How can I inherit a class at project/global level in C#?

Question

I need some help with Nunit test classes.

I have written [OneTimeSetUp] and [OneTimeTearDown] methods in a class file and these methods get executed before and after every class correctly.

I have also written [SetUp] and [TearDown] methods in a separate class file to be executed before and after every test case, but these methods do not get executed until I inherit this class in my test class. I have 80 test classes so I will have to inherit it in 80 class files, which I want to avoid.