Why does this throw and exception? - asp.net

Question

I have a base controller:
Public MustInherit Class InjuredWorkerController(Of TManager As IInjuredWorkerManagerBase)
Then I have a home controller:
Public Class HomeController
Inherits InjuredWorkerController(Of IInjuredWorkerManager)
IInjuredWorkerManager inherits IInjuredWorkerManagerBase
Why does this throw a cast exception:
Dim manager = CType(filterContext.Controller, InjuredWorkerController(Of IInjuredWorkerManagerBase)).Manager
Unable to cast object of type 'MyClaim.Controllers.HomeController' to type 'MyClaim.Controllers.InjuredWorkerController`1[SAIF.Web.Mvc.MyClaim.IInjuredWorkerManagerBase]'.

Answer 1

You need to extract an interface for your InjuredWorkerController to make it work, since co- and contravariance only works with interfaces and delegates.
This code compiles and runs (C# console app, I'm not fluent in VB.Net...):
using System;
namespace TestApplication
{
public interface IInjuredWorkerController<out TManager>
where TManager : IInjuredWorkerManagerBase
{
TManager Manager { get; }
}
public abstract class InjuredWorkerController<TManager>
: IInjuredWorkerController<TManager>
where TManager : IInjuredWorkerManagerBase, new()
{
protected InjuredWorkerController()
{
Manager = new TManager();
}
public TManager Manager { get; private set; }
}
public interface IInjuredWorkerManagerBase
{
string Name { get; }
}
public interface IInjuredWorkerManager
: IInjuredWorkerManagerBase {}
public class InjuredWorkerManager : IInjuredWorkerManager
{
public string Name
{
get { return "Homer"; }
}
}
public class HomeController
: InjuredWorkerController<InjuredWorkerManager> {}
internal class Program
{
private static void Main()
{
var controller = new HomeController();
var manager = ((IInjuredWorkerController<IInjuredWorkerManagerBase>)controller).Manager;
Console.Out.WriteLine(manager.Name);
Console.ReadKey();
}
}
}
Eric Lippert's blog series on the subject is a must read.