How to use Inheritance when using Generic Constraints

Question

I\'m struggling with some Generic constraint issues when trying to implement a library that allows inheritance and hoping someone can help.

I\'m trying to build up a cl

Answer 1

OK, let’s examine the first one. The error is:

The type 'V' cannot be used as type parameter 'V' in the generic type or method 'Test.Generic_Route'. There is no implicit reference conversion from 'V' to 'Test.Generic_Visit'.

It complains about this declaration:

public abstract class Generic_TSPRoute : Generic_Route
    where V : Concrete_TSPVisit
    where E : Concrete_TSPNode

This establishes two definitions:

• V is a Concrete_TSPVisit (or a descendent of it)

• E is a Concrete_TSPNode (or a descendent of it)

Now let’s see what Generic_Route lets us put in:

public class Generic_Route
    where V : Generic_Visit
    where E : GenericElement

The second constraint is fine because Concrete_TSPNode is a GenericElement. The first one is problematic: Remember that E is a Concrete_TSPNode or a descendent of it, therefore Generic_Visit could be:

• Generic_Visit, or

• Generic_Visit

However, we also know from earlier that V is a Concrete_TSPVisit (or a descendent of it).

• Concrete_TSPVisit inherits from Generic_TSPVisit

• Generic_TSPVisit inherits from Generic_Visit

Notice something? This requires it to be a Generic_Visit. It is emphatically not allowed to be Generic_Visit.

In other words, imagine I write this:

var route = new Generic_TSPRoute();

According to your hierarchy, Concrete_TSPVisit is a Generic_Visit and therefore has a property that looks like

public Concrete_TSPNode Element { get; set; }

If I retrieve a value from this property, it is only guaranteed to be a Concrete_TSPNode but not necessarily a Concrete_TSPNode_Subclass.

EDIT:

I’ll leave this answer because it explains the reason for the compiler error, but Enigmativity’s answer actually provides the solution to the problem.