.Net的DateTimePicker没有提供空白显示的功能，一旦选择了日期，也不可删除而显示空白，且在非Windows XP主体环境下也没有Flat显示功能，直接放在界面上还真不太美观，于是着手改造了一下。
     如果想出时显示时日期为空，则设置Format属性为Custom，设置CustomFormat属性为空格" "即可：

using System;
using System.Windows.Forms;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Runtime.InteropServices;
using Srims.UI.Controls;
namespace DrawFlat
{
    [ToolboxBitmap(typeof(System.Windows.Forms.DateTimePicker))]
    public class FlatDateTimePicker: DateTimePicker
    {
        ComboInfoHelper

        [DllImport("user32.dll", EntryPoint="SendMessageA")]
        private static extern int SendMessage (IntPtr hwnd, int wMsg, IntPtr wParam, int lParam);

        [DllImport("user32")]
        private static extern IntPtr GetWindowDC (IntPtr hWnd );

        [DllImport("user32")]
        private static extern int ReleaseDC(IntPtr hWnd, IntPtr hDC );

        const int WM_ERASEBKGND = 0x14;
        const int WM_PAINT = 0xF;
        const int WM_NC_HITTEST = 0x84;
        const int WM_NC_PAINT = 0x85;
        const int WM_PRINTCLIENT = 0x318;
        const int WM_SETCURSOR = 0x20;


        private Pen BorderPen  = new Pen(Color.Black, 2);
        private Pen BorderPenControl  = new Pen(SystemColors.ControlDark, 2);
        private bool DroppedDown = false;
        private int InvalidateSince = 0;
        private static int DropDownButtonWidth = 17;

        static FlatDateTimePicker()
        {
            // 2 pixel extra is for the 3D border around the pulldown button on the left and right
            DropDownButtonWidth = ComboInfoHelper.GetComboDropDownWidth()+2 ;    
        }

        public FlatDateTimePicker()
            : base()
        {
            this.SetStyle(ControlStyles.DoubleBuffer, true);
            //this.SetStyle(ControlStyles.AllPaintingInWmPaint, true);
        }
        protected override void OnValueChanged(EventArgs eventargs)
        {
            base.OnValueChanged (eventargs);
            this.Invalidate();
        }

        protected override void WndProc(ref Message m)
        {
            //IntPtr hDC = IntPtr.Zero;
            //Graphics gdc = null;
            //switch (m.Msg)
            //{
            //    case WM_NC_PAINT:
            //        hDC = GetWindowDC(m.HWnd);
            //        gdc = Graphics.FromHdc(hDC);
            //        SendMessage(this.Handle, WM_ERASEBKGND, hDC, 0);
            //        SendPrintClientMsg();
            //        SendMessage(this.Handle, WM_PAINT, IntPtr.Zero, 0);
            //        OverrideControlBorder(gdc);
            //        //PaintFlatControlBorder(this, gdc);
            //        m.Result = (IntPtr)1;    // indicate msg has been processed
            //        ReleaseDC(m.HWnd, hDC);
            //        gdc.Dispose();
            //        break;
            //    case WM_PAINT:
            //        base.WndProc(ref m);
            //        hDC = GetWindowDC(m.HWnd);
            //        gdc = Graphics.FromHdc(hDC);
            //        OverrideDropDown(gdc);
            //        OverrideControlBorder(gdc);
            //        //PaintFlatControlBorder(this, gdc);
            //        ReleaseDC(m.HWnd, hDC);
            //        gdc.Dispose();
            //        break;
            //    /*
            //    // We don't need this anymore, handle by WM_SETCURSOR
            //    case WM_NC_HITTEST: 
            //        base.WndProc(ref m);
            //        if (DroppedDown)
            //        {
            //            OverrideDropDown(gdc);
            //            OverrideControlBorder(gdc);
            //        }
            //        break;
            //    */
            //    //case WM_SETCURSOR:
            //    //    base.WndProc(ref m);
            //    //    // The value 3 is discovered by trial on error, and cover all kinds of scenarios
            //    //    // InvalidateSince < 2 wil have problem if the control is not in focus and dropdown is clicked
            //    //    if (DroppedDown && InvalidateSince < 3)
            //    //    {
            //    //        Invalidate();
            //    //        InvalidateSince++;
            //    //    }
            //    //    break;
            //    default:
            //        base.WndProc(ref m);
            //        break;
            //}
            IntPtr hDC = GetWindowDC(m.HWnd);
            Graphics gdc = Graphics.FromHdc(hDC);
            switch (m.Msg)
            {
                case WM_NC_PAINT:
                    SendMessage(this.Handle, WM_ERASEBKGND, hDC, 0);
                    SendPrintClientMsg();
                    SendMessage(this.Handle, WM_PAINT, IntPtr.Zero, 0);
                    OverrideControlBorder(gdc);

