Behavior of 1.0/1.1 managed apps on 64bit machines

As I alluded to in my previous post there are multiple ways that we can go in terms of supporting legacy 1.0/1.1 assemblies on a Win64 machine. The context of the 1.0/1.1 support story is made somewhat simpler by the current plan of having both a v2.0 32bit CLR and a v2.0 64bit CLR on the box but no 1.0/1.1 CLR bits.

I mentioned that the 1.0/1.1 compilers didn't know anything about “bitness”. Basically they spit out a PE image that said “Hey! I'm managed! Run me with the CLR!” (gross simplification), whereas the v2.0 compilers produce images that range from “Hey! I'm managed, and I can run everywhere!!” to “Hey! I'm managed and I only run on x86!” etc...

This brings us to the fundamental question of this post -- what to do with 1.0/1.1 assemblies?

Option 1: call them “Legacy” assemblies since they don't know about “bitness”. Require them to run in the WOW64 under the 32bit CLR as we can't say for sure that the developer who created them was thinking about 64bit compatibility when they were created (remember that many of these were created years before even a 64bit alpha of .NET was available at PDC last year). Additionally, make the loader get angry and spew something along the lines of “BAD_IMAGE_FORMAT” if you try to load a legacy assembly in a native 64bit managed process just as if you had tried to load a v2.0 assembly marked x86 only.

Option 2: treat them like the v2.0 notion of MSIL assemblies, allow them to be used from both 32bit and 64bit managed processes. By default if they are an exe kick off the 64bit CLR when someone tries to start them. This would cause them to run as a 64bit process even though their creators probably didn't have that potential in mind when the code was written and tested.


Cases can be made for both sides. Right now the more conservative approach is “Option 1” which is what we are leaning towards. But there are definitely some negatives to that, the primary one in my mind being that is makes the transition to 64bit harder for groups that have dependencies on a lot of managed code that they don't own but are willing to do the testing legwork to make sure they work in 64bit mode anyway. In effect it makes the 1.0/1.1 managed code assemblies much like 32bit native code components as dependencies for moving your app to 64bit because in that scenario we won't let you load 1.0/1.1 assemblies in your 64bit process.

One of the great things about managed code is that frequently there isn't much if any work to be done to move it to 64bit. But given “Option 1” above we would at least require the work of a recompile (though someone could imagine a tool that would be frightfully dangerous which would modify 1.0/1.1 headers to look like 2.0 headers to pretend to be a v2.0 compiled MSIL image... Please don't do this!!). If you don't own the managed code you're using that means waiting for whoever does to recompile and give you the properly tagd version before you can move your app to 64bit.

Mind you, that is probably better than the alternative. If we were to just load up 1.0/1.1 images in a 64bit process expecting that they should be the equivalent of v2.0’s MSIL (which is what the compilers are currently producing as a default) you could end up with all manner of random execution failure, usually related to calling into some native 32bit code or other... “Option 2” would allow those who are willing do the legwork in testing to do their due diligence, test their application in a 64bit environment thoroughly even though it might contain 1.0/1.1 components and be able to say with reasonable confidence that their customers wont have problems running 64bit native. The fact that I id “willing to do the legwork in testing” and “due diligence” etc.. should be setting off huge danger signals in your head. How many people are willing to toughly test some component they paid for, isn’t that part of what you paid for??

There are of course all manner of “in-between” scenarios, few of which are supportable or justifiable, so for the purposes of this debate lets stick to these two options.


The main reason I started writing this post however wasn't to make up your mind but to poll your thoughts…