private void btn_login_Click(object sender, EventArgs e)
 {
     strUsername = tb_username.Text;
     strPassword = tb_passsword.Text;
     if (Validation.isValidUser(strUsername, strPassword))
     {
         this.Hide();
         frm_Main main = new frm_Main(strUsername);
         main.Show();
     }
     else
     {
         tb_passsword.Text = "";
         tb_username.Text  = "";
         MessageBox.Show("Username or Password is invalid!", "Warning", MessageBoxButtons.OK, MessageBoxIcon.Warning);
     }
 }