Return an inherited type from a method

Question

Suppose I have the following classes defined:

Public Class BaseClass
    ...
End Class

Public Class DerivedClass
    Inherits BaseClass

   ... Extra Fields, me         


        

Answer 1

Have a look at this:

http://msdn.microsoft.com/en-us/library/dd799517(v=vs.110).aspx

Understanding Covariance and Contravariance will clear things up a bit :)

• Covariance

Enables you to use a more specific type than originally specified. You can assign an instance of IEnumerable (IEnumerable(Of Derived) in Visual Basic) to a variable of type IEnumerable.

Example:

IEnumerable<Derived> d = new List<Derived>();
IEnumerable<Base> b = d;

• Contravariance

Enables you to use a more generic (less derived) type than originally specified. You can assign an instance of IEnumerable (IEnumerable(Of Base) in Visual Basic) to a variable of type IEnumerable.

Example:

Action<Base> b = (target) => { Console.WriteLine(target.GetType().Name); };
Action<Derived> d = b;
d(new Derived());

• Invariance

Means that you can use only the type originally specified; so an invariant generic type parameter is neither covariant nor contravariant. You cannot assign an instance of IEnumerable (IEnumerable(Of Base) in Visual Basic) to a variable of type IEnumerable or vice versa.