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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Drawing;
using System.Windows.Forms;
 
namespace WindowsFormsApp1
{
    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/MM/dd";
            Value = DateTime.Now;
            Checked = true;
            IsNullValue = false;
        }
 
        public new DateTime? Value { get; set; }
 
        protected override void OnKeyDown(KeyEventArgs e)
        {
            base.OnKeyDown(e);
 
            if(e.KeyCode == Keys.Delete)
            {
                if (!IsNullValue)
                {
                    SavedValue = Value;
                    SavedFormat = Format;
                    SavedCustomFormat = CustomFormat;
 
                    Value = null;
                    CustomFormat = " ";
 
                    IsNullValue = true;
                }
            }
            else
            {
                if (IsNullValue)
                {
                    CustomFormat = "yyyy/MM/dd";
                    Format = SavedFormat;
                    Value = SavedValue;
 
                    IsNullValue = false;
                }
            }
        }
    }
}

prog.cs(7,22): 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(11,36): error CS0246: The type or namespace name `DateTimePicker' could not be found. Are you missing an assembly reference?
prog.cs(14,17): error CS0246: The type or namespace name `DateTimePickerFormat' could not be found. Are you missing an assembly reference?
prog.cs(27,30): warning CS0109: The member `WindowsFormsApp1.NullableDateTimePicker.Value' does not hide an inherited member. The new keyword is not required
prog.cs(29,43): error CS0246: The type or namespace name `KeyEventArgs' could not be found. Are you missing an assembly reference?
Compilation failed: 4 error(s), 1 warnings

Standard output is empty