What is the purpose of the "Prefer 32-bit" setting in Visual Studio and how does it actually work?

c# .net visual-studio compilation cpu-architecture

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Solution 1

Microsoft has a blog entry What AnyCPU Really Means As Of .NET 4.5 and Visual Studio 11:

In .NET 4.5 and Visual Studio 11 the cheese has been moved. The default for most .NET projects is again AnyCPU, but there is more than one meaning to AnyCPU now. There is an additional sub-type of AnyCPU, “Any CPU 32-bit preferred”, which is the new default (overall, there are now five options for the /platform C# compiler switch: x86, Itanium, x64, anycpu, and anycpu32bitpreferred). When using the "Prefer 32-Bit" flavor of AnyCPU, the semantics are as follows:

If the process runs on a 32-bit Windows system, it runs as a 32-bit process. IL is compiled to x86 machine code.

If the process runs on a 64-bit Windows system, it runs as a 32-bit process. IL is compiled to x86 machine code.

If the process runs on an ARM Windows system, it runs as a 32-bit process. IL is compiled to ARM machine code.

The difference, then, between “Any CPU 32-bit preferred” and “x86” is only this: a .NET application compiled to x86 will fail to run on an ARM Windows system, but an “Any CPU 32-bit preferred” application will run successfully.

Solution 2

Here's a simple answer:

Note: AnyCPU-32bitPreferred is only available in .Net version 4.5 and higher.

90,187

Aaron

Passionate software engineer with strong knowledge and experience in Linux, security, and databases.

Updated on December 26, 2020

Comments

Aaron over 3 years

It is unclear to me how the compiler will automatically know to compile for 64-bit when it needs to. How does it know when it can confidently target 32-bit?

I am mainly curious about how the compiler knows which architecture to target when compiling. Does it analyze the code and make a decision based on what it finds?
- Marc Gravell over 11 years
  
  stackoverflow.com/questions/7508965/…
- Aaron over 11 years
  
  Ah, thanks. Didn't see that before. I am still curious as to how the compiler automatically knows which architecture to target. Any ideas?
JP Richardson over 11 years

I'm not sure that's accurate. As, it's my understanding that .NET executables regardless of 32 or 64 are still limited around 2 GB per process.
Aaron over 11 years

Any idea on how the compiler knows which architecture to target?
Peru over 11 years

Edited my answer. But not Sure if this is what you are looking for :)
Alexei Levenkov over 11 years

@Aaron, compiler essentially sets flag for runtime to decide if it is ok to load assembly (i.e. block x86-only assembly to be loaded in x64 process) and how to start the process (for new EXE) based on flags. I believe IL is the same for both flavors.
Peru over 11 years

@JPRichardson Yes you are right. But in .net 4.5 you have the option to increase the size. refer MSDN
Alexei Levenkov over 11 years

@JPRichardson, neither 32 nor 64 bit .Net executable limited to 2GB per process - first of all per-process address space is OS level restriction (2/3+GB for 32bit process and much more for 64bit), second even 32bit version can use more than 2GB if "LargeAddressAware" flag is set on the executable. The only 2GB restrictions I know are about array/allocation sizes that are limited by Int32 range (about 2GB).
Lee Grissom almost 11 years

+1. Also, the "Prefer 32-bit" checkbox is only enabled for .NET 4.5+ executable projects.
Bruno Martinez almost 11 years

Another advantage of anycpu32bitspreferred is that another .exe running in 64 bits can load that assembly.
Dave over 10 years

Personally I think it is horrible they set this by default with no Tools setting to turn it off. Even worse, you can't search for it since not in the csproj files unless turned off! Probably added because of the Office Automation incompatibilities with CPUAny on a x64 machine with most folks installing 32 bit Office.
Brian David Berman over 7 years

Wait, so there is no configuration that will lead to a 64-bit process?
Lex Li over 7 years

@BrianDavidBerman there is, if you set false to 32-but preferred and set x64 or Any CPU on a 64-bit machine.
jjxtra over 6 years

There are performance implications to running as 32 bit on a 64 bit operating systems. I've noticed double and long operations taking a lot longer for example.
jjxtra over 6 years

There are performance implications to running as 32 bit on a 64 bit operating systems. I've noticed double and long operations taking a lot longer for example.
Nic over 6 years

The difference between x86 and Any CPU 32 bit preferred is that in the latter case the largeaddressaware flag is set on the executable. This means that the 32 bit process running on a 64 bit OS can use 2GB of memory in x86 mode and 4GB of memory in Any CPU 32 bit preferred mode.
jrh almost 6 years

As a followup to @Nic's statement, Here's a post with more information about how "prefer 32 bit" sets IMAGE_FILE_LARGE_ADDRESS_AWARE : "Why does 'Any CPU (prefer 32-bit)' allow me to allocate more memory than x86 under .NET 4.5?"
Shiv over 4 years

Changed the default to Prefer 32-bit is a HUGE regression.
Peter Cordes over 4 years

What's the difference between "runs as 32-bit" vs. runs as "WoW64". I thought WoW64 = "Windows (32-bit) on Windows64" and was needed for any 32-bit application to run.
Ran Sagy over 4 years

Is there a source to this? Seemingly everywhere else still tells me the default is anycpu32bitpreferred, which is a vast difference to people running on 64bit windows machines (that's a lot).
Yousha Aleayoub over 4 years

@RanSagy you can simply test it by creating a new project and checking Project -> Properties -> Build tab -> Platform target... but note that AnyCPU-32bitPreferred is only available in .Net version 4.5 and higher. That's why the default is AnyCPU.
Ran Sagy over 4 years

In some cases mine was greyed out; I was just hoping there's some documentation on what happens in .net 4.5+ or .net standard/core (or really, MSBuild 16)