using System;

using System.Numerics;

using System.Collections.Generic;

​

public interface IMyEntityInterface

{

    public int ID { get; set; }

}

​

public struct MyEntityType : IMyEntityInterface {

    public int ID { get; set; }

}

​

public class MyScene

{

    // In the full implementation I use a sparse array, but for simplicity I'm using a regular array here.

    public IMyEntityInterface[] entities = new IMyEntityInterface[3];

    private int entityCount = 0;

    private int lastID = 0;

    public int AddEntity(IMyEntityInterface entity)

    {

        entityCount++;

        lastID++;

        entity.ID = lastID - 1;

        entities[entity.ID] = entity;

        return entity.ID;

    }

​

    public IMyEntityInterface GetEntity(int id)

    {

        return entities[id];

    }

    public void PrintAllEntities()

    {

        Console.WriteLine($"Entities in scene (total {entityCount}):");

        for (int i = 0; i < entityCount; i++)

        {

            Console.WriteLine(entities[i].ID);

        }

    }

}

public class Program

{

    private static void CopyEntityByID(ref MyScene scene, int id)

    {

        IMyEntityInterface entity = scene.GetEntity(id);

        scene.AddEntity(entity);

    }

    public static void Main()

    {

        MyScene scene = new();

        MyEntityType entityA = new();

        scene.AddEntity(entityA);

        // Now we have 1 entity in the scene with ID 0.

        Console.WriteLine("First entity ID:");

        Console.WriteLine(scene.entities[0].ID);

        // ... and now we try to copy it.

        CopyEntityByID(ref scene, 0);

        // And now we check up on the entities.

        scene.PrintAllEntities();

        // This should print 0, 1, but it prints 1, 1.

        // If I manually create a new entity and add it, it behaves as expected and gets the ID 2.

        MyEntityType entityB = new();

        scene.AddEntity(entityB);

        scene.PrintAllEntities();

    }

}