VB.NET - 介绍了IEnumerator

Question

请看看下面的代码：

'Form1.vb 
Imports System.Data.SqlClient 

Public Class Form1 

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load 
     'ExecuteDataReader(Function(x) New Person With {.URN = x("URN")}) 
     Try 
      Dim results As IEnumerable(Of Person) = ExecuteDataReader(Function(x) New Person With {.URN = x("URN")}) 
      For Each c As Person In results 'Line 4 
      Next 
     Catch ex As Exception 

     Finally 

     End Try 

    End Sub 

    Public Function ExecuteDataReader(ByVal castRow As Func(Of IDataRecord, Person)) As IEnumerable(Of Person) 
     Try 
      Dim objCon As New SqlConnection("Data Source=IANSCOMPUTER;Initial Catalog=Test;Integrated Security=True") 
      Dim objCommand As New SqlCommand 
      Dim objDR As SqlDataReader 
      objCon.Open() 
      objCommand.Connection = objCon 
      objCommand.CommandText = "SELECT URN FROM Person" 
      objDR = objCommand.ExecuteReader() 
      Do While objDR.Read 
       castRow(objDR) 
      Loop 
     Catch ex As Exception 

     End Try 

    End Function 
End Class 

'Person.vb 
Public Class Person 
    'Implements IEnumerator, IEnumerable 
    Public URN As String 
End Class 

为什么我是新来IEnumberators结果变量上线4空。我使用的.NET版本（3.5）不允许使用Yield关键字。

更新 Damien_The_Unbeliever纠正了代码。你认为这种模式适用于数据逻辑层吗？我相信我有四种选择：

1）返回数据表而不是数据读取器到业务逻辑 图层。然后我可以将代码封装在Using语句中。
2）使用Damien_The_Unbeliever的答案（不包括使用语句中的 可丢弃对象）中描述的模式 将业务逻辑层的数据读取器返回。
3）将数据返回读者 业务对象层，只有当 DataReader的是即dr = cmd.ExecuteReader(CommandBehavior.CloseConnection)
4关闭关闭连接）不要有数据访问层。在需要时，在业务逻辑 层中打开和关闭连接。我相信这会让代码更易于维护。

如果还有其他选择，请让我知道。

Answer 1

试试这个：

Public Function ExecuteDataReader(ByVal castRow As Func(Of IDataRecord, Person)) As IEnumerable(Of Person) 
    Using objCon As New SqlConnection("Data Source=IANSCOMPUTER;Initial Catalog=Test;Integrated Security=True") 
     Using objCommand as New SqlCommand("SELECT URN FROM Person",objCon) 
     Dim objDR As SqlDataReader 
     objCon.Open() 
     objDR = objCommand.ExecuteReader() 
     Dim ret as New List(Of Person) 
     Do While objDR.Read 
      ret.Add(castRow(objDR)) 
     Loop 
     Return ret 
     End Using 
    End Using 
End Function 

其中A）移除的“错误处理”，它默默吞下错误的坏榜样，B）包裹SqlCommand和SqlConnection对象，使他们得到妥善处置，以及c）其实从函数返回一些东西，这是你出错的地方。