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Resources, Managed, Native, and ALink

  • Artigo

So most of you have figured out by now that making your application run with different languages can be very hard.  The overall design is always the same: place all content that might change for different languages in some sort of runtime-selectable package, and then load everything dynamically rather than just hard-coding in constants.  Now before  C# and the CLR, classic windows used a special section in the PE file to do this.  The “.rsrc” section was the designated dumping ground for such things.  It is designed to be extensible, and localizable.  There are 2 obvious and common things in just about any native resource section: the application icon and the version resource.  When you use windows explorer to browse through a bunch of .DLLs or .EXEs, the shell cracks open the resource section and looks for the application icon.  It it's there it uses it as the icon for the file, otherwise it uses one of it's built-in icons.  If you've got a really slow or old machine you can see this in action, all the files will start with default icons and then one-by-one get updated to the right one.  The version resource is a little bit trickier.  If you right-click on the file and then choose properties, if the file has a version resource, there will be a version tab that shows it's values.  Most setup/install applications also look for this resource so they can make sure to upgrade files to the latest-and-greatest.  The ugly part comes in because of how this resource is actually stored.  Specifically the file version is stored as both a number (2 sequential DWORDs usually represented as 4 unsigned 16-bit numbers) and as a string.  Nothing requires them to match, and some tools specifically use the string to contain extra information (like build time stamp, machine, user, etc.).  Native resources usually start life as an .RC file, then get compiled to a .RES file using rc.exe, then get converted to a .OBJ file using cvtres.exe, and finally get linked into your native application just like every other .OBJ file.  Enough of the native resources, they're all obsolete anyway now that we're in the managed world, right?  Wrong, but I'll get back to that later.

Managed resources are a completely new and different beast.  For most of you (hopefully) you never really have to know how this works because the designer and IDE project system take care of this for you magically (at least until the magic wears out and starts smelling more like a bug or a hack).  Basically managed resources come in 2 file formats: .RESX and .RESOURCES.  Neither of them share anything with native resource except the same end purpose.  The designer allows you to edit the .RESX file graphically.  Then at build time the IDE does it magic and invokes resgen to compile the .RESX file into a .resources file.  Finally the C# (or VB or C++) compiler embeds (or links) the file into the final output.  The .RESOURCES (and the .RESX) file itself is very structured with strong type information for everything stored in it.  There is no chance for accidentally loading an icon as a string, just like the native resources.  However, at the metadata level, the managed resources are just an arbitrary blob in the assembly.  They come in 2 flavors: embedded and linked.  Linked means, “some other stand-alone file in this assembly” and there is metadata to indicate which file, and that whole file is considered the 'resource'.  Embedded means, “the resource is stored in this file at offset XXX”.  At that offset you will find a 32-bit unsigned integer that gives the size of the resource, which starts immediately after that.  Notice the complete lack of types?  To the CLR resources are just arbitrary blobs that identified by strings.  Now the frameworks and designers have tried to enforce a convention on top of that.  Specifically, they assume that the resource names are:

  1. Not case sensitive.
  2. Correspond to <typename>.<locale>.resources naming convention where typename is the fully-qualified typename that will be consuming the resources (usually a winforms class).
  3. Always be the structured .RESOURCES file format.

As for #1, they generally do a case sensitive lookup since it is faster.  Then only if that fails will they do a case-insensitive lookup and report some vague exception if they find duplicates that differ only by casing.  So as long as the casing matches , you may never know you have a 'problem'.  This is especially problematic because several tools rely on the compiler's default which is to use the resource filename as the resource name.  (If I could roll back time, I would have never allowed such an awful default).  Since some file systems preserve casing and some don't, this could result in different resource names, just because the file system changed!

As for #2 this is generally where the IDE's magic falls apart.  Rather than the designer just spitting out the name it used, and then resgen reading it out of the .RESX file (or any number of other sensible options), the project system has to sniff your sources and guess what name the designer used.  It does this by using the “Default Namespace” project property setting (which doesn't always match the actual namespace used int eh source file), and then assumes that everybody only puts on class in each source file, and thus uses the first class in the source file.  It combines that information to produce the proper .RESOURCES filename which is then used as the string identifier in metadata because they don't bother to override the default.

As for #3, they actually do check!  There is a special magic number at the start of .RESOURCES files that they use to validate them.  Although I personally think it would be better if this were somehow encoded in metadata...

Now how does ALink fit into this, and what about native resources?

Well not having a version resource would break most, if not all, installers who check for this information to determine if it's safe to upgrade or not.  Not to mention the IT experts and QA teams who have grown accustomed to using this information when comparing files.  We could have written new tools and APIs that would just gather the information out of the metadata, but that just wasn't back-wards compatible enough for the powers that be.  Instead they decided that all managed code should be forced to have a version resource that duplicates the same information into the old native resource section to better interop with old code.  Since ALink was already the common-dumping ground, it got this task.  Basically it just tracks all of the assembly metadata, and then produces a .RES file that contains a native version resource and optionally an application icon that the user specifies.  If you don't like the default version resource the command-line compiler allows you to supply your own, but if you do that, then you have to make sure the version resource in there matches the assembly version info.

Now there are a few really advanced people who realize that adding a whole section to a PE file just to get a version number is sometimes a fair bit of waste, especially on the compact frameworks.  Well since there's no way to turn off the auto-generation of the version resource, it might look like you're stuck, but I have found a work-around.  By creating an ultra small .OBJ file that has effectively an empty .RSRC section it fools the compiler into not emitting any .RSRC section!  So here the .OBJ file: https://mysite.verizon.net/grantri/empty.obj

--Grant

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