A quick working example of Covariance and Contravariance in generics for a reference of when in and out C# keywords can be used.
There are a lot of useful blog posts explaining Covariance and Contravariance, e.g. mathemetical model behind it, and all the technical details.
Below is a simple code snippet illustrating what effect in and out C# keywords have when used in generics. Lines that cause compilation error are commented out.