The other day I had a reader ask me about the difference between 32 and 64 bit builds. Here is his email to me.

I have your Using MSBuild book and I have been learning much with it. One aspect that I cannot seem to find online or in the book is that of the build server environment. Specifically how the choice of Server OS affects your builds and subsequent testing environments. The aspects of most of our build environments are TFS2008 and MSbuild servers running on Server 2003 32bit Standard some have VSTS Team Test edition 2008.

Here are some questions I am looking for answers on.

If my dev team is writing an app for run on 64 bit servers and they will be running VS Team test 2008 on the same build server then do they want VS 2008 64-bit installed as their build server? What could the pitfalls be if they are running builds and tests or just build for that matter on Server 2008 32-bit or 2003 32-bit? Or even just running builds?

If a team is developing an app for 32 bit server 2003 but your build machine is Server2008 64-bit, what could go wrong there? Also in many of the many combinations here are there any false hopes, such as building 32bit on a 64 machine may work but…

Additionally what is to be expected of WOW in this context 64-bit will run Windows on Windows on some apps I think but is this a best practice?

There are so many combinations of the way that an environment could be setup. A blog post exploring these different setups would be great if you could manage it. I just want to make sure my teams are getting what they need and are getting the expected results. Thanks much for your time.

I don't have too much knowledge in this area myself so I asked a couple other guys (William Bartholomew, JB Brown, Jim Lamb, and Grant Holliday) and here is some related info.

Assuming this is a .NET application the bitness of the build machine has no effect on the bitness of the produced application, this is controlled entirely by the project's compilation settings. Basically:

1. If the assembly is configured for Any CPU then it will run as x64 on a 64-bit machine and as x86 on a 32-bit machine.

2. If the assembly is configured for x86 then it will run as WOW64 on a 64-bit machine (i.e. a 32-bit process) and as x86 on a 32-bit machine.

3. If the assembly is configured for x64 then it will run as x64 on a 64-bit machine and will fail to run on a 32-bit machine. For some related info on this see Visual Studio: Why is there no 64 bit version? (yet)

The bitness of a process is determined by the compilation settings for the entry point (i.e. the executable)... You need to ensure that dependencies are compiled with an appropriate bitness for their host process. For example, devenv.exe always runs as a 32-bit process so if you create a Visual Studio add-in that's compiled for 64-bit only then it will never be loadable by Visual Studio. Basically it comes down to that a process can always load a dependency that's compiled as Any CPU or the same bitness as the process.

A few extra points about deployment and runtime.

4 - Extra considerations need to be made if you are dealing with multiple web sites under IIS 6. Since IIS 6 is a single process it is the entry point and determines the 64 bitness of the process it can only be in 1 "bitness" - so all sites under IIS have to be compatible with that bitness choice - IE - no running a 32 bit website and a 64 bit website under the same IIS 6 process. For more info on this see How to switch between the 32-bit versions of ASP.NET 1.1 and the 64-bit version of ASP.NET 2.0 on a 64-bit version of Windows.

5. While a 64 bit entry point/process may load a dependency that is compiled in Any CPU or x64 - it doesn't guarantee that those dependencies are run-time 64 bit process compatible. For example they may use COM-interop which will pass all compile time checks but fail at runtime. You'll need to do your due-diligence here on COM interop and 3rd party assemblies being used.

6. If your assemblies need regsrv32'd and run as a 32 bit WOW process remember that you need to use the 32 bit version of regsvr32.exe to get it registered into the right hive on that 64 bit OS. 

7. There is a similar thing to #6 for installutil.exe - a 64 bit version and a 32 bit WOW version

Sayed Ibrahim Hashimi