我错过了什么？

得到错误：

Cannot implicitly convert type ‘System.Linq.IOrderedEnumerable’ to ‘System.Linq.IQueryable’

代码段(删除了一些部分)：

public class QuickbooksRespository<TEntity>
         where TEntity : class,Intuit.Ipp.Data.IEntity,new()
    {
    public virtual IQueryable<TEntity> GetAll(
        int page,int pageSize,Func<TEntity,object> orderbyascending = null,object> orderbydescending = null)
    {
        int skip = Math.Max(pageSize * (page - 1),0);

        IQueryable<TEntity> results = _qbQueryService
                .Select(all => all);

        if (orderbyascending != null)
        {
            results = results.OrderBy(orderbyascending);
        }

        if (orderbydescending != null)
        {
            results = results.OrderByDescending(orderbydescending);
        }

        return results
                .Skip(skip)
                .Take(pageSize);
    }
}