NUnit will automatically get all types marked with TestFixture
attribute from test assembly when you load it (it does not matter whether fixtures in one .cs file or in separate). Then NUnit will search for methods marked with Test
or TestCase
attributes and load them. If NUnit don't see some test, then make sure you loaded latest version of your test assembly.
NOTE: If you are using NUnit test runner, then there is nice setting Reload when test assembly changes. With this option turned on NUnit will automatically reload test assembly when you rebuild it in Visual Studio.