WPF datepicker validation error appears when binding value is null

Question

I have a WPF application where I am using a date picker that is bound to an entity framework(with SQL server) entity's date field. I bind it as follows:

<DatePicker x:Name="dtComplete" Style="{StaticResource FTC_DatePicker}" Grid.Column="2" Grid.Row="7" Grid.ColumnSpan="3"                                            
Text="{Binding dtComplete, Mode=TwoWay, ValidatesOnDataErrors=True}"/>

The binding works fine and can be updated to the entity.

My problem is that when the underlying database field is null, I get the select a date watermark, which I want, but that watermark is being validated and coming back as not being of a date format. I want to keep the watermark, but not have the validation trigger until the user changes the input. Also, I want to keep ValidatesOnDataErrors=True because I use that somewhere else to evaluate business logic.

To see what I mean, here is a form that uses the datepicker with a null value for the date, you can see the validation error:

The output window when debugging gives the following validation conversion error:

System.Windows.Data Error: 7 : ConvertBack cannot convert value '' (type 'String'). BindingExpression:Path=dtComplete; DataItem='job_BF0D6052EEADCDA3C2B6D1A174D77C322D5AB16A035F214705610767131A80F0' (HashCode=17930557); target element is 'DatePicker' (Name='dtComplete'); target property is 'Text' (type 'String') FormatException:'System.FormatException: String was not recognized as a valid DateTime. at System.DateTime.Parse(String s, IFormatProvider provider) at System.Convert.ToDateTime(String value, IFormatProvider provider) at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider) at System.Windows.Data.BindingExpression.ConvertBackHelper(IValueConverter converter, Object value, Type sourceType, Object parameter, CultureInfo culture)'

Can someone help me get rid of this validation error until the user changes the input?

Thanks in advance

Answer 1

Figured it out:

I was binding to the text value of hte datePicker, so it was trying to validate the watermark text as a date, but what I really should have been doing was binding to the selectedDate Property as follows:

<DatePicker x:Name="dtComplete" Style="{StaticResource FTC_DatePicker}" Grid.Column="2" Grid.Row="7" Grid.ColumnSpan="3"                                                                                        
 SelectedDate="{Binding dtComplete, Mode=TwoWay, ValidatesOnDataErrors=True}" />

This now behaves as I want, the select a date watermark is still there and the binding date value works. Its all too obvious now. Simple fix