                    m.Result = (IntPtr)1; // indicate msg has been processed
                    break;
                case WM_PAINT: base.WndProc(ref m);
                    OverrideControlBorder(gdc);
                    OverrideDropDown(gdc);
                    break;
                case WM_NC_HITTEST:
                    base.WndProc(ref m);
                    if (DroppedDown)
                        this.Invalidate(this.ClientRectangle, false);
                    break;
                default:
                    base.WndProc(ref m);
                    break;
            }
            ReleaseDC(m.HWnd, hDC);
            gdc.Dispose();    

            
        }
        public static void PaintFlatDropDown(Control ctrl, Graphics g)
        {
            Rectangle rect = new Rectangle(ctrl.Width - DropDownButtonWidth, 0, DropDownButtonWidth, ctrl.Height);
            ControlPaint.DrawComboButton(g, rect, ButtonState.Flat);
        }
        private void SendPrintClientMsg()
        {
            // We send this message for the control to redraw the client area
            Graphics gClient = this.CreateGraphics();
            IntPtr ptrClientDC = gClient.GetHdc();
            SendMessage(this.Handle, WM_PRINTCLIENT, ptrClientDC, 0);
            gClient.ReleaseHdc(ptrClientDC);
            gClient.Dispose();
        }
        public void SetText(string text)
        {
            if (text != "")
            {
                this.Format = DateTimePickerFormat.Long;

                this.Text = text;
            }
            else
            {
                this.Format = DateTimePickerFormat.Custom;
            }
        }
        private void OverrideDropDown(Graphics g)
        {
            if (!this.ShowUpDown)
            {
                Rectangle rect = new Rectangle(this.Width-DropDownButtonWidth, 0, DropDownButtonWidth, this.Height);
                ControlPaint.DrawComboButton(g, rect, ButtonState.Flat);    
            }
        }

        private void OverrideControlBorder(Graphics g)
        {
            //this.Width = this.Width + 4;

            if (this.Focused == false || this.Enabled == false )
                g.DrawRectangle(BorderPenControl, new Rectangle(0, 0, this.Width, this.Height));
            else
                g.DrawRectangle(BorderPen, new Rectangle(0, 0, this.Width, this.Height));
        }
        protected override void OnKeyUp(KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Back || e.KeyCode == Keys.Delete)
            {
                this.Format = DateTimePickerFormat.Custom;
                this.CustomFormat = " ";
                // OnValueChanged(EventArgs.Empty);
            }
            base.OnKeyUp(e);
        }
        protected override void OnDropDown(EventArgs eventargs)
        {
            InvalidateSince = 0;
            DroppedDown = true;
            base.OnDropDown (eventargs);
        }
        protected override void OnCloseUp(EventArgs eventargs)
        {
            DroppedDown = false;
            this.Format = DateTimePickerFormat.Long;
            base.OnCloseUp (eventargs);
        }
    
        protected override void OnLostFocus(System.EventArgs e)
        {
            base.OnLostFocus(e);
            this.Invalidate();
        }

        protected override void OnGotFocus(System.EventArgs e)
        {
            base.OnGotFocus(e);
            this.Invalidate();
        }        
        protected override void OnResize(EventArgs e)
        {
            base.OnResize (e);
            this.Invalidate();
        }

    }
}