Linq to Objects 查询 LLBLGen 投影

Question

我正在尝试使用推荐的 LLBLGen 语法来查询投影 (http://www.llblgen.com/documentation/3.5/LLBLGen%20Pro%20RTF/Using%20the%20generated%20code/Adapter/gencode_usingentityview_adapter.htm#projections)

IEntityView2 view = table.DefaultView;
List<A1AllocationHelp1TableDTO> something = 
    (from c in view
     select new A1AllocationHelp1TableDTO
     {
         RecordStatus = c.RecordStatus,
         UniqueIdent = c.UniqueIdent
     }).ToList();

但我在“选择”时收到此错误：

The type arguments for method 'IEnumerable<TResult>
System.Linq.Enumerable.Select<TSource, TResult>(this IEnumerable<TSource>,
Func<TSource, TResult>)' cannot be inferred from the query.

有趣的是，在 VB.Net 中同样可以正常工作

Dim view As IEntityView2 = table.DefaultView
Dim something As List(Of A1AllocationHelp1TableDTO) = _
    (From c In view
     Select New A1AllocationHelp1TableDTO With _
          {
              .RecordStatus = c.RecordStatus, _
              .UniqueIdent = c.UniqueIdent
          }).ToList()

我正在使用 VS2010、.NET 4 和 LLBLGen 2.6。 不知道如何解决这个问题，谁能帮帮我？

谢谢

编辑：

IEntityView2 由 LLBLGen 生成，这是它的定义

public interface IEntityView2 : IEnumerable
{
    bool AllowEdit { get; set; }
    bool AllowNew { get; set; }
    bool AllowRemove { get; set; }
    int Count { get; }
    PostCollectionChangeAction DataChangeAction { get; set; }
    IPredicate Filter { get; set; }
    IEntityCollection2 RelatedCollection { get; }
    ISortExpression Sorter { get; set; }
    IEntity2 this[int index] { get; }
    event ListChangedEventHandler ListChanged;
    bool Contains(IEntity2 value);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, DataTable destination);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, IEntityCollection2 destination);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, IEntityDataProjector projector);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, DataTable destination, bool allowDuplicates);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, IEntityCollection2 destination, bool allowDuplicates);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, IEntityDataProjector projector, bool allowDuplicates);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, DataTable destination, bool allowDuplicates, IPredicate filter);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, IEntityCollection2 destination, bool allowDuplicates, IPredicate filter);
    void CreateProjection(List<IEntityPropertyProjector> propertyProjectors, IEntityDataProjector projector, bool allowDuplicates, IPredicate filter);
    int IndexOf(IEntity2 value);
    IEntityCollection2 ToEntityCollection();
    IEntityCollection2 ToEntityCollection(int startIndex);
}

Answer 1

IEntityView2 继承了非泛型接口IEnumerable。但是，Select 方法需要通用版本。这就是您收到错误消息的原因。

假设您要访问的属性是在IEntity2 上定义的，以下将起作用：

view.Cast<IEntity2>()
    .Select(c => new A1AllocationHelp1TableDTO
           {
               RecordStatus = c.RecordStatus,
               UniqueIdent = c.UniqueIdent
           })
    .ToList();

它可以在 VB.NET 中工作，因为它使用后期绑定。您可以在以下示例中轻松看到这一点：

Dim view As IEntityView2 = table.DefaultView
Dim something As List(Of A1AllocationHelp1TableDTO) = _
(From c In view
 Select New A1AllocationHelp1TableDTO With _
      {
          .RecordStatus = c.IDontExist _
      }).ToList()

我正在使用一个不存在的属性 (IDontExist)。此代码仍会编译，但在运行时会抛出异常：

MissingMemberException: Public member 'IDontExist' on type 'IEntity2' not found.
   at Microsoft.VisualBasic.CompilerServices.Symbols.Container.GetMembers(String& MemberName, Boolean ReportErrors)
   at Microsoft.VisualBasic.CompilerServices.NewLateBinding.ObjectLateGet(Object Instance, Type Type, String MemberName, Object[] Arguments, String[] ArgumentNames, Type[] TypeArguments, Boolean[] CopyBack)
   at Microsoft.VisualBasic.CompilerServices.NewLateBinding.LateGet(Object Instance, Type Type, String MemberName, Object[] Arguments, String[] ArgumentNames, Type[] TypeArguments, Boolean[] CopyBack)
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source) 

Answer 2

.DefaultView 返回一个类型化的视图，它实现了 IEntityView2，但你不应该将它强制转换为 IEntityView2，因为你会丢失泛型类型。所以你应该这样做：

List<A1AllocationHelp1TableDTO> something = 
    (from c in table.DefaultView
     select new A1AllocationHelp1TableDTO
     {
         RecordStatus = c.RecordStatus,
         UniqueIdent = c.UniqueIdent
     }).ToList();

这样，编译器就知道视图的泛型类型。