public void InsertInto(string qs_insert)
        {
            try
            {
                conn = new MySqlConnection(cs);
                conn.Open();

                cmd = new MySqlCommand();
                cmd.Connection = conn;
                cmd.CommandText = qs_insert;
                cmd.ExecuteNonQuery();
            }
            catch (MySqlException ex)
            {
                MessageBox.Show(ex.ToString());
            }
            finally
            {
                if (conn != null)
                {
                    conn.Close();
                }
            }
        }

using(MySqlConnection conn = new MySqlConnetion(cs)
{
    conn.open();
    //setup and execute query

} //conn gets closed here

public void insert_command(string username, string password, string fullname)
    {
        using (var con = new MySqlConnection { ConnectionString = "Your Connection String.." })
        {
            using (var command = new MySqlCommand { Connection = con })
            {
                con.Open();

                command.CommandText = @"INSERT INTO usertable SET username=@username, password=@password, fullname=@fullname";
                command.Parameters.AddWithValue("@usename", username);
                command.Parameters.AddWithValue("@password", password);
                command.ExecuteNonQuery();
                command.Parameters.Clear();

            } // command close

        } // connection close

    }