插入Access时如何处理空字段

我正在尝试将表单中的值插入到Access数据库中，但字段“值为”时出现错误Null。即使使用，我仍然会收到错误消息= DBNull.Value。

前8个值不能是Null因为这些是必填字段，但是如果该字段为空，则一旦到达Cost，就会出现此错误。

577行是 da.Update(ds, "EquipList")

这是我的代码：

 Try
        Dim con As New OleDb.OleDbConnection
        Dim ds As New DataSet
        Dim da As OleDb.OleDbDataAdapter
        Dim sql As String
        con.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|\equip_full.mdb;Jet OLEDB:Database Password=matt"
        con.Open()

        sql = "SELECT * FROM EquipList"
        da = New OleDb.OleDbDataAdapter(sql, con)

        da.Fill(ds, "EquipList")
        Dim cb As New OleDb.OleDbCommandBuilder(da)
        Dim dsNewRow As DataRow
        dsNewRow = ds.Tables("EquipList").NewRow()

        ' Equipment Information fields

        dsNewRow.Item(0) = UniqueID
        dsNewRow.Item(1) = InvNumber
        dsNewRow.Item(2) = Item

        dsNewRow.Item(3) = Type

        dsNewRow.Item(4) = Description

        dsNewRow.Item(5) = Manufacturer
        dsNewRow.Item(6) = Model_No
        dsNewRow.Item(7) = Serial_No
        If Cost = "" Then
            dsNewRow.Item(8) = DBNull.Value
        Else
            dsNewRow.Item(8) = Cost
        End If
        If Cost_Centre = "" Then
            dsNewRow.Item(9) = DBNull.Value
        Else
            dsNewRow.Item(9) = Cost_Centre
        End If

        'dsNewRow 10 to 39

        ds.Tables("EquipList").Rows.Add(dsNewRow)
        da.Update(ds, "EquipList")
        con.Close()

        MsgBox(Description & " has successfully been added to the system.")

 Catch ex As Exception
        MsgBox("Unable to connect to the database " & ex.ToString)
        Exit Sub
 End Try

更新：我的代码很好，这只是MsgBox的问题。MsgBox无法显示为空的值

删除MsgBox可以解决此问题。无论如何，它只是在那里进行测试。