如何使用逆变参数将通用接口转换为基本类型？

Question

我想开发一个通用的命令处理器。我想创建实现给定接口的命令处理程序类。我将使用控制反转来根据收到的命令类型创建适当类的实例。然后我想以通用的方式调用该类的“Execute”方法。

我能够使用covariant类型参数进行这项工作，但在这种情况下，我不能使用泛型类型参数作为方法参数。

这似乎是一种逆变方法应该起作用，因为它允许我根据需要声明方法参数，但不幸的是该类的实例无法转换为基本接口。

下面的代码举例说明了问题：

using System; 
using System.Diagnostics; 

namespace ConsoleApplication2 
{ 
    // Command classes 

    public class CommandMessage 
    { 
     public DateTime IssuedAt { get; set; } 
    } 

    public class CreateOrderMessage : CommandMessage 
    { 
     public string CustomerName { get; set; } 
    } 

    // Covariant solution 

    public interface ICommandMessageHandler1<out T> where T : CommandMessage 
    { 
     void Execute(CommandMessage command); 
    } 

    public class CreateOrderHandler1 : ICommandMessageHandler1<CreateOrderMessage> 
    { 
     public void Execute(CommandMessage command) 
     { 
      // An explicit typecast is required 
      var createOrderMessage = (CreateOrderMessage) command; 
      Debug.WriteLine("CustomerName: " + createOrderMessage.CustomerName); 
     } 
    } 

    // Contravariant attempt (doesn't work) 

    public interface ICommandMessageHandler2<in T> where T : CommandMessage 
    { 
     void Execute(T command); 
    } 

    public class CreateOrderHandler2 : ICommandMessageHandler2<CreateOrderMessage> 
    { 
     public void Execute(CreateOrderMessage command) 
     { 
      // Ideally, no typecast would be required 
      Debug.WriteLine("CustomerName: " + command.CustomerName); 
     } 
    } 

    class Program 
    { 
     static void Main(string[] args) 
     { 
      var message = new CreateOrderMessage {CustomerName = "ACME"}; 

      // This code works 
      var handler1 = new CreateOrderHandler1(); 
      ICommandMessageHandler1<CreateOrderMessage> handler1b = handler1; 
      var handler1c = (ICommandMessageHandler1<CommandMessage>) handler1; 
      handler1c.Execute(message); 

      // This code throws InvalidCastException 
      var handler2 = new CreateOrderHandler2(); 
      ICommandMessageHandler2<CreateOrderMessage> handler2b = handler2; 
      var handler2c = (ICommandMessageHandler2<CommandMessage>)handler2; // throws InvalidCastException 
      handler2c.Execute(message); 
     } 
    } 
} 

Answer 1

您可以out泛型参数只投接口通用接口与更具体参数。例如。 ICommandMessageHandler1<CommandMessage>可能被铸造为ICommandMessageHandler2<CreateOrderMessage>（Execute(CommandMessage command)也将接受CreateOrderMessage），但反之亦然。

试着想想，例如，如果在你的代码中投掷InvalidCastException会被允许，如果你叫handler2c.Execute(new CommandMessage())会发生什么？

Answer 2

接口ICommandMessageHandler1<T>和ICommandMessageHandler2<T>彼此不相关。仅仅因为两者都有一个方法Execute不会使它们兼容。这将是duck-typing，这在c＃中不受支持。

Answer 3

也许我真的没有得到你想要做的，但是这对我没有任何typecasts正常工作。

public class CommandMessage 
{ 
    public DateTime IssuedAt 
    { 
     get; 
     set; 
    } 
} 

public class CreateOrderMessage : CommandMessage 
{ 
    public string CustomerName 
    { 
     get; 
     set; 
    } 
} 

public interface ICommandMessageHandler2<in T> where T : CommandMessage 
{ 
    void Execute(T command); 
} 
public class CreateOrderHandler2 : ICommandMessageHandler2<CreateOrderMessage> 
{ 
    public void Execute(CreateOrderMessage command) 
    { 
     // No typecast is required 
     Debug.WriteLine("CustomerName: " + command.CustomerName); 
    } 
} 

class Program 
{ 
    static void Main(string[] args) 
    { 
     var message = new CreateOrderMessage 
     { 
      CustomerName = "ACME" 
     }; 

     // This code throws InvalidCastException 
     var handler2 = (ICommandMessageHandler2<CreateOrderMessage>)new CreateOrderHandler2(); 
     handler2.Execute(message); 
    } 
}