“视图模型”列表返回 Null答案

【问题标题】：List of "View Models" returns Null“视图模型”列表返回 Null
【发布时间】：2017-08-03 21:48:46
【问题描述】：

我使用 ViewModel 的列表实例从查询中收集数据并尝试在视图中显示。但是对象“CustomerVMlist”返回 null 且没有结果，customerlist 向我显示 4 个查询结果。而我的观点完全是空的。

public class CustomerVM
{

    public int CustomerID { get; set; }
    public string Name { get; set; }
    public string Address { get; set; }

    public string Mobileno { get; set; }
    public Nullable<System.DateTime> OrderDate { get; set; }
    public Nullable<decimal> OrderPrice { get; set; }
}

索引函数

public ActionResult Index()
{
    List<CustomerVM> CustomerVMlist = new List<CustomerVM>(); // to hold list of Customer and order details

    var customerlist =(from Cust in db.Customers
                       join Ord in db.Orders 
                       on Cust.CustomerID equals Ord.CustomerID
                       select new 
                       { 
                           Cust.Name, 
                           Cust.Mobileno, 
                           Cust.Address, 
                           Ord.OrderDate, Ord.OrderPrice
                       }).ToList();

    foreach (var item in customerlist) 
    {
        CustomerVM VMOBJ = new CustomerVM();
        VMOBJ.Name= item.Name;
        VMOBJ.Mobileno = item.Mobileno;
        VMOBJ.Address = item.Address;
        VMOBJ.OrderDate = item.OrderDate;
        VMOBJ.OrderPrice = item.OrderPrice;
    }

    return View(CustomerVMlist);// returns null 

}

【问题讨论】：

标签： c# linq linq-to-sql asp.net-mvc-5

【解决方案1】：

您需要将项目添加到列表中

 foreach (var item in customerlist) {
    CustomerVM VMOBJ = new CustomerVM();
    VMOBJ.Name= item.Name;
    VMOBJ.Mobileno = item.Mobileno;
    VMOBJ.Address = item.Address;
    VMOBJ.OrderDate = item.OrderDate;
    VMOBJ.OrderPrice = item.OrderPrice;
    CustomerVMlist.Add(VMOBJ);
 }

【讨论】：

【解决方案2】：

您忘记在列表中添加您创建的对象。

foreach (var item in customerlist) 
{
    CustomerVM VMOBJ = new CustomerVM();
    VMOBJ.Name= item.Name;
    VMOBJ.Mobileno = item.Mobileno;
    VMOBJ.Address = item.Address;
    VMOBJ.OrderDate = item.OrderDate;
    VMOBJ.OrderPrice = item.OrderPrice;

    // after having set all the values you add the newly created object to the list.
    CustomerVMlist.Add(VMOBJ);
}

您可以使用更少的代码行来实现相同的目标，如下所示：

public ActionResult Index()
{

    var CustomerVMlist = (from Cust in db.Customers
                          join Ord in db.Orders 
                          on Cust.CustomerID equals Ord.CustomerID
                          select new CustomerVM
                          { 
                              Name = Cust.Name, 
                              Mobileno = Cust.Mobileno, 
                              Address = Cust.Address, 
                              OrderDate = Ord.OrderDate, 
                              OrderPrice = Ord.OrderPrice
                          }).ToList();

    return View(CustomerVMlist)
}

如您所见，我们可以将结果投影到所需的类型CustomerVM，而无需循环遍历结果集（使用foreach），一一创建所需的对象并将它们添加到列表中。

【讨论】：

同意。谢谢。