可以为空值、可以删除显示日期的DateTimePicker
.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(); } }
}