C#•4w 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

Tofaa•4w ago

Please ping me if anyone replies

Angius•4w 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

Tofaa•4w 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•4w ago

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

Tofaa•4w ago

wdym

MODiX•4w 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•4w 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

Tofaa•4w ago

yeah but im not sure how that would help my case

Tofaa•4w ago

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

Angius•4w 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;
    }

Tofaa•4w 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•4w ago

And the listener would be, say,

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

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

Tofaa•4w ago

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

Angius•4w ago

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

Tofaa•4w ago

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

Angius•4w ago

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

Tofaa•4w ago

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

Angius•4w ago

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

Tofaa•4w ago

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

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Help using generics and maps