Why is the checkbox 'Prefer 32-bit' disabled in Visual Studio 2012?

Question

I came across a situation in which I would like to set the Prefer 32-bit to true. But in Visual Studio 2012, it is showing as disabled. And no matter what I do

Answer 1

This setting may also be disabled if your project is a unit test project. It turns out the setting wasn't my issue anyway. I got tests running by changing the Visual Studio test runner to run tests as a 64-bit process.

In Visual Studio Community 2015, I found this in the Test menu:
Test > Test Settings > Default Processor Architecture > X64

Here's more info from Microsoft:
https://msdn.microsoft.com/en-us/library/ee782531.aspx