VB.Net中如何遍历匿名类型的IEnumerable集合？

问题场景 #

从SQL Server获取DataTable数据，按VendorID分组并拼接字段多值的代码运行正常：

Dim dt As DataTable = GetVendorData()
Dim results = From p In dt.AsEnumerable()
              Group p By VendorID = p.Field(Of String)("VenID") Into g = Group
              Select New With {
                  Key .VendorID = VendorID,
                  Key .FundNumber = String.Join(";", From i In g Select i.Field(Of String)("FundNumber")),
                  Key .Statefund = String.Join(";", From i In g Select i.Field(Of String)("Statefund")),
                  Key .State = String.Join(";", From i In g Select i.Field(Of String)("State"))
              }

但尝试用如下方式遍历时报错：

For Each result As DataRow In results
    Dim x As String = result.Field(Of String)("FundNumber"))
Next

错误信息：

System.InvalidCastException
HResult=0x80004002
Message=Unable to cast object of type 'VB$AnonymousType_1`4[System.String,System.String,System.String,System.String]' to type 'System.Data.DataRow'

解决方法 #

错误核心是results是匿名类型的IEnumerable集合，而非DataRow集合，不能用DataRow类型强制接收。正确遍历方式有两种：

方式1：使用类型推断（推荐）

让VB自动推断匿名类型，直接通过属性名访问字段值：

For Each result In results
    Dim x As String = result.FundNumber
    ' 可直接访问其他定义的属性
    Dim vendorId As String = result.VendorID
    Dim stateFund As String = result.Statefund
    Dim state As String = result.State
Next

方式2：用Object类型接收（不推荐，无编译时类型检查）

若必须指定类型，用Object接收后直接访问属性（VB支持后期绑定）：

For Each result As Object In results
    Dim x As String = result.FundNumber
Next

补充说明 #

LINQ的Select New With {...}生成的是匿名类型实例，每个实例包含你定义的VendorID、FundNumber等强类型属性，直接通过.访问即可，无需使用DataRow专属的Field方法。

内容的提问来源于stack exchange，提问作者Massey