What determines if a .NET app that targets the "AnyCpu" platform runs at 32 or 64 bits?

Question

I have a .NET solution with several .NET Framework projects inside, two of which generate executables (one is a Windows Service, the other is a WPF app).

Both of those projects target the "Any CPU" platform and do NOT have the "prefer 32bit" flag switched on in the build options.

However, I noticed through the task manager that while the WPF app starts by default in 64bit mode, the service starts in 32bit mode instead.

My question is: why the difference? What determines if a .NET executable that doesn't target a specific platform starts in 32 or 64 bit mode? Is it because it is a Windows Service specifically?

EDIT: re-opening, because the linked supposed "duplicate" question doesn't actually explain why a service would start in 32bit mode on a 64bit machine.

Do both the WPF app and the service target .NET Framework 4.8? — canton7, Jan 27 '22 at 11:42
Is that dup relevant? My understanding of OP's situation is that they have two exes, both targetting the same runtime, both with the same bitness settings, which are being run as different bitness — canton7, Jan 27 '22 at 11:46
@canton7: you are correct. The dupe question doesn't answer mine, it doesn't explain why one would start in 32bit mode and the other with the same settings on the same machine starts in 64 bit mode. I voted to reopen. — Master_T, Jan 27 '22 at 11:49
Can you just run `corflags` on the service, see what it says? — canton7, Jan 27 '22 at 11:50
@canton7: thanks for your suggestion... turns out the 32BITPREF flag WAS set, even if I disabled it in visual studio. I had to close visual studio, delete the bin and obj folders and rebuild, now it's launching in 64bit mode! Thanks! — Master_T, Jan 27 '22 at 11:55
(I've voted to close this as "Cannot Reproduce or caused by a typo", as it's unlikely to be helpful to others) — canton7, Jan 27 '22 at 11:58

score 0 · Accepted Answer · answered Jan 27 '22 at 11:57

Thanks to @canton7 for pointing me in the right direction.

Turns out that the "prefer 32bit" flag was still active, even if I disabled it from visual studio. Probably a caching issue, I closed visual studio, deleted the bin folders, reopened and rebuilt the solution, the service now correctly starts in 64bit mode.

What determines if a .NET app that targets the "AnyCpu" platform runs at 32 or 64 bits?

1 Answers1