vb.net - 参数 'Expressions'无法转换为 “DBNull”类型

Question

我严重地陷入了错误。当我尝试从Menu.vb调用表单(frmRecipe)时，它在尝试从配方表中获取最大ID的行上给出了错误，这是我的以下代码

 Function Get_Max_Id() As String
        Try
            Dim CMD As New SqlCommand("SELECT MAX(Recipe_ID) FROM Recipe", CONN)
            CMD.CommandType = CommandType.Text
            Dim dr As SqlDataReader
            dr = CMD.ExecuteReader
            If dr.Read Then
                Return Format(Val(dr(0)).ToString + 1, "0000")
            Else
                Return "0001"
            End If
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Function

Recipe_id是数据库中Varchar的类型

Answer 1

尝试这个:

Return Format(Val(dr(0).ToString) + 1, "0000")