namespace EnablingMARS.Models
{
    public class Product
    {
        public int Id { get; set; }
        public string Title { get; set; }
        public string Desc { get; set; }
        public float Price { get; set; }
        public Category Category { get; set; }

    }

    public enum Category
    {
        Cate1,
        Cate2,
        Cate3
    }
}

namespace EnablingMARS.Models
{
    public class ProductDbContext : DbContext
    {
        public ProductDbContext() : base("EnablingMARS") {}
        public DbSet<Product> Products { get; set; }

    }
}

MyContext.Products.Add(new Product()
 {
                Title = "title1",
                Desc = "desc",
                Price = 4500f,
                Category = Category.Cate1
   });
MyContext.SaveChanges();

foreach (var product in _dvContext.Products.Where(category => category.Category == Category.Cate1))
{
     product.Price = 50000;
     MyContext.SaveChanges();
}

There is already an open DataReader associated with this Command which must be closed first.

"Data Source=(LocalDB)\v11.0;Initial Catalog=EnablingMARS; MultipleActiveResultSets=true"