GameController.cs

void Start()
 { 
     // ...
     _contexts.game.OnEntityCreated += AddId;
     _contexts.input.OnEntityCreated += AddId;
     // ...
 }

 private void AddId(IContext context, IEntity entity)
 {
     GameEntity gameEntity = entity as GameEntity;
     if (gameEntity != null) gameEntity.AddId(gameEntity.creationIndex);
     InputEntity inputEntity = entity as InputEntity;
     if (inputEntity != null) inputEntity.AddId(inputEntity.creationIndex);
 }

public class IdComponent : IComponent 
{
   [PrimaryEntityIndex]
   public int value;
}