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();

但是我在'select'上收到了这个错误：

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，这在VB.Net中运行得很好

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。无法弄清楚如何解决这个问题，任何人都可以帮忙吗？

由于

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继承非通用接口IEnumerable。但Select方法需要通用版本。这就是你得到错误的原因。

假设您要访问的属性是在IEntity2上定义的，则以下内容将起作用：

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

它适用于VB.NET，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()

我正在使用不存在的属性（IDontExist）。此代码仍将编译，但在运行时抛出异常：

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返回一个类型化视图，它实现了IEntityView2，但你不应该将其转换为IEntityView2，因为你失去了泛型类型。所以你应该做的：

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。

Linq to Objects查询LLBLGen投影

2 个答案: