vb.net - 执行查询时抛出异常

Question

我有 vb.net 应用程序表单，它包含 id 作为组合框。单击编辑按钮并选择其他ID后，我发现错误如下图所示。那该怎么办？

Private Sub searchparfum()
    Dim dt As New DataTable
    Dim ds As New DataSet
    ds.Tables.Add(dt)
    Dim da As New OleDbDataAdapter("select num from parfum", MaConnection)
    da.Fill(dt)
    Dim r As DataRow
    cmb_parfum.AutoCompleteCustomSource.Clear()
    For Each r In dt.Rows
        cmb_parfum.AutoCompleteCustomSource.Add(r.Item(0).ToString)
    Next
    cmb_parfum.AutoCompleteMode = AutoCompleteMode.SuggestAppend
    cmb_parfum.AutoCompleteSource = AutoCompleteSource.CustomSource
End Sub

Private Sub cmb_parfum_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmb_parfum.SelectedIndexChanged
    qry = "select * from parfum where num ='" & (cmb_parfum.Text) & "' "
    cmd = New OleDbCommand(qry, MaConnection)
    cmd.ExecuteNonQuery()
    dr = cmd.ExecuteReader
    If dr.Read Then

        txt_num.Text = dr("num")
        lab_nom.Text = dr("nom")
        lab_prix.Text = dr("prix")
        stock_par.Text = dr("stock")
        txt_ventes.Text = dr("ventes")
        photo.Text = dr("photo")

    End If
End Sub

Answer 1

正如错误消息所说，您没有与数据库的开放连接。添加一个 open 和 close 并可能将其包装在 Try .. Catch 中，如下所示：

    cmd = New OleDbCommand(qry, MaConnection)
    Try
        MaConnection.Open()
        cmd.ExecuteNonQuery()
        dr = cmd.ExecuteReader
        If dr.Read Then
            txt_num.Text = dr("num")
            lab_nom.Text = dr("nom")
            lab_prix.Text = dr("prix")
            stock_par.Text = dr("stock")
            txt_ventes.Text = dr("ventes")
            photo.Text = dr("photo")
        End If
    Catch ex As Exception
        ' Handle any exception
    Finally
        MaConnection.Close()
    End Try

还要考虑在 SQL 命令中使用参数

Answer 2

命令和连接之类的数据库对象应在使用它们的方法中声明，以便正确处理它们。Using...End Using即使有错误，块也会为您执行此操作。

你不会使用cmd.ExecuteNonQuery()forSelect声明。仅用于Update, InsertorDelete命令。

您不希望在更新用户界面时连接保持打开状态。因此，我们加载了一个数据表，它在使用数据时不需要连接保持打开状态。阅读器确实需要一个开放的连接。

photo 真的是字符串类型吗？

Private ConStr As String = "Your connection string"

Private Sub cmb_parfum_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmb_parfum.SelectedIndexChanged
    Dim dt As New DataTable
    Dim qry = "select * from parfum where num = @Parfum;"
    Using cn As New OleDbConnection(ConStr),
            cmd As New OleDbCommand(qry, cn)
        cmd.Parameters.Add("@Parfum", OleDbType.VarWChar).Value = cmb_parfum.Text
        cn.Open()
        Using dr = cmd.ExecuteReader
            dt.Load(dr)
        End Using
    End Using 'connection is closed and disposed and command is disposed
    If dt.Rows.Count > 0 Then
        txt_num.Text = dt(0)("num").ToString
        lab_nom.Text = dt(0)("nom").ToString
        lab_prix.Text = dt(0)("prix").ToString
        stock_par.Text = dt(0)("stock").ToString
        txt_ventes.Text = dt(0)("ventes").ToString
        photo.Text = dt(0)("photo").ToString
    End If
End Sub