C# 转换列表在 linq 查询中抛出异常

Question

我有一个关于列表到列表问题的问题。

我的模特：

public partial class OrderDetail
{
    public int Id { get; set; }
    public int OrderId { get; set; }
    public string SparePartName { get; set; }
    public Nullable<decimal> SparePartPrice { get; set; }
    public Nullable<decimal> Labor { get; set; }
    public Nullable<decimal> Total { get; set; }
    public string Comment { get; set; }
}

public partial class Orders
{
    public int Id { get; set; }
    public int CustomerId { get; set; }
    public int StorageId { get; set; }
    public int UserId { get; set; }
    public Nullable<System.DateTime> DateEntry { get; set; }
    public Nullable<System.DateTime> DateRelease { get; set; }
    public Nullable<System.DateTime> DateEstimatedRelease { get; set; }
    public Nullable<System.DateTime> DateUpdate { get; set; }
    public Nullable<decimal> Price { get; set; }
    public int Status { get; set; }
    public string FollowCode { get; set; }
    public string DeviceBrand { get; set; }
    public string DeviceModel { get; set; }
    public string DeviceSerialNumber { get; set; }
    public string DeviceComplaint { get; set; }
    public string Comment { get; set; }
}

我的视图模型：

public class VMOrderEdit
{
    public int Id { get; set; }
    public int StorageId { get; set; }
    public int UserId { get; set; }
    public string StorageName { get; set; }
    public string CustomerName { get; set; }
    public string CustomerTelephone { get; set; }
    public DateTime? DateEntry { get; set; }
    public DateTime? DateUpdate { get; set; }
    public DateTime? DateRelease { get; set; }
    public DateTime? DateEstimatedRelease { get; set; }
    public decimal? Price { get; set; }
    public string StatusName { get; set; }
    public string FollowCode { get; set; }
    public string DeviceBrand { get; set; }
    public string DeviceModel { get; set; }
    public string DeviceSerialNumber { get; set; }
    public string DeviceComplaint { get; set; }
    public string Comment { get; set; }
    public int MyProperty { get; set; }
    public List<VMOrderEditDetails> Details { get; set; }
}
public class VMOrderEditDetails
{
    public int Id { get; set; }
    public int OrderId { get; set; }
    public string SparePartName { get; set; }
    public decimal SparePartPrice { get; set; }
    public decimal Labor { get; set; }
    public decimal Total { get; set; }
    public string Comment { get; set; }
}

我的控制器：

List<VMOrderEdit> OrderEditObj = (from o in DB.Orders
                                  join sn in DB.OrderStatusName on o.Status equals sn.Id
                                  join st in DB.Storages on o.StorageId equals st.Id
                                  join c in DB.Customers on o.CustomerId equals c.Id
                                  where o.Id == Id
                                  select new VMOrderEdit()
                                  {
                                      Id = o.Id,
                                      StorageId = o.StorageId,
                                      StorageName = st.Name,
                                      UserId = o.UserId,
                                      CustomerName = c.NameSurname,
                                      CustomerTelephone = c.Telephone,
                                      DateEntry = o.DateEntry,
                                      DateUpdate = o.DateUpdate,
                                      DateRelease = o.DateRelease,
                                      Price = o.Price,
                                      StatusName = sn.Name,
                                      FollowCode = o.FollowCode,
                                      DeviceBrand = o.DeviceBrand,
                                      DeviceModel = o.DeviceModel,
                                      DeviceSerialNumber = o.DeviceSerialNumber,
                                      DeviceComplaint = o.DeviceComplaint,
                                      Comment = o.Comment,
                                      Details = DB.OrderDetail.Where(x => x.OrderId == o.Id).ToList()
                                      }
                               ).ToList();

在这一行：

Details = DB.OrderDetail.Where(x => x.OrderId == o.Id).ToList()

visual studio 给出这个错误：

不能隐式转换类型 'System.Collections.Generic.List' 到 'System.Collections.Generic.List

我该如何处理这个错误？

Answer 1

您试图将OrderDetail 列表分配给VMOrderEditDetails，这注定会失败。

您的 LINQ 查询应如下所示。我还没有对 Select 进行可空检查和转换。它可以由您完成，并且非常简单。如果有帮助，请标记为答案。

 Details = DB.OrderDetail.Where(x => x.OrderId == o.Id).Select(x=>new VMOrderEditDetails{
    Id = x.Id,
    OrderId = x.OrderId,
    SparePartName = x.SparePartName,
    SparePartPrice = x.SparePartPrice,
    Labor = x.Labor,
    Total = x.Total,
    Comment = x.Comment
}).ToList()

Answer 2

DB 返回一个OrderDetails 的对象，但Details 列表属性中的项目是VMOrderEditDetails 类型的..

您必须在分配之前将每个项目从 OrderDetails 转换为 VMOrderEditDetails。

1. 您可以在 .Where 之后添加 .Select 以转换为该行中 .ToList() 之前的类型，但 除外（参见选项 2 的代码）。

   Details = DB.OrderDetail.Where(x => x.OrderId == o.Id)
                           .Select(x => new VMOrderEditDetails { /* your conversion*/})
                           .ToList()
   

2. 更好的解决方案是在此之前添加一个额外的GroupJoin，然后只在.Select 中进行转换，而不是检索数据：

   List<VMOrderEdit> OrderEditObj = (from o in DB.Orders
                                     join sn in DB.OrderStatusName on o.Status equals sn.Id
                                     join st in DB.Storages on o.StorageId equals st.Id
                                     join c in DB.Customers on o.CustomerId equals c.Id
                                     join d in DB.OrderDetails on o.Id equals d.OrderId into d 
                                     where o.Id == Id
                                     select new VMOrderEdit()
                                     {
                                         Id = o.Id,
                                         StorageId = o.StorageId,
                                         //All the other properties
                                         Comment = o.Comment,
                                         Details = d.Select(x => new VMOrderEditDetails{ /*your convertion*/ }).ToList()
                                     }).ToList();
   

3. 因为您使用的是实体框架，所以更好的选择是 用导航属性替换所有joins，然后仍然在底部进行转换。

Answer 3

这样试试，

Details = DB.OrderDetail.Where(x => x.OrderId == o.Id)
            .Select(x => new VMOrderEditDetails {
                       Id = x.Id,
                       OrderId = x.OrderId,
                       SparePartName = x.SparePartName,
                       SparePartPrice = x.SparePartPrice.Value,
                       Labor = x.Labor.Value,
                       Total = x.Total.Value,
                       Comment = x.Comment
                   }).ToList()

Answer 4

DB.OrderDetail
  .Where(x => x.OrderId == o.Id)
  .ToList()

应该是

DB.OrderDetail
  .Where(x => x.OrderId == o.Id)
  .Select(x => new VMOrderEditDetails () 
  { 
      Id = x.OrderId
  })
  .ToList<VMOrderEditDetails>()

因为types 不同

Answer 5

Details 是 List<VMOrderEditDetails> 类型，但您正尝试为其分配 List<OrderDetail> 值，所以要这样设置

Details = DB.VMOrderEditDetails.Where(x => x.OrderId == o.Id).ToList()