How do I specify the platform for MSBuild?

If you want to build your solution for x86 and x64, your solution must be configured for both platforms. Actually you just have an Any CPU configuration.

How to check the available configuration for a project

To check the available configuration for a given project, open the project file (*.csproj for example) and look for a PropertyGroup with the right Condition.

If you want to build in Release mode for x86, you must have something like this in your project file:

<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|x86' ">
  ...
</PropertyGroup>

How to create and edit the configuration in Visual Studio

_{(source: microsoft.com)}

_{(source: msdn.com)}

_{(source: msdn.com)}

How to create and edit the configuration (on MSDN)

In MSBuild or Teamcity use command line

MSBuild yourproject.sln /property:Configuration=Release /property:Platform=x64

or use shorter form:

MSBuild yourproject.sln /p:Configuration=Release /p:Platform=x64

However you need to set up platform in your project anyway, see the answer by Julien Hoarau.

Hopefully this helps someone out there.

For platform I was specifying "Any CPU", changed it to "AnyCPU" and that fixed the problem.

msbuild C:\Users\Project\Project.publishproj /p:Platform="AnyCPU"  /p:DeployOnBuild=true /p:PublishProfile=local /p:Configuration=Debug

If you look at your .csproj file you'll see the correct platform name to use.

If you're trying to do this from the command line, you may be encountering an issue where a machine-wide environment variable 'Platform' is being set for you and working against you. I can reproduce this if I use the VS2012 Command window instead of a regular windows Command window.

At the command prompt type:

set platform

In a VS2012 Command window, I have a value of 'X64' preset. That seems to interfere with whatever is in my solution file.

In a regular Command window, the 'set' command results in a "variable not defined" message...which is good.

If the result of your 'set' command above returns no environment variable value, you should be good to go.