[英]How to cast generic interfaces using contravariant parameters to a base type?
我正在尝试开发通用命令处理器。 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,根据收到的命令类型,动态地创建适当类的实例。 然后,我想以一种通用的方式调用该类的“ Execute”方法。
我可以使用协变类型参数来完成这项工作,但是在这种情况下,我不能将泛型类型参数用作方法参数。
似乎应该使用一个协变方法,因为它允许我根据需要声明方法参数,但是不幸的是,该类的实例无法转换为基本接口。
下面的代码示例了此问题:
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);
}
}
}
你可以施放通用接口out
泛型参数只接口更具体的参数。 例如,可以将ICommandMessageHandler1<CommandMessage>
强制转换为ICommandMessageHandler2<CreateOrderMessage>
( Execute(CommandMessage command)
也将接受CreateOrderMessage
),但反之亦然。
试想一下,例如,如果允许在代码中抛出InvalidCastException
,那么如果您调用handler2c.Execute(new CommandMessage())
会发生什么?
接口ICommandMessageHandler1<T>
和ICommandMessageHandler2<T>
彼此不相关。 仅仅因为它们都有方法Execute
并不能使它们兼容。 这将是鸭子式的,c#不支持。
也许我真的没有得到您想要的功能,但这对我来说没有任何类型转换的效果很好。
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);
}
}
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