Popup Container

Use it where a combo box is the right shape but a list is the wrong content. You supply the content as any Control in PopupContentHost; when the user has made a choice in it, set DisplayText to show the result and call ClosePopup.

The two halves

The text box takes DisplayTextVisualToTotalSizeRatio per cent of the width (80 by default) and the button the rest. The text box is styled with the TextDisplayVisual... properties; the button with the PopupCommandVisual... properties – its colours in the normal, hover and pressed states, and either a down-arrow image (built in, or your own) or a short text.

The drop-down

A click on the button calls Show, which opens the drop-down under the editor (or at DisplayLocation) at DropDownSize; a second click closes it, as does a click anywhere else. With EnableDropDownResizing on, a strip along its foot lets the user drag it taller or, from the corner, larger, and the size they choose is kept for next time (ShouldRememberUserSizedDropDownSize). DropDownPopupOpened, DropDownPopupClosing and DropDownPopupClosed follow it.

A "Due date" editor whose drop-down is a MonthCalendar.

using System;
using System.Drawing;
using System.Windows.Forms;
using Binarymission.WinForms.Controls.ContainerPopups;

public class TaskForm : Form
{
    private readonly PopupEditor _dueDate = new PopupEditor();
    private readonly MonthCalendar _calendar = new MonthCalendar { MaxSelectionCount = 1 };

    public TaskForm()
    {
        _dueDate.Location = new Point(16, 16);
        _dueDate.Size = new Size(220, 30);
        _dueDate.DisplayText = "Choose a date";
        _dueDate.TextDisplayVisualFont = new Font("Segoe UI", 10f);
        _dueDate.PopupCommandVisualNormalBackColor = Color.SteelBlue;
        _dueDate.PopupCommandVisualBorderColor = Color.SteelBlue;
        _dueDate.PopupCommandVisualHoveredBackColor = Color.RoyalBlue;
        _dueDate.PopupCommandVisualBorderColorOnHover = Color.RoyalBlue;
        _dueDate.TextDisplayVisualBorderColor = Color.SteelBlue;

        _dueDate.PopupContentHost = _calendar;
        _dueDate.EnableDropDownResizing = false;

        _calendar.DateSelected += (sender, e) =>
        {
            _dueDate.DisplayText = e.Start.ToString("ddd d MMM yyyy");
            _dueDate.ClosePopup();
        };

        Controls.Add(_dueDate);
    }
}