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using System;
using System.Windows.Forms;
 
namespace CSP.Views.CustomControls
{
    public class NullableDateTimePicker : DateTimePicker
    {
        private DateTime? SavedValue { get; set; }
        private DateTimePickerFormat SavedFormat { get; set; }
        private string SavedCustomFormat { get; set; }
        private bool IsNullValue { get; set; }
 
        public NullableDateTimePicker()
        {
            Format = DateTimePickerFormat.Custom;
            CustomFormat = "yyyy年MM月dd日";
            Value = DateTime.Now;
            Text = Value.ToString();
            IsNullValue = false;
        }
 
        public NullableDateTimePicker(DateTime dateTime)
        {
            Format = DateTimePickerFormat.Custom;
            CustomFormat = "yyyy年MM月dd日";
            Value = dateTime;
            Text = Value.ToString();
            IsNullValue = false;
        }
 
        public new DateTime? Value
        {
            get;
            set;
        }
 
        protected override void OnKeyDown(KeyEventArgs e)
        {
            base.OnKeyDown(e);
 
            if (e.KeyCode == Keys.Delete)
            {
                if (!IsNullValue)
                {
                    SavedFormat = Format;
                    SavedCustomFormat = CustomFormat;
                    SavedValue = Value;
 
                    Value = null;
                    CustomFormat = " ";
 
                    IsNullValue = true;
                }
            }
            else
            {
                if (IsNullValue)
                {
                    Format = SavedFormat;
                    CustomFormat = SavedCustomFormat;
                    Value = SavedValue;
 
                    IsNullValue = false;
                }
            }
        }
 
        protected override void OnTextChanged(EventArgs e)
        {
            if (String.IsNullOrWhiteSpace(Text))
            {
                Value = null;
                IsNullValue = true;
            }
            else
            {
                Value = DateTime.Parse(Text);
                IsNullValue = false;
            }
        }
 
        protected override void OnValueChanged(EventArgs e)
        {
            if (IsNullValue)
            {
                Format = SavedFormat;
                CustomFormat = SavedCustomFormat;
 
                IsNullValue = false;
            }
 
            if (!String.IsNullOrWhiteSpace(Text))
                Value = DateTime.Parse(Text);
            else
                Value = null;
        }
    }
}

prog.cs(2,22): error CS0234: The type or namespace name `Forms' does not exist in the namespace `System.Windows'. Are you missing `System.Windows.Forms' assembly reference?
prog.cs(6,43): error CS0246: The type or namespace name `DateTimePicker' could not be found. Are you missing an assembly reference?
prog.cs(9,17): error CS0246: The type or namespace name `DateTimePickerFormat' could not be found. Are you missing an assembly reference?
prog.cs(31,30): warning CS0109: The member `CSP.Views.CustomControls.NullableDateTimePicker.Value' does not hide an inherited member. The new keyword is not required
prog.cs(37,43): error CS0246: The type or namespace name `KeyEventArgs' could not be found. Are you missing an assembly reference?
prog.cs(68,33): error CS0115: `CSP.Views.CustomControls.NullableDateTimePicker.OnTextChanged(System.EventArgs)' is marked as an override but no suitable method found to override
prog.cs(82,33): error CS0115: `CSP.Views.CustomControls.NullableDateTimePicker.OnValueChanged(System.EventArgs)' is marked as an override but no suitable method found to override
Compilation failed: 6 error(s), 1 warnings

Standard output is empty