Why there's no possible conversion (generic type restricted with interface)?

Asked Feb 28 '18 at 18:11

Active Feb 28 '18 at 18:17

Viewed 67 times

I've got following classes.

interface ICommand {. . .} 

class ShutdownCommand : ICommand { . . .}

class Message<T> where T : ICommand { . . .}

class Communicator
{
    void Send(Message<ICommand> a_msg) { . . .}
}

When I call following code, I get an error that there's no possible conversion from the type Message<ShutdownCommand> to the Message<ICommand>. It's not intuitive for me since ShutdownCommand inherits from ICommand.

var message = new Message<ShutdownCommand>();
m_communicator.SendCommand(message);

I know I can solve such problem with adding non-generic interface for a Message class and use it as parameter type in Send() method.
Regardless it could be fixed in the way I described above... I'm curious why there's no possible conversion if from the code above arises conversion should be possible.

edited Feb 28 '18 at 18:17

Camilo Terevinto

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asked Feb 28 '18 at 18:11

juliuszelsner

6

Learn about covariance. – SLaks Feb 28 '18 at 18:11
1

One simple fix is to make your Send method generic: `void Send(Message a_msg) where T:ICommand { . . .}` – Ian Mercer Feb 28 '18 at 18:15
Notice that `ShutdownCommand` does not *inherit* from `ICommand`, it *implements* it – Camilo Terevinto Feb 28 '18 at 18:16
1

This is a good interview question, to instantly check presence of mind..he he he – Newton Sheikh Feb 28 '18 at 18:16

Why there's no possible conversion (generic type restricted with interface)?

0 Answers0