benefits (ex: by calling ToArray).

    However, I'm not going to spend a lot of time second-guessing myself; I think it was a great experience. :) The following sums up my thoughts on the matter pretty nicely:

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