A DateTimePicker to change only month and year

Question

UI should remain same I don't want to do this:

dateTimePicker1.Format = DateTimePickerFormat.Custom;
dateTimePicker1.CustomFormat = "mm-yyyy";
dateTimePicker1.ShowUpDown = true; 

A User can pick the year and month, change DateTimePicker to Month/Year picker, the MonthCalendar too.
I don't want to use a third-party control but I will if it will be the only answer point me to any free month/year picker control.

Answer 1

Here's a simple DateTimePicker custom control that pre-defines a custom DateTime format and modified the MonthCalendar view, configuring it show just the Months and the Year when is opened.

I've added a public property, ShowToday, that allows to specify whether the MonthCalendar popup should show the Today date at the bottom of the Calendar interface.

The custom control overrides OnDropDown to get the Handle of the MonthCalendar popup, sending a DTM_GETMONTHCAL message and changes its current view sending a MCM_SETCURRENTVIEW, specifying MCMV_YEAR as the value parameter.

This is how it look like:

---

using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Windows.Forms;

[DesignerCategory("Code")]
class DateTimePickerYearMonth : DateTimePicker
{
    public DateTimePickerYearMonth() {
        this.CustomFormat = "MM-yyyy";
        this.Format = DateTimePickerFormat.Custom;
        this.Value = DateTime.Now;
    }

    [
        Browsable(true), EditorBrowsable(EditorBrowsableState.Always), DefaultValue(false),
        Category("Appearance"), 
        Description("Shows or hides \"Today\" date at the bottom of the Calendar Control")
    ]
    public bool ShowToday {
        get => m_ShowToday;
        set {
            if (value != m_ShowToday) {
                m_ShowToday = value;
                ShowMonCalToday(m_ShowToday);
            }
        }
    }

    protected override void OnHandleCreated(EventArgs e)
    {
        base.OnHandleCreated(e);
        ShowMonCalToday(m_ShowToday);
    }

    protected override void OnDropDown(EventArgs e)
    {
        var hWnd = SendMessage(this.Handle, DTM_GETMONTHCAL, 0, 0);
        if (hWnd != IntPtr.Zero) {
            SendMessage(hWnd, MCM_SETCURRENTVIEW, 0, (int)MonCalView.MCMV_YEAR);
        }
        base.OnDropDown(e);
    }

    private void ShowMonCalToday(bool show)
    {
        int styles = SendMessage(this.Handle, DTM_GETMCSTYLE, 0, 0).ToInt32();
        styles = show ? styles &~(int)MonCalStyles.MCS_NOTODAY : styles | (int)MonCalStyles.MCS_NOTODAY;
        SendMessage(this.Handle, DTM_SETMCSTYLE, 0, styles);
    }
        
    [DllImport("user32.dll")]
    static extern IntPtr SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);

    private const int DTM_FIRST = 0x1000;
    private const int DTM_GETMONTHCAL = DTM_FIRST + 8;
    private const int DTM_SETMCSTYLE = DTM_FIRST + 11;
    private const int DTM_GETMCSTYLE = DTM_FIRST + 12;
    private const int MCM_FIRST = 0x1000;
    private const int MCM_GETCURRENTVIEW = MCM_FIRST + 22;
    private const int MCM_SETCURRENTVIEW = MCM_FIRST + 32;
    private bool m_ShowToday = false;

    public enum MonCalView : int
    {
        MCMV_MONTH = 0,
        MCMV_YEAR = 1,
        MCMV_DECADE = 2,
        MCMV_CENTURY = 3
    }

    public enum MonCalStyles : int
    {
        MCS_DAYSTATE = 0x0001,
        MCS_MULTISELECT = 0x0002,
        MCS_WEEKNUMBERS = 0x0004,
        MCS_NOTODAYCIRCLE = 0x0008,
        MCS_NOTODAY = 0x0010,
        MCS_NOTRAILINGDATES = 0x0040,
        MCS_SHORTDAYSOFWEEK = 0x0080,
        MCS_NOSELCHANGEONNAV = 0x0100
    }
}

Answer 2

The custom format is to be used to determine the way the date should be captured in the system/database irrespective of what the user selected, it is not used to restrict the user to choosing either month or year.