How to run unit tests (MSTest) in parallel?

Most of the answers on this page forget to mention that MSTest parallelizes tests in separate assemblies. You have to split your unittests into multiple .dll's to paralelize it.

But! The recent version - MSTest V2 - now CAN parallelize "in-assembly" (yay!) you just need to install a couple of nuget packages in your test project - TestFramework and TestAdapter - like described here https://blogs.msdn.microsoft.com/devops/2018/01/30/mstest-v2-in-assembly-parallel-test-execution/

And then simply add this to your test project

[assembly: Parallelize(Workers = 4, Scope = ExecutionScope.ClassLevel)]

EDIT: You can also disable parallel execution for a specific test using [DoNotParallelize] on a test method.

You can get up to 5 by using the method from the Visual Studio Team Test Blog

Keep in mind that there may be concurrency issues using this, as MSTest doesn't completely isolate each test (statics carry over, for example, making things interesting for code meant to run once).

(No idea why the limit is 5, but MSTest will not run them in parallel if parallelTestCount is set to more than 5. As per the comments below, this rule apparently changes with Visual Studio 2013)