不能更改定义对象标识的LINQ到SQL成员

'通用插入

Public Shared Sub Add(Of T As Class)(ByVal entity As T)

    Using db As New APIUDataContext()

        db.GetTable(Of T)().InsertOnSubmit(entity)
        db.SubmitChanges()

    End Using

'通用更新

Public Shared Sub Update(Of T As Class)(ByVal oldEntity As T, ByVal newEntity As T)

    Dim db As New DemoDataContext()

    db.GetTable(Of T)().Attach(newEntity, oldEntity)
    db.SubmitChanges()

End Sub

    Dim oldEntity As New TestAuthor
    oldEntity.Id = 4
    oldEntity.FirstName = "James"

    Dim newEntity As New TestAuthor
    newEntity.FirstName = TextBox1.Text
    newEntity.LastName = TextBox2.Text

    GenericCUD.Update(oldEntity, newEntity)

来自常规更新的错误消息。

类型为“TestAuthor”的对象的成员“Id”的值已更改。 无法更改定义对象标识的成员。 考虑添加具有新标识的新对象,并删除现有的对象。

修改通用更新需要什么?非常感谢。