爲什麼這會拋出異常？

Question

我有一個基本的控制器：

Public MustInherit Class InjuredWorkerController(Of TManager As IInjuredWorkerManagerBase) 

然後，我有一個家居控制器：

Public Class HomeController 
    Inherits InjuredWorkerController(Of IInjuredWorkerManager) 

IInjuredWorkerManager繼承IInjuredWorkerManagerBase

爲什麼這個拋出一個轉換異常：

Dim manager = CType(filterContext.Controller, InjuredWorkerController(Of IInjuredWorkerManagerBase)).Manager 

無法將'MyClaim.Controllers.HomeController'類型的對象轉換成'MyClaim.Controllers.InjuredWorkerController`1 [SAIF.Web.Mvc.MyClaim.IInjuredWorkerManagerBase]'。

Answer 1

您需要爲您的InjuredWorkerController提取一個接口才能使其工作，因爲協同和逆變僅適用於接口和委託。

此代碼可以編譯和運行（C＃控制檯應用程序，我不精通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關於這個問題是一個必須閱讀。