Return an inherited type from a method - vb.net

Question

Suppose I have the following classes defined:
Public Class BaseClass
...
End Class
Public Class DerivedClass
Inherits BaseClass
... Extra Fields, methods, etc ...
End Class
And then, in my code, I have a function with a signature of:
Public Function DoSomething(...) As List(Of BaseClass)
And when I try and return an object of type List(Of DerivedClass) from it, I get the error:
Value of type 'System.Collections.Generic.List(Of BaseClass)' cannot be converted to 'System.Collections.Generic.List(Of DerivedClass)'
I know not all the extra fields of the DerivedClass will be filled, but it would give me what I needed.
Is there a way to do this, or is this just considered bad programming practice? And, if so, what would be the right way to do this?

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.