为什么编译器认为这是一个对象而不是 DataRow？
我使用 LINQ 查询将 DataTable 对象内的数据转换为自定义 POCO 对象的简单 IEnumerable。
我的 LINQ 查询是：
Dim dtMessages As DataTable
'...dtMessages is instantiated ByRef in a helper data access routine... '
Dim qry = From dr As DataRow In dtMessages.Rows
Select New OutboxMsg With {
.ComputerID = dr(dcComputerID),
.MessageData = dr(dcMessageData),
.OutBoxID = dr(dcOutBoxID),
.OutBoxReceivedID = dr(dcOutBoxReceivedID),
.TrxDate = dr(dcTrxDate)
}
但是，编译器在 dr As DataRow 下抛出一条警告消息，其中包含以下消息：
Option Strict On 不允许从“Object”到“System.Data.DataRow”的隐式转换。
为什么我会收到此错误？我需要做什么来修复它？我本以为dtMessages.Rows返回一个DataRow类型的集合。这不正确吗？
I'm using a LINQ query to translate data inside a DataTable object to be a simple IEnumerable of a custom POCO object.
My LINQ query is:
Dim dtMessages As DataTable
'...dtMessages is instantiated ByRef in a helper data access routine... '
Dim qry = From dr As DataRow In dtMessages.Rows
Select New OutboxMsg With {
.ComputerID = dr(dcComputerID),
.MessageData = dr(dcMessageData),
.OutBoxID = dr(dcOutBoxID),
.OutBoxReceivedID = dr(dcOutBoxReceivedID),
.TrxDate = dr(dcTrxDate)
}
However, the compiler is throwing a warning message under dr As DataRow with the message:
Option Strict On disallows implicit conversions from 'Object' to 'System.Data.DataRow'.
Why am I getting this error and what do I need to do to fix it? I would have thought that dtMessages.Rows returned a collection of type DataRow. Is this not correct?
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请注意
DataTable.Rows- 它是一个DataRowCollection，仅实现IEnumerable，而不是IEnumerable(Of DataRow)。幸运的是，
DataTableExtensions中有一个扩展方法 允许您调用AsEnumerable( )而不是行；AsEnumerable返回一个IEnumerable(Of DataRow)：（与使用
dt.Rows.Cast(Of DataRow)相比，我更喜欢这个，因为它提供的更少对可能失败的印象，它更适合DataTable。）Lok at
DataTable.Rows- it's aDataRowCollectionwhich only implementsIEnumerable, notIEnumerable(Of DataRow).Fortunately, there's an extension method in
DataTableExtensionswhich lets you callAsEnumerable()instead ofRows;AsEnumerablereturns anIEnumerable(Of DataRow):(I prefer this over using
dt.Rows.Cast(Of DataRow), as it gives less of an impression of something which might fail. It's more tailored toDataTable. Both will work though.)System.DataTable类型早于 .Net 中的泛型，因此返回普通的旧IEnumerable而不是IEnumerable(Of DataRow)，即使在它们是 DataRow 的实例。因此，上述查询中dr的类型是Object，而不是DataRow。您可以通过使用
Cast扩展方法使集合的类型显式来解决此问题The type
System.DataTablepredates generics in .Net and hence returns a plain oldIEnumerableinstead of anIEnumerable(Of DataRow)even though under the hood they are instances ofDataRow. Hence the type ofdrin the above query isObjectand notDataRow.You can work around this by using the
Castextension method to make the type of the collection explicitDataTable.Rows属性返回DataRow的集合，但该集合不实现IEnumerable，而是实现IEnumerablecode>，其工作方式为IEnumerable您可以使用
dtMessages.Rows.Cast() 将集合项显式转换为DataRow。The
DataTable.Rowsproperty returns a collection ofDataRow, however that collection doesn't implementIEnumerable<DataRow>butIEnumerable, which works asIEnumerable<Object>.You can explicitly cast the collection items to
DataRowusingdtMessages.Rows.Cast<DataRow>().