无法将dbSet隐式转换为ObjectQuery

Question

这是我的代码，它不能像这样工作，请帮帮我!!!

public class BL: IBL, IDisposable
{

    private TradeSoftDBEntities entities;

    public List<Roles> TakeRoles()
    {

        if (entities == null) 
            entities = new TradeSoftDBEntities();

        ObjectQuery<Roles> roles = entities.Rolis;

        roles.MergeOption = MergeOption.NoTracking;