What does an “in” generic parameter do?

Question

Saw this signature today:

public interface ISomeInterface

What impact does the in parameter have?

Answer 1

That signifies generic contravariance. The opposite is covariance (keyword out).

What this means is that when an interface is contravariant (in), then the interface can be implicitly converted to a generic type when the type parameter inherits T.

Conversely for covariance out, the interface can be implicitly converted to a generic type where the type parameter is a 'lesser' type in the type hierarchy.