admin

写在前面：在.net中让DateTimePicker显示一个空的时间值，也许是很容易的事情，但事实上，也不是很容易的，至少，没有经过努力的修改和重画，你是达不到的。在修改这个控件的时候，你又常常发现会有bug。下面介绍一个方法，至少源作者是经过研究的，没有发现多少问题。

源网址
http://www.codeproject.com/KB/selection/Nullable_DateTimePicker.aspx
你可以在上述的网址找到源代码和英文的说明。

效果图
下图中，你只要按一下Delete键就可以将DateTimePicker的值设置为空，实际的值是DateTime.MinValue，这一个简单的效果，也许是很多程序员想看到。不过学习一下这个控件。
 

源代码
最重要的两个重载方法是下面俩。
protected override void OnCloseUp(EventArgs eventargs)
protected override void OnKeyDown(KeyEventArgs e)
DateTimePicker本身是支持自定义格式的，DateTimePickerFormat.Custom，就是声明使用自定义格式的。
  public new DateTime Value
  {
   get
   {
    if (bIsNull)
     return DateTime.MinValue;
    else
     return base.Value;
   }
   set
   {
    if (value == DateTime.MinValue)
    {
     if (bIsNull == false)
     {
      oldFormat = this.Format;
      oldCustomFormat = this.CustomFormat;
      bIsNull = true;
     }

     this.Format = DateTimePickerFormat.Custom;
     this.CustomFormat = " ";
    }
    else
    {
     if (bIsNull)
     {
      this.Format = oldFormat;
      this.CustomFormat = oldCustomFormat;
      bIsNull = false;
     }
     base.Value = value;
    }
   }
  }

admin

  .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#region ComboInfoHelper
        internal class ComboInfoHelper
        {
            [DllImport("user32")]
            private static extern bool GetComboBoxInfo(IntPtr hwndCombo, ref ComboBoxInfo info);

            RECT struct#region RECT struct
            [StructLayout(LayoutKind.Sequential)]
            private struct RECT
            {
                public int Left;
                public int Top;
                public int Right;
                public int Bottom;
            }
            #endregion

            ComboBoxInfo Struct#region ComboBoxInfo Struct
            [StructLayout(LayoutKind.Sequential)]
            private struct ComboBoxInfo
            {
                public int cbSize;
                public RECT rcItem;
                public RECT rcButton;
                public IntPtr stateButton;
                public IntPtr hwndCombo;
                public IntPtr hwndEdit;
                public IntPtr hwndList;
            }
            #endregion

            public static int GetComboDropDownWidth()
            {
                ComboBox cb = new ComboBox();
                //DateTimePicker cb = new DateTimePicker();
                int width = GetComboDropDownWidth(cb.Handle);
                cb.Dispose();
                return width;
            }
            public static int GetComboDropDownWidth(IntPtr handle)
            {
                ComboBoxInfo cbi = new ComboBoxInfo();
                cbi.cbSize = Marshal.SizeOf(cbi);
                GetComboBoxInfo(handle, ref cbi);
                int width = cbi.rcButton.Right - cbi.rcButton.Left;
                return width;
            }
        }
        #endregion

        [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 en BorderPen  = new en(Color.Black, 2);
        private en BorderPenControl  = new en(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 aintFlatDropDown(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();
        }

    }
}