public class category {

    [Key]

    public int CategoryId { get; set; }

    [DisplayName("耗材分类名称")]

    public string Title { get; set; }

    //分类下数量 不生成字段

    [NotMapped]

    public int Count { get; set; }

}

public class product {

    [Key]

    public int productId { get; set; }

    public int? CategoryId { get; set; }

}

select * from category

var Categories = from s in db.Categories

                             select s;

SELECT Category.* ,

                  (SELECT COUNT(*) 

                   FROM product 

                   WHERE product.CategoryId=Category.CategoryId) as Count

 FROM Category