ASP.NETMVC---分部类的使用

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因为最近需要使用C#，所以特意花了几天时间看了下C#的基本语法。其中，分部类型就引起了我的注意。

分部类型是C#中一个很神奇的地方，它允许我们将一个类型的定义散布在各个文件中。像是下面这样的例子：

//Example1.cs
public partial class Example{
     public string mName = "Example";
}

//Example2.cs
public partial class Example{
    public void Show(){
         Console.WriteLine(mName);
    }
}

然后我们通过一个测试类来测试：

public static void Main(string[] args){
    Example example = new Example();
    example.Show();
}

从这里我们可以看出，我们可以将数据放在一个分部类型中，然后将方法放在另一个同名的分部类型中。记得，必须是同名，否则它们之间的数据和方法并不认为是共享的。
既然有这个东西，我们第一个念头肯定是：有啥用啊？

使用分部类型的第一个反应就是破坏了封装性，原本类型就是封装完成一个逻辑所需要的一系列数据和行为，但是使用分部类型，我们就可以将逻辑进行“分部”，就像上面一样。但这并不是问题，上面的例子只是为了交代分部类的特性而已，实际上的分部类并不会这样写，更多时候，是将一个完整的解决方案中的多个逻辑功能散布在多个分部类型中。这样的好处是显而易见的，我们可以随时修改或者替换这些功能，不需要跑到解决方案中特意找出相应位置的代码。

当然，分部类型的好处包括源码控制，源码拆分，这些都是比较抽象的话题，很难一下子明白，这里我也不班门弄斧，各位只要到MSDN上就能找到更多的资料。

分部类型还有很多的限制。如果一个分部类型的访问权限是public，那么我们就不能将其他分部类型的访问权限设置为private，也就是说，它们的访问权限必须是一样的(毕竟它们之后是要合并为同一个类型)。由于C#是单根继承的，所以，当一个分部类型继承自一个基类的时候，其他分部类型也只能继承自同一个基类。至于接口，就比较随便了，而且，我们还可以在另一个分部类型中实现我们本该实现的接口方法。泛型类型中，所有的分部类型的约束都必须是一样的。

分部类型并不是一个难以理解的特性，但实际使用的时候却很令新手费解，像是我下面的例子：

public partial class ShoppingCarts
    {
        StoreContext storeDb = new StoreContext();
        string ShoppingCartId { get; set; }
        public const string CartSessionKey = "CartId";
        public static ShoppingCarts GetCart(HttpContextBase context)
        {
            var cart = new ShoppingCarts();
            cart.ShoppingCartId = cart.GetCartId(context);
            return cart;
        }

        //Helper method to simplify shopping cart calls
        public static ShoppingCarts GetCart(Controller controller)
        {
            return GetCart(controller.HttpContext);
        }

        public void AddToCart(Album album)
        {

            //Get the matching cart and album instances
            var cartItem = storeDb.Carts.SingleOrDefault(c => (c.CartId == ShoppingCartId) && (c.AlbumId == album.AlbumId));
            if (cartItem == null)
            {

                //Create a new cart item if no cart item exists
                cartItem = new Cart
                {
                    AlbumId = album.AlbumId,
                    CartId = ShoppingCartId,
                    Count = 1,
                    DateCreated = DateTime.Now
                };
                storeDb.Carts.Add(cartItem);
            }
            else
            {

                //If the item does exist in the cart,then add one to the quantity
                cartItem.Count++;
            }

            //Save changes
            storeDb.SaveChanges();
        }

        public int RemoveFromCart(int id)
        {

            //Get the Cart
            var cartItem = storeDb.Carts.Single(cart => (cart.CartId == ShoppingCartId) && (cart.RecordId == id));
            int itemCount = 0;
            if (cartItem != null)
            {
                if (cartItem.Count > 1)
                {
                    cartItem.Count--;
                    itemCount = cartItem.Count;
                }
                else
                {
                    storeDb.Carts.Remove(cartItem);
                }

                //Save changes
                storeDb.SaveChanges();
            }
            return itemCount;
        }

        public void EmptyCart()
        {
            var cartItems = storeDb.Carts.Where(cart => cart.CartId == ShoppingCartId);
            foreach (var cartItem in cartItems)
            {
                storeDb.Carts.Remove(cartItem);
            }

