在运行时转换为未知的派生类型？

Question

我有这个命令总线实现，通常这样调用：

bus.Invoke(new FocusCommand());

这工作正常，但现在有一种情况是要调用的命令是动态的并且会抛出错误：

// Command to invoke is resolved at runtime.
Command parsedCommand = ParseCommand(input);
bus.Invoke(parsedCommand);

实施的简化版本如下所示：

using System;
using System.Diagnostics;

public class Command
{
}

public class FocusCommand : Command
{
}

public interface ICommandHandler<TCommand> where TCommand : Command
{
  void Handle(TCommand command);
}

public class FocusHandler : ICommandHandler<FocusCommand>
{
  public void Handle(FocusCommand command)
  {
    Debug.WriteLine("FocusHandler was called.");
  }
}

public class Bus
{
  // Sends command to appropriate command handler.
  public void Invoke<T>(T command) where T : Command
  {
    object handlerInstance = GetHandlerInstanceForCommand(command);

    (handlerInstance as ICommandHandler<T>).Handle(command); // <--- Error is thrown here in 2nd case.
  }

  // Returns an instance of the corresponding handler for a command.  In the real
  // application, this gets the handler instance from the DI framework using
  // `Provider.GetRequiredService`, which always returns an `object`.
  private object GetHandlerInstanceForCommand<T>(T command) where T : Command
  {
    if (command.GetType() == typeof(FocusCommand))
      return new FocusHandler();
    else
      throw new Exception();
  }
}

static class Program
{
  /// <summary>
  ///  The main entry point for the application.
  /// </summary>
  [STAThread]
  static void Main()
  {
    Debug.WriteLine("Application started");

    Bus bus = new Bus();
    bus.Invoke<FocusCommand>(new FocusCommand()); // Is called successfully.

    Command parsedCommand = ParseCommand("FocusCommand");
    bus.Invoke<Command>(parsedCommand); // Throws error.
  }

  // Returns different commands based on input.
  private static Command ParseCommand(string input)
  {
    if (input == "FocusCommand")
      return new FocusCommand();
    else
      throw new Exception();
  }
}

调用 bus.Invoke<Command>(parsedCommand) 时，在 Bus.Invoke 内转换 handlerInstance as ICommandHandler<T> 失败。

如何更改 Bus.Invoke 以使用派生的 Command 类型未知的命令？

Answer 1

这与 covariance and contravariance 有关。

转换失败的原因是，如果您被允许执行您正在尝试执行的操作，您将能够将不兼容的类型传递给您的处理程序。例如，您的方法将允许这样：

ICommandHandler<Command> focusHandler = new FocusHandler();
Command closeCommand = new CloseCommand();
focusHandler.Handle(closeCommand);

处理编译时未知类型的一种方法是使用反射。您提到了 Provider.GetRequiredService，所以我假设您使用的是 Microsoft.Extensions.DependencyInjection。您可以执行以下操作：

将所有处理程序注册为其实际类型
使用反射从实际命令类型创建实际处理程序类型
使用该类型从 DI 获取处理程序
使用反射调用处理程序

像这样注册您的处理程序：

services.AddTransient<ICommandHandler<FocusCommand>, FocusHandler>();
services.AddTransient<ICommandHandler<CloseCommand>, CloseHandler>();

像这样实现您的 Invoke 方法：

public void Invoke(Command command)
{
    Type commandType = command.GetType();
    Type handlerType = typeof(ICommandHandler<>).MakeGenericType(commandType);
    var handler = serviceProvider.GetService(handlerType);
    var mi = handlerType.GetMethod("Handle", 0, new[] { commandType });
    mi.Invoke(handler, new[] { command });
}

在运行时转换为未知的派生类型？

Casting to unknown derived type at runtime?

.net

c#

reflection

casting

system.reflection