This is a bit off-topic, but why are you using the data adapter at all. As far as the code is shown, you create a command, set the parameters and execute the command. Adapter isn't necessary for this. So if you simply need to insert a row from few text boxes and you don't use any other adapter functionality in your code, you could possibly simplify this to:
con = new SqlConnection(dbconnection.constring);
SqlCommand cmd = new SqlCommand("INSERT INTO item VALUES (@item_name,@price)", con);
cmd.Parameters.Add("@item_name", SqlDbType.NVarChar).Value = textBox1.Text;
cmd.Parameters.Add("@price", SqlDbType.Money).Value = textBox2.Text;
con.Open();
cmd.ExecuteNonQuery();
con.Close();