What is default msbuild platform

Question

How does msbuild chose a platform if it is not specified? It seems to me that for some solutions it selects "Mixed Platforms" for others "x86".

I switch on the diagnostics level of logging and the only thing I can see is that "Initial Properties" at the beginning contain e.g. "Platform = Mixed Platforms" without any explanation why.

To preempt some answers, I know that I can override the platform manually. That is not an issue. I need to know what msbuild does when it is NOT specified.

Answer 1

MSBuild does not choose but whatever MSBuild project it is building may default certain properties. I am assuming that your question relates to how MSBuild builds a solution file.

msbuild.exe "somesolution.sln" /t:Build

You need to look at the projects that make up the solution, in there you will see the properties that are set. For example you will probably see the following at the top of the project file:

<PropertyGroup>
  <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
  <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
  ...
</PropertyGroup>

This shows a PropertyGroup containing amongst others two properties, Configuration and Platform. Their values are set based on a Condition. The condition says says that if no value has been set for the property Configuration it should default to 'Debug'. Likewise if nothing is set for Platform it should default to AnyCPU.

You may also see a Conditional PropertyGroup:

<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
  ...
</PropertyGroup>

What this condition says is that if the property Configuration and Platform match Debug and AnyCPU then it should apply all of the properties contained within.

A point to note is that the property names are just an arbitrary name and the values are just strings. However when building .Net projects there is a convention to which these properties and their values are a part. To see what the default values are you do not need to open each project in a text editor. You can go into Visual Studio and look at the solution configuration.

Answer 2

This may help: I was researching this and finally tracked down the default platform for my install, by looking in Microsoft.Cpp.Default.props (line 21 in this version of Visual Studio), which lives in Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V120:

<Platform Condition="'$(Platform)' == ''">Win32</Platform>

This means that under VS12 (Visual Studio 2013) MSBuild will pick Win32 as the platform if no other platform is explicitly specified. As noted in some other questions, setting an environment variable named Platform will change the default to the value you set.

Important note: If you invoke MSBuild on a Visual Studio solution file (*.sln) rather than a project file, and you don't specify a platform in the MSBuild arguments, then it appears that MSBuild will choose the platform automatically based on the first entry under the SolutionConfigurationPlatforms global section in the solution file. I haven't found this documented anywhere but from experimentation it appears to be the case. This means that editing your project file and providing a different default Platform property (as described above), MSBuild will ignore this default, because it will have chosen the platform already before it even starts looking at the project. Invoking MSBuild directly on the project file seems to bypass this behavior.