            //Save changes
            storeDb.SaveChanges();
        }

        public List<Cart> GetCartItems()
        {
            return storeDb.Carts.Where(cart => cart.CartId == ShoppingCartId).ToList();
        }

        public int GetCount()
        {

            //Get the count of each item in the cart and sum them up
            int? count = (from cartItems in storeDb.Carts
                          where cartItems.CartId == ShoppingCartId
                          select (int?)cartItems.Count).Sum();

            //Return 0 if all entries are null
            return count ?? 0;
        }

        public decimal GetTotal()
        {

            //Multiply album price by count of that album to get
            //the current price for each of albums in the cart
            //sum all album price totals to get the cart total
            decimal? total = (from cartItems in storeDb.Carts
                              where cartItems.CartId == ShoppingCartId
                              select (int?)cartItems.Count * cartItems.Album.Price).Sum();
            return total ?? decimal.Zero;
        }

        public int CreateOrder(Order order)
        {
            decimal orderTotal = 0;
            var cartItems = GetCartItems();

            //Iterate over the items in the cart, adding the order details for each
            foreach (var item in cartItems)
            {
                var orderDetail = new OrderDetail
                {
                    AlbumId = item.AlbumId,
                    OrderId = order.OrderId,
                    UnitPrice = item.Album.Price,
                    Quantity = item.Count
                };

                //Set the order total of the shopping cart
                orderTotal += (item.Count * item.Album.Price);
                storeDb.OrderDetails.Add(orderDetail);
            }

            //Set the order's total to the orderTotal count
            order.Total = orderTotal;

            //Save the order
            storeDb.SaveChanges();

            //Empty the shopping cart
            EmptyCart();

            //Return the OrderId as the confirmation number
            return order.OrderId;
        }

        //We are using HttpContextBase to allow access to cookies
        public string GetCartId(HttpContextBase context)
        {
            if (context.Session[CartSessionKey] == null)
            {
                if (!string.IsNullOrWhiteSpace(context.User.Identity.Name))
                {
                    context.Session[CartSessionKey] = context.User.Identity.Name;
                }
                else
                {

                    //Generate a new random GUID using System.Guid class
                    Guid tempCartId = Guid.NewGuid();

                    //Send tempCartId to client as a cookie
                    context.Session[CartSessionKey] = tempCartId.ToString();
                }
            }
            return context.Session[CartSessionKey].ToString();
        }

        //When a user has logged in, migrate their shopping cart to
        //be associated with their username
        public void MigrateCart(string userName)
        {
            var shoppingCart = storeDb.Carts.Where(c => c.CartId == ShoppingCartId);
            foreach (Cart item in shoppingCart)
            {
                item.CartId = userName;
            }
            storeDb.SaveChanges();
        }
    }

还有一个不同名的分部类型：

 [Bind(Exclude = "OrderId")]
    public partial class Order
    {
        [ScaffoldColumn(false)]
        public int OrderId { get; set; }
        
        [ScaffoldColumn(false)]
        public string UserName { get; set; }

        [Required(ErrorMessage = "First Name is required")]
        [DisplayName("First Name")]
        [StringLength(160)]
        public string FirstName { get; set; }

        [Required(ErrorMessage = "Last Name is required")]
        [DisplayName("Last Name")]
        [StringLength(160)]
        public string LastName { get; set; }

        [Required(ErrorMessage = "Address is required")]
        [StringLength(70)]
        public string Address { get; set; }

        [Required(ErrorMessage = "City is required")]
        [StringLength(40)]
        public string City { get; set; }

        [Required(ErrorMessage = "State is required")]
        [StringLength(40)]
        public string State { get; set; }

        [Required(ErrorMessage = "PostalCode is required")]
        [DisplayName("PostalCode")]
        [StringLength(10)]
        public string PostalCode { get; set; }

        [Required(ErrorMessage = "Country is required")]
        [StringLength(40)]
        public string Country { get; set; }

        [Required(ErrorMessage = "Phone is required")]
        [StringLength(24)]
        public string Phone { get; set; }

        [Required(ErrorMessage = "Email is required")]
        [DisplayName("Email Address")]
        [RegularExpression(@"[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+[A-Za-z]{2,4}", ErrorMessage = "Email is not valid")]
        [DataType(DataType.EmailAddress)]
        public string Email { get; set; }

        [ScaffoldColumn(false)]
        public decimal Total { get; set; }

        [ScaffoldColumn(false)]
        public System.DateTime OrderDate { get; set; }

        public List<OrderDetail> OrderDetails { get; set; }

    }