• With generic types, you can use the in and out in C# keywords to define if a type is covariant or contravariant.
  • Like this in C#:

interface IRepo <in TContentType>

Covariance

• The out keyword in C# marks a type parameter as covariant
• An object that is instantiated with a more derived type argument can be assigned to an object instantiated with a less derived type argument. 
• Like this in C#:

interface IRepo <out TContentType>

IEnumerable<string> strings = new List<string>();
IEnumerable<object> objects = strings;

• The second line of code above is possible because of covariance.

Contravariance

• The in keyword in C# marks a type parameter as contravariant
• An object that is instantiated with a less derived type argument can be assigned to an object instantiated with a more derived type argument. 
• Like this in C#:

interface IRepo <in TContentType>

• Assume that I have this method: 

static void SetObject(object o){}

• I can now do this:

Action<object> actObject = SetObject;
Action<string> actString = actObject;

• The second line of code above is possible because of contravariance.

More here: https://blogs.msdn.microsoft.com/csharpfaq/2010/02/16/covariance-and-contravariance-faq/