C#•6mo ago

Help using generics and maps

I've been trying to make a basic listener consumer system for a Minecraft server implementation im making in c# from scratch. I come from a java background so c# is fairly similar but im stuck on trying to make a generic map of an unknown type at runtime.

private final Map<Class<? extends ClientPacket>, PacketConsumer>[] listeners = new Map[5];

public <T extends ClientPacket> void setListener(@NotNull ConnectionState state, @NotNull Class<T> packetClass, @NotNull PacketConsumer<T> consumer) {
        this.listeners[state.ordinal()].put(packetClass, consumer);
    }
@FunctionalInterface
public interface PacketConsumer<T extends ClientPacket> {
  void consume(T packet, PlayerConnection connection);
}

private final Map<Class<? extends ClientPacket>, PacketConsumer>[] listeners = new Map[5];

public <T extends ClientPacket> void setListener(@NotNull ConnectionState state, @NotNull Class<T> packetClass, @NotNull PacketConsumer<T> consumer) {
        this.listeners[state.ordinal()].put(packetClass, consumer);
    }
@FunctionalInterface
public interface PacketConsumer<T extends ClientPacket> {
  void consume(T packet, PlayerConnection connection);
}

I'm trying to rewrite this to c#. I have

    private readonly ConcurrentDictionary<Type, Action<IClientPacket, Connection>>[] _listeners = [];
    public void SetListener<T>(ConnectionState state, Type type, Action<T, Connection> listener) where T : IClientPacket
    {
        var dict = _listeners[(int)state];
        dict[type] = listener; // cannot set this because its not IClientPacket, but rather T
    }

    private readonly ConcurrentDictionary<Type, Action<IClientPacket, Connection>>[] _listeners = [];
    public void SetListener<T>(ConnectionState state, Type type, Action<T, Connection> listener) where T : IClientPacket
    {
        var dict = _listeners[(int)state];
        dict[type] = listener; // cannot set this because its not IClientPacket, but rather T
    }

Any help would be fantastic, thank you :D

19 Replies

TofaaOP•6mo ago

Please ping me if anyone replies

Angius•6mo ago

Can't you just use the interface instead of a generic parameter even in the listener? Thing with Java is that generics are fake and don't exist during runtime, they just get replaced with an Object and a bunch of casts C# does not erase generics, so it's a bit more strict Stuff like type covariance/contravariance come into play

TofaaOP•6mo ago

I could but then i'd need to define a class extending it at all times since Functional interfaces dont exist and c# has deligates instead Most of the listeners i need are one/two liners so it'd be a lot of unnecessary boilerplate But if that's my only alternative ig

Angius•6mo ago

Worst case scenario, use object and validate that it's of that interface ¯\_(ツ)_/¯

TofaaOP•6mo ago

wdym

MODiX•6mo ago

Angius

REPL Result: Success

interface IFoo { }
class Foo : IFoo { }

object f = new Foo();

if (f is IFoo ifoo)
{
    Console.WriteLine("It's an IFoo");
}

interface IFoo { }
class Foo : IFoo { }

object f = new Foo();

if (f is IFoo ifoo)
{
    Console.WriteLine("It's an IFoo");
}

Console Output

It's an IFoo

It's an IFoo

Compile: 473.945ms | Execution: 39.387ms | React with ❌ to remove this embed.

Angius•6mo ago

You can use pattern matching to check that an object is an IFoo and cast it to an IFoo in one go This is more like what Java would do under the hood

TofaaOP•6mo ago

yeah but im not sure how that would help my case

TofaaOP•6mo ago

you cant also put unknown generics into c# collections either so i couldnt convert it to an interface

Angius•6mo ago

    private readonly ConcurrentDictionary<Type, Action<object, Connection>>[] _listeners = [];
    public void SetListener(ConnectionState state, Type type, Action<object, Connection> listener)
    {
        var dict = _listeners[(int)state];
        dict[type] = listener;
    }

    private readonly ConcurrentDictionary<Type, Action<object, Connection>>[] _listeners = [];
    public void SetListener(ConnectionState state, Type type, Action<object, Connection> listener)
    {
        var dict = _listeners[(int)state];
        dict[type] = listener;
    }

TofaaOP•6mo ago

unless u mean i make it store the object type then cast it Ah yeah LOL Still seems like a very hacky workaround

Angius•6mo ago

And the listener would be, say,

Connection (object a) => {
    if (a is IClientPacket icp)
    {
        // ...
    }
}

Connection (object a) => {
    if (a is IClientPacket icp)
    {
        // ...
    }
}

TofaaOP•6mo ago

I got it that's rather annoying but nothing i cando Thank you for your help :D

Angius•6mo ago

It's about the only thing I could think of, maybe someone else will have a better solution

TofaaOP•6mo ago

I'll do it for now and see if i can find a better fix down the line

Angius•6mo ago

Maybe covariant/contravariant generics would help here, idk Making the T an in or out generic, etc

TofaaOP•6mo ago

I'd need to look into how that works in c#

Angius•6mo ago

I'll be honest, I don't fully understand co/contravariance myself lol

TofaaOP•6mo ago

I remember the concept from kotlin but its still mad hacky LOL

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Help using generics and maps