building x64 c# project with msbuild results in 32bit targets

Question

I'm using Visual Studio 2010 to build a c# project that has both x86 and x64 targets. When I build using the IDE, I get the correct result of x64 and x86 targets.

When I use msbuild on the command line, I get everything built in x86, even though i specify x64 on the command line.

I didn't have this problem until I upgraded from .Net 4.0 to .Net 4.5.

With .Net 4.0 I was able to get my x64 targets even if I specified x86 on the command line.

I did a build in the .Net 4.0 environment and then another build in the .Net 4.5 environment and piped the output into log files. I noticed some differences in the log files but I think this is what is causing my issue:

In .Net 4.0 I see this line in the log file:

/reference:C:\Windows\Microsoft.Net\assembly\GAC_MSIL\Microsoft.CSharp\v4.0_4.0.0.0__b03f5f7f11d50a3a\Microsoft.CSharp.dll

In .Net 4.5 I see this line in the log file: /r:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0\Microsoft.CSharp.dll"

Any ideas? It seams .Net 4.5 is using an x86 specific path.

Answer 1

This could happen if your solution's x64 configuration is configured to build project as AnyCPU. A new configuration option in .Net 4.5, Prefer 32 bit, makes executables with Prefer32bit flag to start up as 32-bit processes on 64 bit machine. Since Prefer32bit flag is default in MSBuild targets, you will see the behavior you describe, i.e. upgrading from .Net 4.0 to .Net 4.5 will have your AnyCPU project to switch from 64 bit to 32 bit.

To make project (not solution) built for x64, specify it on command line:

msbuild project.csproj /p:Platform=x64

Notice, the command line is for .csproj, not for .sln.

Alternatively, verify you solution configuration in Configuration Manger and make sure it gets built as x64.