SqlConnection connect = new SqlConnection("Data Source=DESKTOP-58TQHKQ;Initial Catalog=phonebook;Integrated Security=True");

        public void login()
        {
            connect.Open();

            //SqlCommand commander = new SqlCommand("SELECT * FROM table_login WHERE username='@username' and password='@password'", connect);

            //commander.Parameters.AddWithValue("@username", txt_username.Text);
            //commander.Parameters.AddWithValue("@passowrd", txt_password.Text);

            SqlDataAdapter data_adapter = new SqlDataAdapter("SELECT COUNT(*) FROM table_login WHERE username = @username AND password = @password", connect);
            SqlCommand commander = new SqlCommand(data_adapter.ToString(), connect);
            commander.Parameters.AddWithValue("username", txt_username.Text);
            commander.Parameters.AddWithValue("password", txt_password.Text);

            DataSet data_set = new  DataSet();
            data_adapter.Fill(data_set);

            //if (data_table.Rows[0][0].ToString() == "1")

            if (data_set != null)

                if (data_set.Tables[0].Rows.Count > 0)
            {
                this.Hide();
                Form1 frm1 = new Form1();
                frm1.Show();

            }
            else
            {
                MessageBox.Show("Invaild entry, Try Again");
            }

            connect.Close();
            }