How to make a “popup” (hint, drop-down, popup) window in Winforms?

Question

How can i make, what i will call, a \"popup\" window\" in WinForms?

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Since i used my own made-up word \"popup\", let me give examples of this so-called \"p

Answer 1

You want an owned window. In your main form:

private void showPopup_Click(object sender, EventArgs e)
{
    PopupForm popupForm = new PopupForm();
    // Make "this" the owner of form2
    popupForm.Show(this);
}

PopupForm should look like this:

public partial class PopupForm : Form
{
    private bool _activating = false;

    public PopupForm()
    {
        InitializeComponent();
    }

    // Ensure the popup isn't activated when it is first shown
    protected override bool ShowWithoutActivation
    {
        get
        {
            return true;
        }
    }

    private const int WM_NCACTIVATE = 0x86;

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    public static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam);

    protected override void WndProc(ref Message m)
    {
        // The popup needs to be activated for the user to interact with it,
        // but we want to keep the owner window's appearance the same.
        if ((m.Msg == WM_NCACTIVATE) && !_activating && (m.WParam != IntPtr.Zero))
        {
            // The popup is being activated, ensure parent keeps activated appearance
            _activating = true;
            SendMessage(this.Owner.Handle, WM_NCACTIVATE, (IntPtr) 1, IntPtr.Zero);
            _activating = false;
            // Call base.WndProc here if you want the appearance of the popup to change
        }
        else
        {
            base.WndProc(ref m);
        }
    }
}

And make sure that PopupForm.ShowInTaskbar = false.