How do I make the main form show only if the user login is successful from a second form?

Question

I am trying to make a form that verifies user login. I want that form to show first then, if the user login is successful, the main form should show.

this is what I have tried(in the second form):

 private void button1_Click(object sender, EventArgs e)
    {
        string funame = "thename";
        string dep = "thedep";
        string unm = "theusername";
        string ups = "thepassword";

        User cs = new User(funame, dep, unm, ups);

        if (cs.validateLogin(unm, ups));
        {
            MessageBox.Show("Welcome " + funame + " of " + dep);

            frmPurchaseDiscountedItem fpd = new frmPurchaseDiscountedItem();
            fpd.Show();
        }

        

    }

The problem is the main form always pops out first.

It should be something like:

*2nd form pops up then, if user is verified, main form pops up

Answer 1

Here is a workaround.

You don't need to change "start form" in Program.cs. You can open the main window at the beginning and hide it. Then create a Login instance and show it.

public partial class MainForm : Form
{
    public MainForm()
    {
        InitializeComponent();

        Login login = new Login(this);
        // hide MainForm
        this.Hide();
        // show Login
        login.ShowDialog();
    }
}

If login button in Login form clicked, show the MainForm and close Login form.

public partial class Login : Form
{
    private MainForm mainform = null;

    public Login(MainForm main)
    {
        InitializeComponent();
        // get MainForm instance
        this.mainform = main;
    }

    // if success
    bool success = true;

    private void btnLogin_Click(object sender, EventArgs e)
    {
        if(success) // check login here
        {
            // show main form
            mainform.Visible = true;
            // close login form
            this.Close();
        }
    }
}

In addition, if you click the X button in the upper right corner of the Login form, it will also show the hidden MainForm (if user doesn't want to login, close the application). We can avoid it by overriding WndProc.

protected override void WndProc(ref Message msg)
{
    const int WM_SYSCOMMAND = 0x0112;
    const int SC_CLOSE = 0xF060;
    if (msg.Msg == WM_SYSCOMMAND && ((int)msg.WParam == SC_CLOSE))
    {
        // click the 'X' button to close the application
        Application.Exit();
        return;
    }
    base.WndProc(ref msg);
}