DatePicker在验证类中发送日期，即使我没有选择任何日期

Question

问题在Validation类中，我试图验证用户是否未选择任何日期。但是每次Date-picker控件每次在DateTime类型的变量（即开始时间，结束时间等）中发送选定的日期时，我都会使用INOtifyPropertyChanged和IDataErrorInfo进行验证。 这是DatePIcker控件的WPF代码

 <DatePicker Name="dpStartDate"  Height="30" Width="Auto">
            <DatePicker.DisplayDate>
                <Binding Path="startdate"
                     ValidatesOnDataErrors="True"   
                     UpdateSourceTrigger="PropertyChanged" NotifyOnValidationError="True">

                    <Binding.ValidationRules>
                        <!-- Checks for exceptions during the setting of the source property.
                        Alternatively, set ValidatesOnExceptions to True on the Binding.-->
                        <ExceptionValidationRule/>
                    </Binding.ValidationRules>
                </Binding>
            </DatePicker.DisplayDate>
        </DatePicker>

下面的代码是用于验证的Model类     类TimingViewModel：INotifyPropertyChanged，IDataErrorInfo     {         公共事件PropertyChangedEventHandler PropertyChanged =委托{};

    private DateTime  _startdate { get; set; }
    private DateTime _enddate { get; set; }
    private DateTime _starttime { get; set; }
    private DateTime _endtime { get; set; }
    private string _intervaltime { get; set; }

    public bool ValidationEnabled { get; set; }
    public DateTime startdate
    {
        get
        {
            return _startdate;
        }
        set
        {

            this._startdate = value;
            PropertyChanged(this, new PropertyChangedEventArgs("startdate"));
        }
    }
    public DateTime enddate
    {
        get
        {
            return _enddate;
        }
        set
        {

            this._enddate = value;
            PropertyChanged(this, new PropertyChangedEventArgs("enddate"));
        }
    }
    public DateTime starttime
    {
        get
        {
            return _starttime;
        }
        set
        {

            this._starttime = value;
            PropertyChanged(this, new PropertyChangedEventArgs("starttime"));
        }
    }
    public DateTime endtime
    {
        get
        {
            return _endtime;
        }
        set
        {

            this._endtime = value;
            PropertyChanged(this, new PropertyChangedEventArgs("endtime"));
        }
    }
    public string intervaltime
    {
        get
        {
            return _intervaltime;
        }
        set
        {

            this._intervaltime = value;
            PropertyChanged(this, new PropertyChangedEventArgs("intervaltime"));
        }
    }
    public string Error
    {
        get { return null; }
    }
    public string this[string propName]
    {
        get
        {
            string result = string.Empty;
            if (ValidationEnabled)
            {
                if (propName == "startdate")
                {
                    if (startdate.ToShortDateString() == null)
                    {
                        result = "StartDate is required.";
                    }
                }

                else if (propName == "enddate")
                {
                    if (enddate.Date == null || startdate.Date > enddate.Date)
                    {
                        result = "End Date must not be less than start date";
                    }
                }
                else if (propName == "starttime")
                {
                    if (starttime.TimeOfDay == null)
                    {
                        result = "Start Time Must Not be Empty";
                    }
                }
                else if (propName == "endtime")
                {
                    if (starttime.TimeOfDay > endtime.TimeOfDay || endtime.TimeOfDay == null)
                    {
                        result = "End Time Must not be Empty";
                    }
                }
                else if (propName == "intervaltime")
                {
                    if (string.IsNullOrEmpty(this.intervaltime))
                    {
                        result = "Interval Time must not be Empty";
                    }
                }
            }

            return result;
        }
    }
}

在后面的代码中，我检查了这样的验证。但不会产生错误

        if (Validation.GetHasError(dpStartDate) || Validation.GetHasError(dpEndDate) || Validation.GetHasError(cbStarttime)
            || Validation.GetHasError(cbEndtime) || Validation.GetHasError(cbIntervalTime))
        {
            return;
        }