Specified cast is not valid exception in ms access query using C#

hey guys m having this wierd exception of cast though my datatypes are correct in db:

string sql =
            string.Format(
                @"select aim_network_id,aim_n开发者_开发知识库etwork_name,oxinetwork_id,pack_id,pack_name,p_face_value,pm_prefix from Operator where aim_network_id='{0}'",
                gridbackOffice["aim_network_id", gridbackOffice.CurrentCell.RowIndex].Value);
        OleDbCommand getSelectedGridDatecmd = new OleDbCommand(sql, conn);
        OleDbDataReader reader = getSelectedGridDatecmd.ExecuteReader();
        while (reader.Read())
        {
            txtAimNetworkID.Text = reader.GetString(0);
            txtAimNetworkName.Text = reader.GetString(1);
            txtPARNetworkID.Text = reader.GetString(2);
            txtPARFaceValue.Text = reader["p_face_value"].ToString();
           //in above line if i'm doing this `reader.GetString(5)` then i'm getting specified cast exception and that to randomly i.e some time it works fine and suddenly sometime gives this exception
            txtPARPackID.Text = reader.GetString(3);
            txtPARPackName.Text = reader.GetString(4);
            txtPARPMPrefix.Text = reader["pm_prefix"].ToString();
        }

I'm little bit confused if m using this reader["p_face_value"].ToString() then my code is running very smoothly but whats the issue with using this reader.GetString(5) , according to me both method return string, nebody had faced this error b4 ? ....Error is at 4th and 7th line in while loop.

Exception:Specified cast is not valid (InvalidCastException unhandled)

According to MSDN, OleDbDataReader.GetString() does not perform any conversions before attempting to cast to a string - therefore the data retrieved must already be a string.

If there is a chance that the value in that column could be null, the docs suggest that you should check if the value is null first:

if (  !reader.IsDBNull(5) ) {
    txtPARFaceValue.Text = reader.GetString(5); 
}

Calling reader["p_face_value"] on a null value returns DBNull - and when you call ToString() on DBNull, you get an empty string.

From MSDN:

No conversions are performed; therefore the data retrieved must already be a string.

If the column is not a string type, you'll need to use the .ToString() method to convert it.

What is the datatype of p_face_value in your database?

Based on the error description given it seems that this is not a string type, so when you call:

reader.GetString(5)

the code errors out as it cannot convert whatever type it is to a string. The .ToString() method will work as this does not use a cast.

You should use GetString only when the column is a string-equivalent type in the database (like varchar), in your case "p_face_value" seems to be a numeric type, therefore it cannot simply convert it to a string.

The way you're doing it right now is the right way.