System.InvalidCastException：使用postgres datareader在C＃中指定的强制转换无效

Question

I am trying to pull data from database using postgre in C# and putting the values returned in label controls. I keep getting System.InvalidCasaeExeception. The database field is an integer so I a using a data reader to get the value.

here is my code

private void Get_Defects()
    {
        NpgsqlConnection conn = Connection.getConnection();

        try
        {
            conn.Open();
            NpgsqlCommand cmd = new NpgsqlCommand("select * from defect where defect_id >= :MinID and defect_id <= :MaxID and location_id = 102 and top_or_bottom = :TopBottom;", conn);
            cmd.Parameters.Add(new NpgsqlParameter("MinID", MinID));
            cmd.Parameters.Add(new NpgsqlParameter("MaxID", MaxID));
            cmd.Parameters.Add(new NpgsqlParameter("TopBottom", TopBottom));

            NpgsqlDataReader dr = cmd.ExecuteReader();

            if (dr.Read())
            {
                lblCrookedPart.Text = dr.GetInt32(12).ToString();
            }

        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
        finally
        {
            conn.Close();
        }
    }

}

Not sure why it wont work. I pulled the first element and it displays correctly. some of the data is integers but have null values in the DB. I tried an element with data but I get the cast exception error. Please help.

Answer 1

I got it working, thank you all for you help and tips!!

here is a snippit of the working code

conn.Open();
            NpgsqlDataAdapter da = new NpgsqlDataAdapter("select * from defect where defect_id >= '"+MinID+"' and defect_id <= '"+MaxID+"' and location_id = 102 and top_or_bottom = '"+TopBottom+"';", conn);
            ds.Reset();
            da.Fill(ds);
            dt = ds.Tables[0];

            //Used to sum the column values

            object CrookedPartTotal = dt.Compute("Sum(crooked_part)", "defect_id >= '" + MinID + "' and defect_id <= '" + MaxID + "'");
            if (CrookedPartTotal.ToString() == "")
                lblCrookedPart.Text = "0";
            else
                lblCrookedPart.Text = CrookedPartTotal.ToString();


            object TooMuchSolder = dt.Compute("Sum(too_much_solder)", "defect_id >= '" + MinID + "' and defect_id <= '" + MaxID + "'");
            if (TooMuchSolder.ToString() == "")
                lblTooMuchSolder.Text = "0";
            else
                lblTooMuchSolder.Text = TooMuchSolder.ToString();

Answer 2

You should check that the value is not null. Try the following instead:

lblCrookedPart.Text = (dr.IsDBNull(12)) ? "NULL" : dr.GetInt32(12).ToString();

Answer 3

Try something like this

NpgsqlDataReader da = default(NpgsqlDataReader);
NpgsqlCommand cmd = new NpgsqlCommand("select * from myTable", GenConnection);
string strVAL = null;
da = cmd.ExecuteReader;
if (da.HasRows) {
    while (da.Read) {
        strVAL = (Information.IsDBNull(da["field"]) ? 0 : da["field"]).ToString;
    }
}