可设置空值(Nullable)的DateTimePicker

由于WinForm自带的DateTimePicker不能设置空值(Null)，所以我基于原来的DateTimePicker做了扩展。

若有bug，可反馈，我再修改。

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace WinFormTest
{
    public class DateTimePicker2 : DateTimePicker
    {
        const string NullableFormat = " ";
        bool isSelfSetting;
        string originalCustomFormat;
        bool originalCustomFormatInitialized;
        DateTimePickerFormat? originalFormat;

        bool IsNullableState
        {
            get { return this.Format == DateTimePickerFormat.Custom && this.CustomFormat == NullableFormat; }
        }

        void SetNullable(bool nullable)
        {
            if (!this.originalFormat.HasValue)
            {
                this.originalFormat = this.Format;
            }
            if (!this.originalCustomFormatInitialized)
            {
                this.originalCustomFormat = this.CustomFormat;
                this.originalCustomFormatInitialized = true;
            }

            this.isSelfSetting = true;
            this.Format = nullable ? DateTimePickerFormat.Custom : this.originalFormat.Value;
            this.CustomFormat = nullable ? NullableFormat : this.originalCustomFormat;
            this.isSelfSetting = false;
        }

        #region Properties

        [Browsable(false)]
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public DateTime? Value2
        {
            get { return this.IsNullableState ? (DateTime?)null : Value; }
            set
            {
                if (value.HasValue)
                    Value = value.Value;
                SetNullable(!value.HasValue);
            }
        }

        public new DateTimePickerFormat Format
        {
            get { return base.Format; }
            set
            {
                if (!this.isSelfSetting)
                    this.originalFormat = value;
                base.Format = value;
            }
        }

        public new string CustomFormat
        {
            get { return base.CustomFormat; }
            set
            {
                if (!this.isSelfSetting)
                {
                    this.originalCustomFormat = value;
                    this.originalCustomFormatInitialized = true;
                }
                base.CustomFormat = value;
            }
        }

        #endregion

        #region Events

        protected override void OnGotFocus(EventArgs e)
        {
            if (this.IsNullableState)
                SetNullable(false);
            base.OnGotFocus(e);
        }

        protected override void OnMouseDown(MouseEventArgs e)
        {
            if (this.IsNullableState)
                SetNullable(false);
            base.OnMouseDown(e);
        }

        protected override void OnKeyDown(KeyEventArgs e)
        {
            SetNullable(e.KeyCode == Keys.Back || e.KeyCode == Keys.Delete);
            base.OnKeyDown(e);
        }

        #endregion
    }
}