C#•7mo ago

Issue with generic interfaces (covariance/contravariance)

Hello! Currently, I have these interfaces within my project:

c#
public interface IResult<out TResponse>
{
    public bool Success { get; }
    public TResponse? Response { get; }
}

public interface ICommand<in TOptions, out TResponse>
{
    IResult<TResponse?> Execute(TOptions options);
}

public interface ICommand<in TOptions>
{
    IResult<object?> Execute(TOptions options);
}

public interface ICommand
{
    IResult<object?> Execute(object options);
}

public class CommandOptions
{

}

public class Command : ICommand<CommandOptions>
{
    public IResult<object?> Execute(CommandOptions options) 
    {
    }
}

c#
public interface IResult<out TResponse>
{
    public bool Success { get; }
    public TResponse? Response { get; }
}

public interface ICommand<in TOptions, out TResponse>
{
    IResult<TResponse?> Execute(TOptions options);
}

public interface ICommand<in TOptions>
{
    IResult<object?> Execute(TOptions options);
}

public interface ICommand
{
    IResult<object?> Execute(object options);
}

public class CommandOptions
{

}

public class Command : ICommand<CommandOptions>
{
    public IResult<object?> Execute(CommandOptions options) 
    {
    }
}

However, I'm unable to create a command like this:

c#
object options = new CommandOptions();
ICommand<object> command = new Command();
command.Execute(options);

c#
object options = new CommandOptions();
ICommand<object> command = new Command();
command.Execute(options);

Cannot implicitly convert type 'Command' to 'ICommand<object>'. An explicit conversion exists (are you missing a cast?)

Cannot implicitly convert type 'Command' to 'ICommand<object>'. An explicit conversion exists (are you missing a cast?)

... which makes it impossible for me to create a command and options dynamically. I have probably a design issue within my interfaces, so please, if there's anything that should be changed, don't hesitate to make these changes. Thanks! 😁 (Downcasting to ICommand<object> gives me an System.InvalidCastException)

1 Reply

Warcaith•7mo ago

Yeah, it does makes sense... I'll probably just need to rethink how these commands are created. Currently, I was looking to create a factory method that could create the "Command" and corresponding "Options" by a specific unique (string) name, but since I can't return something generic and always have to specify which sort of "TOptions" the command will use, the factory won't work. Yeah, I actually did exactly that after my post, however, when I tried to cast it, it still gave me the exception. I'll see if there's something weird with my implementation Here's basically what I was trying to do:

1. Program.exe MyCommandName
2. MyCommandName - Goes into -> FactoryMethod(MyCommandName)
3. FactoryMethod(MyCommandName) - Returns -> Command, Options

1. Program.exe MyCommandName
2. MyCommandName - Goes into -> FactoryMethod(MyCommandName)
3. FactoryMethod(MyCommandName) - Returns -> Command, Options

But as you said, it won't work, since I can't return anything generic enough. Do you happen to know why it doesn't work to cast to the interface? Yeah, right, seems a bit unsafe.. 😄 However, I do have this abstract class:

public abstract class Command<TOptions, TResponse> : ICommand<TOptions, TResponse>
{
    public abstract IResult<TResponse?> Execute(TOptions options);

    public IResult<TResponse?> Execute(object options)
    {
        return Execute((TOptions)options);
    }
}

public abstract class Command<TOptions> : ICommand<TOptions>
{
    public abstract IResult<object?> Execute(TOptions options);

    public IResult<object?> Execute(object options)
    {
        return Execute((TOptions)options);
    }
}

public abstract class Command : ICommand
{
    public abstract IResult<object?> Execute(object options);
}

public abstract class Command<TOptions, TResponse> : ICommand<TOptions, TResponse>
{
    public abstract IResult<TResponse?> Execute(TOptions options);

    public IResult<TResponse?> Execute(object options)
    {
        return Execute((TOptions)options);
    }
}

public abstract class Command<TOptions> : ICommand<TOptions>
{
    public abstract IResult<object?> Execute(TOptions options);

    public IResult<object?> Execute(object options)
    {
        return Execute((TOptions)options);
    }
}

public abstract class Command : ICommand
{
    public abstract IResult<object?> Execute(object options);
}

I mean, I can't cast to that class either.. is it because it's an abstract class? They do have relation For example, this doesn't work:

c#
class MyCommand : Command<CommandOptions>
{
    ...
}

ICommand<object> command = (Command<object>)new MyCommand();

c#
class MyCommand : Command<CommandOptions>
{
    ...
}

ICommand<object> command = (Command<object>)new MyCommand();

Hmm, I see... Yeah, either way, there's something missing from my design that I need to figure out 😄 Would this make sense @eXtensible Style Sheets?

c#
private static IResult<object?> ExecuteCommand(string name, string[] args)
    {
        switch(name)
        {
            case "Deploy":
                return (new DeployCommand())
                    .Execute(CreateOptions<DeployOptions>(args));
            case "Launch":
                return (new LaunchCommand())
                    .Execute(CreateOptions<LaunchOptions>(args));
            default:
                throw new ArgumentException();
        }
    }
}

c#
private static IResult<object?> ExecuteCommand(string name, string[] args)
    {
        switch(name)
        {
            case "Deploy":
                return (new DeployCommand())
                    .Execute(CreateOptions<DeployOptions>(args));
            case "Launch":
                return (new LaunchCommand())
                    .Execute(CreateOptions<LaunchOptions>(args));
            default:
                throw new ArgumentException();
        }
    }
}

Yeah, but I do need to register them somewhere. Any suggestion for making it better? I'm eager to learn 🙂 I mean, I need a registry somewhere, of some sort. All commands are of type "Command", this one:

C#
public abstract class Command<TOptions, TResponse> : ICommand<TOptions, TResponse>
{
    public abstract IResult<TResponse?> Execute(TOptions options);

    public IResult<TResponse?> Execute(object options)
    {
        return Execute((TOptions)options);
    }
}

public abstract class Command<TOptions> : ICommand<TOptions>
{
    public abstract IResult<object?> Execute(TOptions options);

    public IResult<object?> Execute(object options)
    {
        return Execute((TOptions)options);
    }
}

public abstract class Command : ICommand
{
    public abstract IResult<object?> Execute(object options);
}

C#
public abstract class Command<TOptions, TResponse> : ICommand<TOptions, TResponse>
{
    public abstract IResult<TResponse?> Execute(TOptions options);

    public IResult<TResponse?> Execute(object options)
    {
        return Execute((TOptions)options);
    }
}

public abstract class Command<TOptions> : ICommand<TOptions>
{
    public abstract IResult<object?> Execute(TOptions options);

    public IResult<object?> Execute(object options)
    {
        return Execute((TOptions)options);
    }
}

public abstract class Command : ICommand
{
    public abstract IResult<object?> Execute(object options);
}

However... now we are back at the casting problems:

c#

public LaunchOptions {}

public LaunchCommand : Command<LaunchOptions> {}

List<Command<object>> commands = new List<Command<object>>();
commands.Add(new LaunchCommand());

c#

public LaunchOptions {}

public LaunchCommand : Command<LaunchOptions> {}

List<Command<object>> commands = new List<Command<object>>();
commands.Add(new LaunchCommand());

How would I then specify which sort of options the command will use? So basically, the "Execute" method will have no options parameter, but the class itself will have the options in a property? Oh, I see!

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Issue with generic interfaces (covariance/contravariance)