การทำ combobox ให้โชว์ 2 column ทำแบบนี้ มัน error ครับ รบกวนช่วยชี้แนะ

   Public Class Treatment
    Dim ds_user As New DataSet
    Dim ds_pat As New DataSet
    Dim ds_dis As New DataSet
    Dim ds_dia As New DataSet

    Sub loaduser()
        Dim strselect As String = "select username, user_fname +'|'+ user_lname AS Fullname"
        strselect &= "From PrivateUser"
        With conn
            If .State = ConnectionState.Open Then .Close()
            .Open()
        End With
        With da
            .SelectCommand = New OleDbCommand(strselect, conn)
            .Fill(ds_user, "PrivateUser")
        End With
        With cmbuser
            .DataSource = ds_user.Tables("PrivateUser")
            .DisplayMember = "Fullname"
            .ValueMember = "username"
        End With
    End Sub