Linq自定义数据错误 [英] Linq to a custom data error

问题描述

我尝试将一组新数据添加到预定义的数据字典中但是我在Return Result.tolist（）中遇到错误消息



错误BC30311List（Of S_pocontRepo.postatus）类型的值无法转换为S_pocontRepo.postatus。

I try to get a new set of data into a predefined data dictionary however I encounter error message at the "Return Result.tolist()"

"Error BC30311 Value of type 'List(Of S_pocontRepo.postatus)' cannot be converted to 'S_pocontRepo.postatus'."

Structure postatus
    Public po_no As String
    Public po_date As String
    Public crcode As String
    Public name As String
    Public qty As Decimal
    Public qty_rec As Decimal
    Public os_qty As Decimal
    Public dt_due As DateTime
    Public status As String
    Public rowid As Integer
End Structure

''Note
''The EXT.NET Grid Store cannot accept EF List as datasource so we need to use LindQ.
''
Public Function GetPOStatusByLinQ(ByVal inPartcode As String) As postatus '' List(Of postatus)
    ''LinQ
    ''-----------------------------------------------------------------------------------------------
    Dim result = From p In Db.s_pocont
                 Group Join c In Db.s_apmaster On c.crcode Equals p.crcode Into podtl_ap = Group
                 From c In podtl_ap.DefaultIfEmpty()
                 Group Join h In Db.s_pohdr On h.po_no Equals p.po_no Into podtl_hdr = Group
                 From h In podtl_hdr.DefaultIfEmpty()
                 Where p.part_code = inPartcode And
                     h.status <> "CANCELLED" And
                     h.status <> "COMPLETED" And
                     p.type <> "D" And
                     p.os_qty > 0
                 Select New postatus With {
                    .po_no = p.po_no,
                    .po_date = h.date,
                    .crcode = p.crcode,
                    .name = c.name,
                    .qty = p.qty,
                    .qty_rec = p.qty_rec,
                    .os_qty = p.os_qty,
                    .dt_due = p.dt_due,
                     .status = h.status,
                     .rowid = p.rowid
                  }

    Return result.ToList()

End Function

我的尝试：



我注意到

What I have tried:

I notice

IEnumerable

数据类型无法转换为VB.net中的List< of，这就是我尝试使用上述方法的原因。但是，到了死胡同。



如何解决这个问题？

TIA

data type is not able to convert to "List<of" in VB.net and that's why I tried to work around with above method. yet, hitting dead end.

How can I resolve this?
TIA

推荐答案

好吧，在你的函数声明中你说你返回 postatus ，

但是在你的代码中你返回一个列表（Postatus）

这就是错误信息告诉你的内容..



你怎么解决这个问题？



更改您的代码以返回单个 postatus ...如果您的查询只返回1个元素而您确定 .Single 会做..

Well, in your function declaration you say you return postatus,
but in your code you return a List(Of postatus)
thats what the error message tells you ..

how can you resolve that?

change your code to return a Single postatus ... if your Query only returns 1 element and you a sure of that a .Single will do..

我建​​议你读一下：类型'< type1>'的值无法转换为'< type2>' [ ^ ]

它可能有助于您理解你做错了什么。



关于你的另一个问题（在评论解决方案＃1）：

I'd suggest to read this: Value of type '<type1>' cannot be converted to '<type2>'[^]
It might help you to understand what you're doing wrong.

As to your another issue (in the comment to the solution #1):

Quote：

当我改为列表（ of postatus），错误消息显示

When I change to List(of postatus), the error message shows "

An exception of type 'System.NotSupportedException' occurred in System.Data.Entity.dll but was not handled in user code<br />
Additional information: Only parameterless constructors and initializers are supported in LINQ to Entities.

此错误当Entity Framework不知道如何将Linq Select（）语句转换为SQL表达式时，将显示消息。如果您需要对不是实体的POCO进行数据转换，您应首先从EF获取相关数据，然后将其转换为POCO，例如：

This error message is displayed when Entity Framework doesn't know how to translate Linq Select() statement into SQL expression. If you need to do a data transformation to a POCO that is not an entity, you should first get the relevant data from EF and then transform it to the POCO, for example:

Public Function GetPOStatusByLinQ(ByVal inPartcode As String) As List(Of postatus)
    ''LinQ
    ''-----------------------------------------------------------------------------------------------
    Dim EfResult = (From p In Db.s_pocont
                 Group Join c In Db.s_apmaster On c.crcode Equals p.crcode Into podtl_ap = Group
                 From c In podtl_ap.DefaultIfEmpty()
                 Group Join h In Db.s_pohdr On h.po_no Equals p.po_no Into podtl_hdr = Group
                 From h In podtl_hdr.DefaultIfEmpty()
                 Where p.part_code = inPartcode And
                     h.status <> "CANCELLED" And
                     h.status <> "COMPLETED" And
                     p.type <> "D" And
                     p.os_qty > 0) _
                 .ToList() 'this causes the query to execute

     'now, you can return POCO model
     Dim PocoResult = (From efr In EfResult
                 Select New postatus With {
                    .po_no = p.po_no,
                    .po_date = h.date,
                    .crcode = p.crcode,
                    .name = c.name,
                    .qty = p.qty,
                    .qty_rec = p.qty_rec,
                    .os_qty = p.os_qty,
                    .dt_due = p.dt_due,
                     .status = h.status,
                     .rowid = p.rowid
                  }) _
                  .ToList()

    Return PocoResult

End Function

如需了解更多信息，请参阅：

使用POCO实体 [ ^ ]

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