当绑定值为null出现WPF日期选择器验证错误 [英] WPF datepicker validation error appears when binding value is null

问题描述

我有一个WPF应用程序，其中我使用绑定到一个实体框架（与SQL服务器）实体的日期字段日期选择器。我把它绑定如下：

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}"/>

的结合工作正常，可更新的实体。

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

我的问题是，当底层的数据库字段为空时，我得到的选择日期水印，这是我想要的，但水印被验证，回来为不是日期格式。我想保持水印，但没有验证触发，直到用户改变输入。另外，我想保持 ValidatesOnDataErrors = TRUE 因为我用的是别的地方评估业务逻辑。

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.

要明白我的意思，这里是一个使用日期选择器与日期null值的一种形式，你可以看到验证错误：  

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:

在调试提供了以下验证转换错误的输出窗口：

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

System.Windows.Data错误：7：ConvertBack不能转换值''   （类型字符串）。 BindingEx pression：路径= dtComplete;   DataItem的='job_BF0D6052EEADCDA3C2B6D1A174D77C322D5AB16A035F214705610767131A80F0   （哈希code = 17930557）;目标元素是'DatePicker的   （名称='dtComplete'）;目标属性是文本（类型字符串）   出现FormatException：System.FormatException：该字符串未被识别为   有效的DateTime。在System.DateTime.Parse（字符串S，   的IFormatProvider提供商）在System.Convert.ToDateTime（字符串   值的IFormatProvider提供商）在   System.Convert.ChangeType（对象的值，类型conversionType，   的IFormatProvider提供商）在   System.Windows.Data.BindingEx pression.ConvertBackHelper（的IValueConverter   转换器，对象的值，类型sourceType的，对象参数，   CultureInfo的文化）

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)'

有人可以帮助我摆脱这个验证错误，直到用户改变输入？

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

在此先感谢

推荐答案

想通了：

我被绑定到兴田日期选择器的文本值，因此它试图验证水印文本作为一个日期，但我真的应该一直在做被绑定到selectedDate属性如下：

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}" />

这现在的行为，因为我想，选择一个日期水印仍然存在，并结合最新价值的作品。所有的太明显了。简单的修复

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

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