protected void btnInsert_Click(object sender, EventArgs e)
    {
        SqlConnection con = dbConnection.GetConnection();
        SqlCommand command = new SqlCommand("select b_name from books where b_name=@name", con);
        command.Parameters.AddWithValue("@name", txtBook.Text);
        con.Open();
        SqlDataReader rdr = command.ExecuteReader();
        if (rdr.Read())
        {
            ltrlInf.Text = "";
            ltrlInf.Text += "This book has been added before";
        }
        else
        {
            rdr.Close();

            SqlCommand cmd2 = new SqlCommand("select c_id from categories where c_name=@catname", con);
            cmd2.Parameters.AddWithValue("@catname", ddlCat.SelectedValue.ToString());
            SqlDataReader rdr2 = cmd2.ExecuteReader();
            
            while (rdr2.Read())
            {
                i = Int32.Parse(rdr2.GetValue(0).ToString());
            }
            
            SqlCommand cmd = new SqlCommand("insert into books (b_name,b_author,b_lang,b_page,c_id,b_stock,b_year,b_thumbnail,s_id) VALUES (@name,@author,@lang,@page,@cid@,@stock,@year,@thumbnail,@sid)", con);
            cmd.Parameters.AddWithValue("@name", txtBook.Text);
            cmd.Parameters.AddWithValue("@author", txtAuthor.Text);
            cmd.Parameters.AddWithValue("@lang", txtLang.Text);
            cmd.Parameters.AddWithValue("@page", txtPage.Text);
            cmd.Parameters.AddWithValue("@cid",i);
            cmd.Parameters.AddWithValue("@stock", txtStock.Text);
            cmd.Parameters.AddWithValue("@year", txtYear.Text);
            cmd.Parameters.AddWithValue("@thumbnail", ".\\" + "thumbs" + "\\" + FileUpload1.FileName);
            cmd.Parameters.AddWithValue("@sid", ddlScat.SelectedValue.ToString());
            rdr2.Close();
            cmd.ExecuteNonQuery(); //ERROR COMES HERE ( Must declare the scalar variable "@cid@". )           
            ltrlInf.Text += "The book has been added";
            con.Close();
        }
    }