Cannot Get Value When OleDbParameter Used - f#

Question

It is not good form to create a query like
let fnam_query =
"select * from file_name_info where fnam_origin = 'invoice_cloud'"
But the code block below has two problems. First, the fnam_readOk returns false from the read.
Second, how can the OleDbParameter be disposed? I tried using use, but got a compile-time error saying OleDbType.Char could not be used within a use.
let fnam_query =
"select * from file_name_info where fnam_origin = '?' "
use fnam_cmd = new OleDbCommand(fnam_query, db_con)
let local_params = new OleDbParameter("fnam_origin", OleDbType.Char)
fnam_cmd.Parameters.Add(local_params) |> ignore
let fnam_reader = fnam_cmd.ExecuteReader ()
let fnam_readOK = fnam_reader.Read ()
let ic_lb_fnam =
if fnam_readOK then
fnam_reader.GetString(2)
else
"ic_lockbox.txt"

Answer 1

There are a lot of problems with this code. One of them, is that you used the OleDbParameter overload that passes a name and value. The line new OleDbParameter("fnam_origin", OleDbType.Char) specifies a parameter with the name fnam_origin and an integer value equal to whatever the underlying value of OleDbType.Char is.
The other problem is that you don't use that parameter at all. '?' is just a string that contains ?.
You don't need to quote parameters in a parameterized query. They aren't placeholders for string replacement. They specify actual, strongly typed parameters, just like an F# function parameter.
Your query should be :
let fnam_query =
"select * from file_name_info where fnam_origin = ? "
You should also use the correct parameter type. Char is used only for fixed-length parameters. You should use VarChar or even better, NVarchar.
Finally, you should pass the parameter value that you want. Your code doesn't specify a parameter value at all.
The entire function should look like this :
let fnam_query = "select * from file_name_info where fnam_origin = ? "
use db_con = new OleDbConnection("...")
use fnam_cmd = new OleDbCommand(fnam_query, db_con)
let local_params = new OleDbParameter("origin", SqlDbType.NVarChar,100)
fnam_cmd.Parameters.Add(local_params) |> ignore
local_params.Value <- "GR"
db_con.Open()
let fnam_reader = fnam_cmd.ExecuteReader ()
let fnam_readOK = fnam_reader.Read ()
...
A better implementation though, would be to create the command once and reuse it with different connections and values:
let build_cmd =
let fnam_query = "select * from file_name_info where fnam_origin = ? "
let fnam_cmd = new OleDbCommand(fnam_query)
let local_params = new OleDbParameter("whatever", SqlDbType.NVarChar,100)
fnam_cmd.Parameters.Add(local_params) |> ignore
fnam_cmd
use db_con = new OleDbConnection("...")
build_cmd.Connection <- db_con
build_cmd.Parameters.[0].Value <- "GR"
db_con.Open()

Answer 2

Based on the excellent answer I got from Panagiotis Kanavos, I've altered a different place in my code where I needed parameters that were not embedded in the query string. I chose to use cmd.Parameters.Add rather than have a separate OleDbParameter val.
(* by looking at the xfer_type, really the arg passed to main,
we can determine the report type parameter for the Access
database. *)
let select_report_type xfer_type =
match xfer_type with
| "/al" -> 0
| "/am" -> 1
| "/ap" -> 2
| "/pm" -> 3
| "/pp" -> 4
| _ -> 99
let query = "select count(*) from ProcessStatus where ReportType = ? and ReportDate = ? and ReportFileName = ? "
use cmd = new OleDbCommand(query , db_con)
cmd.Parameters.Add(new OleDbParameter("ReportType",(OleDbType.VarChar,5))) |> ignore
cmd.Parameters.[0].Value <- ((select_report_type xfer_type).ToString())
cmd.Parameters.Add(new OleDbParameter("ReportDate",OleDbType.VarChar, 11)) |> ignore
cmd.Parameters.[1].Value <- report_date
cmd.Parameters.Add(new OleDbParameter("ReportFileName",OleDbType.VarChar, 100)) |> ignore
cmd.Parameters.[2].Value <- fn
let sql_reader = cmd.ExecuteReader ()
if (sql_reader.Read ()) then