默认转换器何时启动？ [英] When do default converters kick in?
问题描述
使用以下代码，尽管Text属性绑定到DateTime源属性，但我注意到WPF似乎自动将文本转换为DateTime，而无需编写ValueConverter。有人可以说明一下如何完成
With the following code, although Text property is bound to a DateTime source property, I noticed WPF seems to automatically convert the text to a DateTime, without me needing to write a ValueConverter. Can someone please shed some light on how this is done
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:WpfApplication1="clr-namespace:WpfApplication1"
Title="MainWindow" Height="350" Width="525"
>
<StackPanel>
<DatePicker Height="25" Name="datePicker1" Width="213" Text="{Binding Path=DueDate,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" />
</StackPanel>
</Window>
public class P
{
private DateTime? dueDate = DateTime.Now;
public DateTime? DueDate
{
get { return dueDate; }
set
{
dueDate = value;
}
}
}
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
P p = new P();
this.DataContext = p;
}
}
推荐答案
它正在使用基类库中的 DateTimeTypeConverter （编辑：嗯，它本来可以使用TypeConverter，但是从@DeviantSeev的回答他们没有这样做）。
It is using the DateTimeTypeConverter from the Base Class Library ( Well, it could have used a TypeConverter however it appears that from @DeviantSeev's answer that they did not).
您是默认转换器谈论的实际上是 TypeConverters （ MSDN ），并且自v2.0起它们就成为.NET Framework的一部分，并且在整个基类库中都使用了它们。 WPF中TypeConverters的另一个示例是 ThicknessTypeConverter ，用于 Padding ， Margin 和 BorderThickness 属性。它将逗号分隔的字符串转换为 Thickness 对象。
There 'default' converters you are talking about are actually TypeConverters (MSDN) and they have been a part of the .NET Framework since v2.0 and they are used through-out the Base Class Libraries. Another example of TypeConverters in WPF is the ThicknessTypeConverter for Padding, Margin, and BorderThickness properties. It converts a comma-delimited string to a Thickness object.
有大量（如果您想文章） a href = http://www.csharphelp.com/2007/07/c-type-converters-your-friendly-helpers/ rel = nofollow noreferrer>进一步了解它们。
There are plenty of articles available if you want to understand them further.
使用 TypeConverter 有两部分-实现类，然后使用标记您的属性/类型 TypeConverterAttribute 。
There are two parts to using a TypeConverter - implementation of the class and then marking up your properties/types with TypeConverterAttribute.
例如，我最近有一个自定义控件，需要一个 char [] ，我想从 Xaml 像这样：
For example, I recently had a custom control that required a char[] that I wanted to set from Xaml like so:
<AutoCompleteTextBox MultiInputDelimiters=",;. " />
用法
[TypeConverter(typeof(CharArrayTypeConverter))]
public char[] MultiInputDelimiters
{
get { return (char[])GetValue(MultiInputDelimitersProperty); }
set { SetValue(MultiInputDelimitersProperty, value); }
}
实施
public class CharArrayTypeConverter : TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
return (Type.GetTypeCode(sourceType) == TypeCode.String);
}
public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
{
if (value is string)
return ((string)value).ToCharArray();
return value;
}
}
何时使用 TypeConverter ？
When to use a TypeConverter?
您只能使用 TypeDescriptors 如果您要编写自定义控件，则需要使用 TypeDescriptorAttribute 标记该属性。如果转换相当简单，我也只会使用 TypeConverter -如上面的示例中我有一个字符串并想要一个 char [ ] -或是否有多种可能的格式要转换。
You can only use TypeDescriptors if you are writing a custom control as you need to be able to mark-up the property with the TypeDescriptorAttribute. Also I would only use TypeConverter if the conversion is rather a straight-forward - as in the example above where I have a string and want a char[] - or if there are multiple possible formats that I want to convert from.
当您想要更大的灵活性来通过数据驱动或传递来转换值时，编写 IValueConverter 参数。例如，在WPF中非常常见的操作是将 bool 转换为 Visibility ；这样的转换有三种可能的输出（可见，隐藏，崩溃），并且只有两个输入（ true ， false ），很难在 TypeConverter 。
You write IValueConverter when you want more flexibility on how the value to converted by driving it by data or a passing a parameter. For example, a very common action in WPF is converting a bool to Visibility; there are three possible outputs from such a conversion (Visible, Hidden, Collapsed) and with only two inputs (true, false) it difficult to decide this in a TypeConverter.
在我的应用程序中，为了实现这两个输入到三个输出的问题，我编写了一个 BoolToVisibilityConverter ，其中 TrueValue 和 FalseValue 属性，然后在我的全局 ResourceDictionary 。 我明天早上发布代码示例，我现在不在我面前。。
In my applications, to achieve this two inputs to three output problem I have written a single BoolToVisibilityConverter with a TrueValue and FalseValue properties and then I instance it three times in my global ResourceDictionary. I'll post the code sample tomorrow morning, I don't it in front of me right now..
[ValueConversion(typeof(bool), typeof(Visibility))]
public class BooleanToVisibilityConverter : IValueConverter
{
public Visibility FalseCondition { get; set; }
public Visibility TrueCondition { get; set; }
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return ((bool)value) ? TrueCondition : FalseCondition;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
if ((bool)value)
return TrueCondition;
return FalseCondition;
}
}
<converters:BooleanToVisibilityConverter x:Key="BoolToVisibilityConverter" FalseCondition="Collapsed" TrueCondition="Visible"/>
<converters:BooleanToVisibilityConverter x:Key="BoolToVisibilityCollapsedConverter" FalseCondition="Visible" TrueCondition="Collapsed"/>
<converters:BooleanToVisibilityConverter x:Key="BoolToVisibilityHiddenConverter" FalseCondition="Visible" TrueCondition="Hidden"/>
<converters:BooleanToVisibilityConverter x:Key="BoolToVisibilityHiddenWhenFalseConverter" FalseCondition="Hidden" TrueCondition="Visible"/>
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