如何在DateTime为空（01/01/0001）时将加载时的DateTimePicker对象保留为空白（选择日期），但在有值时填充该对象？

Question

I'm trying to get a DateTimePicker object to populate with the default text (Select a date) when it doesn't get any date back from the database. If there is a date in the database, the field will populate with that date.

I wroting code that has a two way bind on SelectedDate option to a DateTime property on in the code-behind. It works properly and populates the field with 01/01/0001 since that is the null of DateTime objects. I've tried to changing it to a OneWayToSource and just bind the date if it is greater than 01/01/0001 but it puts a redbox around the object if it doesn't get a date.

Any suggestion?

Thanks for the help everyone! Here is the solution that I found.

[ValueConversion(typeof(DateTime), typeof(DateTime))]
class DateTimeNullConverter: IValueConverter
    {
         public object Convert (object value, Type targetType, object parameter, Culture culture)
         {
                if (value != null)
                {
                   DateTime dateTime = (DateTime)value;
                   if (dateTime.Year.ToString == "1")
                       return null;
                   else
                      return dateTime;
                }
                else
                {
                      return null;
                }
            }

         public object ConvertBack (object value, Type targetType, object parameter, Culture culture)
         {
              DateTime convertDateTime;
              if (value == null)
              {
                  convertDateTime = new DateTime();
              }
              else
              {
                  convertDateTime = (DateTime) value;
              }

              return convertDateTime;
         }
    }

Answer 1

Create a DateBlankConverter converter that binds to the same control:

 <DatePicker x:Name="DatePickerInstance" 
 Visibility="{Binding ElementName=DatePickerInstance, 
 Converter={StaticResource DateBlankConverter}, ConverterParameter={Binding Date}}"/>

And inside the converter check if the date is null to hide or show the DatePicker, or change the property you need.

Answer 2

You could try with some code in the setter and getter of your property

private DateTime? _date;
public DateTime? Date
{
    get 
    {
        if (null == _date)
        {
            //Set some initial value
            //or just return some default value without setting the property
        }
        return _date; 
    }
    set
    {
        if (value != _date)
        {
            _date = value;
            this.OnPropertyChanged("Date");
        }
    }
}

Answer 3

Thanks for the help everyone! Here is the solution that I found.

[ValueConversion(typeof(DateTime), typeof(DateTime))]
class DateTimeNullConverter: IValueConverter
    {
         public object Convert (object value, Type targetType, object parameter, Culture culture)
         {
                if (value != null)
                {
                   DateTime dateTime = (DateTime)value;
                   if (dateTime.Year.ToString == "1")
                       return null;
                   else
                      return dateTime;
                }
                else
                {
                      return null;
                }
            }

         public object ConvertBack (object value, Type targetType, object parameter, Culture culture)
         {
              DateTime convertDateTime;
              if (value == null)
              {
                  convertDateTime = new DateTime();
              }
              else
              {
                  convertDateTime = (DateTime) value;
              }

              return convertDateTime;
         }
    }