自己写的一个用于解决DateTimePicker的值无法为Null的控件.不过有问题.需要请各位帮忙看一下问题出在哪.

我的代码如下

    /**//// <summary>
    /// 用于处理DBNull问题的DateTimePicker
    /// </summary>
    public class MDateTimePicker : System.Windows.Forms.DateTimePicker
    {
        /**//// <summary>
        /// 用于保存本身的值.可以为Null
        /// </summary>
        private object dateValue;
        /**//// <summary>
        /// 此控件使用DateTimePicker 本身的CheckBox来控制是否为DBNull,如果Checked==false;则为DBNull
        /// 所以要让此控件显示CheckBox;
        /// </summary>
        public MDateTimePicker()
        {
            this.ShowCheckBox = true;
        }
        /**//// <summary>
        /// 隐藏基类的Value
        /// </summary>
        public new object Value
        {
            get
            {
                return this.dateValue;
            }
            set
            {
                if (value != DBNull.Value)
                {
                    base.Value = Convert.ToDateTime(value);
                    this.dateValue = value;
                }
                else
                {
                    this.dateValue = null;
                }
                valueChanged();
            }
        }
        protected override void OnBindingContextChanged(EventArgs e)
        {
            //base.OnBindingContextChanged(e);
            //valueChanged();
        }
        /**//// <summary>
        /// 当值改变时,修改日期的显示格式
        /// </summary>
        /// <param name="eventargs"></param>
        protected override void OnValueChanged(EventArgs eventargs)
        {
            if (this.Checked)
            {
                this.Format = DateTimePickerFormat.Long;
            }
            
        }
        protected override void OnTextChanged(EventArgs e)
        {
            base.OnTextChanged(e);
            valueChanged();
        }
        /**//// <summary>
        /// 当值改变时,修改Checked属性,并修改Format为" ",这样控件就不会显示日期了.看起来像Null
        /// </summary>
        private void valueChanged()
        {
            if (this.Value == DBNull.Value || this.Value==null)
            {
                this.Checked = false;
                this.Format = DateTimePickerFormat.Custom;
                this.CustomFormat = " ";
            }
            else
            {
                this.Checked = true;
                this.Format = DateTimePickerFormat.Long;
                this.dateValue = base.Value;
            }
        }
        /**//// <summary>
        /// 当按下Delete时,将值修改为Null
        /// </summary>
        /// <param name="e"></param>
        protected override void OnKeyDown(KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Delete)
            {
                this.Value = DBNull.Value;
            }
        }
    }

我的问题是:
我在表单中.是这样写的

private MDateTimePicker dtpPurchaseDate;
this.dtpPurchaseDate = new WinUI.MDateTimePicker();

DataSet ds=new PurchaseMainBLL().GetAllList();
this.bindingSource1.DataSource = ds;
this.bindingSource1.DataMember = ds.Tables[0].TableName;

this.dtpPurchaseDate.DataBindings.Add("Value", this.bindingSource1, "PurchaseDate");

当数据库中PurchaseDate字段的值为Null时.此控件可以显示为空.值为日期时.此控件也可以正确显示.
只是当我按DELETE时,控件显示的日期被删除.Value也变成了null.但与它绑定的PurchaseDate的值不会改变.
请各位牛人.帮忙看一下.谢谢!