Tim Van Wassenhove home

Today we ran into a nasty problem with DateTimePickerFormat.Custom. We allow the user to input a month/date with a DateTimePicker as following (nothing fancy)

private void Form1_Load(object sender, EventArgs e)
{
	this.dateTimePicker1.Value = new DateTime(2007, 8, 31);
	this.dateTimePicker1.Format = DateTimePickerFormat.Custom;
	this.dateTimePicker1.CustomFormat = "MM/yyyy";
}

Now, change to 09/2007 and notice that you get an Exception, because the control tries to create an unrepresentable new DateTime(2007, 8+1, 31). Thus, if you’re going to use the DateTimePicker for MM/yyyy input make sure to set it’s value to a DateTimeTime with a day component that exists for all months/years (thus a value between 1 and 28).

Addendum: As usual, super moderator and MVP Hans Passant provided a nice workaround for the problem:)

public class MonthPicker : DateTimePicker 
{
	public MonthPicker() 
	{
		this.Format = DateTimePickerFormat.Custom;
		this.CustomFormat = "MM/yyyy";
		DateTime now = DateTime.Now;
		this.Value = new DateTime(now.Year, now.Month, 1);
	}
	
	protected override void WndProc(ref Message m) 
	{
		if (m.Msg == 0x204e) 
		{
			NMHDR hdr = (NMHDR)m.GetLParam(typeof(NMHDR));
				if (hdr.code == -759) 
				{
					NMDATETIMECHANGE dt = (NMDATETIMECHANGE)m.GetLParam(typeof(NMDATETIMECHANGE));
					this.Value = new DateTime(dt.st.wYear, dt.st.wMonth, 1);
					return;
				}
		}
		base.WndProc(ref m);
	}
	
	// P/Invoke declarations
	[StructLayout(LayoutKind.Sequential)]
	private struct NMHDR 
	{
		public IntPtr hWnd;
		public IntPtr id;
		public int code;
	}
	
	[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
	private class NMDATETIMECHANGE 
	{
		public NMHDR nmhdr;
		public int dwFlags;
		public SYSTEMTIME st;
	}
	
	[StructLayout(LayoutKind.Sequential)]
	private class SYSTEMTIME 
	{
		public short wYear;
		public short wMonth;
		public short wDayOfWeek;
		public short wDay;
		public short wHour;
		public short wMinute;
		public short wSecond;
		public short wMilliseconds;
	}
}