继承的LINQ to SQL类在InsertOnSubmit上抛出NullReferenceException

Question

在我的datacontext，CalcinerDataContext中，我有一个类，MainData和一个简单继承的类NewSample（用于探索此错误）。下面的代码运行没有任何错误：

        Dim cdc As New CalcinerDataAccess.CalcinerDataContext
        Dim newData As New CalcinerDataAccess.MainData
        With newData
            .LotNumber = "Test1"
            .ProcessType = 4
            .SubmitterID = 2
            .Calciner = 1
            .DateEntered = Now
        End With

        Try
            cdc.MainDatas.InsertOnSubmit(newData)

        Catch ex As Exception
            Debug.WriteLine(ex.ToString)
            MsgBox(ex.Message)
        End Try

以下代码会生成以下错误： System.NullReferenceException：未将对象引用设置为对象的实例。    在System.Data.Linq.Table`1.InsertOnSubmit（TEntity实体）。请注意，这两个代码块之间的唯一区别是newData

的实例化

        Dim cdc As New CalcinerDataAccess.CalcinerDataContext
        Dim newData As New CalcinerDataAccess.NewSample
        With newData
            .LotNumber = "Test1"
            .ProcessType = 4
            .SubmitterID = 2
            .Calciner = 1
            .DateEntered = Now
        End With

        Try
            cdc.MainDatas.InsertOnSubmit(newData)

        Catch ex As Exception
            Debug.WriteLine(ex.ToString)
            MsgBox(ex.Message)
        End Try

为什么继承的类会引发异常，但基类不会？