Popup window in winform c#

Just create another form (let's call it formPopup) using Visual Studio. In a button handler write the following code:

var formPopup = new Form();
formPopup.Show(this); // if you need non-modal window

If you need a non-modal window use: formPopup.Show();. If you need a dialog (so your code will hang on this invocation until you close the opened form) use: formPopup.ShowDialog()

This is not so easy because basically popups are not supported in windows forms. Although windows forms is based on win32 and in win32 popup are supported. If you accept a few tricks, following code will set you going with a popup. You decide if you want to put it to good use :

class PopupWindow : Control
{
    private const int WM_ACTIVATE = 0x0006;
    private const int WM_MOUSEACTIVATE = 0x0021;

    private Control ownerControl;

    public PopupWindow(Control ownerControl)
        :base()
    {
        this.ownerControl = ownerControl;
        base.SetTopLevel(true);
    }

    public Control OwnerControl
    {
        get
        {
            return (this.ownerControl as Control);
        }
        set
        {
            this.ownerControl = value;
        }
    }

    protected override CreateParams CreateParams
    {
        get
        {
            CreateParams createParams = base.CreateParams;

            createParams.Style = WindowStyles.WS_POPUP |
                                 WindowStyles.WS_VISIBLE |
                                 WindowStyles.WS_CLIPSIBLINGS |
                                 WindowStyles.WS_CLIPCHILDREN |
                                 WindowStyles.WS_MAXIMIZEBOX |
                                 WindowStyles.WS_BORDER;
            createParams.ExStyle = WindowsExtendedStyles.WS_EX_LEFT |
                                   WindowsExtendedStyles.WS_EX_LTRREADING |
                                   WindowsExtendedStyles.WS_EX_RIGHTSCROLLBAR | 
                                   WindowsExtendedStyles.WS_EX_TOPMOST;

            createParams.Parent = (this.ownerControl != null) ? this.ownerControl.Handle : IntPtr.Zero;
            return createParams;
        }
    }

    [DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
    public static extern IntPtr SetActiveWindow(HandleRef hWnd);

    protected override void WndProc(ref Message m)
    {
        switch (m.Msg)
        {
            case WM_ACTIVATE:
                {
                    if ((int)m.WParam == 1)
                    {
                        //window is being activated
                        if (ownerControl != null)
                        {
                            SetActiveWindow(new HandleRef(this, ownerControl.FindForm().Handle));
                        }
                    }
                    break;
                }
            case WM_MOUSEACTIVATE:
                {
                    m.Result = new IntPtr(MouseActivate.MA_NOACTIVATE);
                    return;
                    //break;
                }
        }
        base.WndProc(ref m);
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        e.Graphics.FillRectangle(SystemBrushes.Info, 0, 0, Width, Height);
        e.Graphics.DrawString((ownerControl as VerticalDateScrollBar).FirstVisibleDate.ToLongDateString(), this.Font, SystemBrushes.InfoText, 2, 2);
    }
}

Experiment with it a bit, you have to play around with its position and its size. Use it wrong and nothing shows.

Forms in C# are classes that inherit the Form base class.

You can show a popup by creating an instance of the class and calling ShowDialog().