command-casting-error.cs

using System;
using System.Diagnostics;

namespace MyProgram
{

  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 `Command` types based on input.
    private static Command ParseCommand(string input)
    {
      if (input == "FocusCommand")
        return new FocusCommand();
      else
        throw new Exception();
    }
  }
}