client.

room.Listen("players/:id", OnPlayerChange);

private void OnPlayerChange(DataChange change)
{
	Debug.Log($"OnPlayerChange: {change.operation} > {change.path["id"]} > {change.value}");

	if (change.operation == "add")
	{
		// extract data
		IndexedDictionary<string, object> value = (IndexedDictionary<string, object>)change.value;

		// this prints (0) - no values in the dictionary
		Debug.LogWarning($"There are {value.Count} entries");

		string id = change.path["id"];
		Vector2 pos = new Vector3(Convert.ToSingle(value["x"]), Convert.ToSingle(value["y"]));
		int hp = Convert.ToInt32(value["hp"]);

		// create and add to list of players
		gameController.CreateActor(id, pos, hp);
	}

	else if (change.operation == "remove")
	{
		// remove player
		gameController.RemoveActor(change.path["id"]);
	}
}

server

import { Client, Room, nosync } from "colyseus";

export class Player
{
    x: number;
    y: number;
    hp: number;

    @nosync client: Client;
    @nosync room: Room;
    
    constructor(client: Client, room: Room) 
    {
      this.client = client;
      this.room = room;
    }    
}

import { EntityMap, Client, Room } from "colyseus";
import { Player } from "./Player";

export class GameState
{
    players: EntityMap<Player> = {};
   
    addPlayer(client: Client, room: Room)
    {
        this.players[client.sessionId] = new Player(client, room);
    }
    
    removePlayer(client: Client)
    {
        delete this.players[client.sessionId];
    }
    
    performAction(client: Client, action: string)
    {
        if (action === "left") 
        {
            this.players[client.sessionId].x -= 1;
        } 
        else if (action === "right") 
        {
            this.players[client.sessionId].x += 1;
        }
    }    
}

export class Player
{
    x: number = 0;
    y: number = 0;
    hp: number = 0;