使用LINQ提取对象ID拼接逗号分隔字符串遇异常求助

从自定义对象集合中提取ID并拼接成逗号分隔字符串时，执行LINQ代码后得到的结果是：
System.Linq.Enumerable+WhereSelectEnumerableIterator2[item,System.Int32] 而非预期的1,2,3`。

自定义类代码 #

Public Class Items
  Implements IEnumerable

   Private _ID As Integer
   Private desc As String
   Private arrItemList As New ArrayList

   Public Property ID() As Integer
    Get
        Return _ID
    End Get
    Set(ByVal Value As Integer)
        _ID = Value
    End Set
   End Property

   Public ReadOnly Property Count() As Integer
    Get
        Return arrItemList.Count
    End Get
   End Property

   Public Sub AddProduct(ByVal obj As item)
    arrItemList.Add(obj)
   End Sub

   Public Function GetEnumerator() As IEnumerator Implements IEnumerable.GetEnumerator
    Return arrItemList.GetEnumerator()
   End Function
End Class

Public Class item
  Public Property ID As Integer
  Public Property desc As String
End Class

当前实现代码 #

Dim objItems As New Items()
objItems.AddProduct(New item With {.ID = 1, .desc = "orange"})
objItems.AddProduct(New item With {.ID = 2, .desc = "apple"})
objItems.AddProduct(New item With {.ID = 3, .desc = "peach"})

Dim query = objItems.Cast(Of item)().Select(Function(o) o.ID)
Dim result As String = String.Join(",", query)

问题原因 #

LINQ查询返回的是延迟执行的迭代器对象，早期版本的String.Join没有针对泛型IEnumerable(Of T)的重载，直接传入迭代器时，会调用对象的ToString()方法返回其类型名称，而非遍历元素进行拼接。

解决方案 #

将LINQ查询结果转换为具体集合类型（如数组或List），让String.Join能遍历元素拼接：

方法1：转换为数组 #

Dim objItems As New Items()
objItems.AddProduct(New item With {.ID = 1, .desc = "orange"})
objItems.AddProduct(New item With {.ID = 2, .desc = "apple"})
objItems.AddProduct(New item With {.ID = 3, .desc = "peach"})

Dim query = objItems.Cast(Of item)().Select(Function(o) o.ID)
Dim result As String = String.Join(",", query.ToArray()) ' 调用ToArray()触发迭代

方法2：转换为List #

Dim result As String = String.Join(",", query.ToList()) ' 调用ToList()触发迭代

优化建议 #

将自定义Items类改为实现泛型IEnumerable(Of item)，并使用泛型List(Of item)替代非泛型ArrayList，避免每次查询时的Cast(Of item)操作：

Public Class Items
    Implements IEnumerable(Of item)

    Private _ID As Integer
    Private desc As String
    Private arrItemList As New List(Of item) ' 替换为泛型List

    Public Property ID() As Integer
        Get
            Return _ID
        End Get
        Set(ByVal Value As Integer)
            _ID = Value
        End Set
    End Property

    Public ReadOnly Property Count() As Integer
        Get
            Return arrItemList.Count
        End Get
    End Property

    Public Sub AddProduct(ByVal obj As item)
        arrItemList.Add(obj)
    End Sub

    Public Function GetEnumerator() As IEnumerator(Of item) Implements IEnumerable(Of item).GetEnumerator
        Return arrItemList.GetEnumerator()
    End Function

    ' 显式实现非泛型IEnumerable的GetEnumerator
    Private Function IEnumerable_GetEnumerator() As IEnumerator Implements IEnumerable.GetEnumerator
        Return GetEnumerator()
    End Function
End Class

优化后的查询代码可简化为：

Dim query = objItems.Select(Function(o) o.ID)
Dim result As String = String.Join(",", query.ToArray())

内容的提问来源于stack exchange，提问作者CDVS