认识类型转换器
有没有想过，当你在xaml中写下"red","blue"等色值时，控件的背景色变成了红色和蓝色，WPF是如何“智能识别出我们的意图的”?
答案是类型转换器：BrushConverter
namespace System.Windows.Media;//
// 摘要:
// Defines objects used to paint graphical objects. Classes that derive from System.Windows.Media.Brush
// describe how the area is painted.
[TypeConverter(typeof(BrushConverter))]
[ValueSerializer(typeof(BrushValueSerializer))]
[Localizability(LocalizationCategory.None, Readability = Readability.Unreadable)]
public abstract class Brush : Animatable, IFormattable, DUCE.IResource
形如这种还有 Margin属性、FontSize属性等等，他们能够让我们简写成下面的形式，都因类型转换器起了作用：
<TextBlockx:Name="infoText"Margin="10"FontSize="12"Foreground="Red"Text="{Binding Fruits, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
自定义类型转换器
public enum MyEnum
{[Description("苹果")]Apple,[Description("橘子")]Orange
}
现在，我有一个枚举类型，需要绑定到界面上的文本控件的文本属性上；由于枚举类型对于用户阅读不太友好，我希望能直接显示枚举成员的DescriptionAttribute的Description值。即：Apple能够展示成苹果。
xaml代码上文已给出，其中Fruits属性为MainWindow中的属性
public MyEnum Fruits { get; set; }
运行起来,发现界面上展示的是Apple并不是苹果~~~
我们需要进一步改写代码，才可以达到上述的效果；参考：类型转换器与XAML，只需两步。
自定义枚举类型转换器
已经有EnumConverter，我们不必派生自TypeConverter；
public class MyEnumConverter : EnumConverter
{public MyEnumConverter(Type type) : base(type){}public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType){var type = value.GetType();var fieldInfo = type.GetField(value.ToString());if (fieldInfo != null){var targetAttr = fieldInfo.GetCustomAttribute<DescriptionAttribute>();if (targetAttr != null){return targetAttr.Description;}}return base.ConvertTo(context, culture, value, destinationType);}
}
引入类型转换器
在自定义的枚举类型MyEnum中通过TypeConverterAttribute引入MyEnumConverter。
[TypeConverter(typeof(MyEnumConverter))]
public enum MyEnum
{[Description("苹果")]Apple,[Description("橘子")]Orange
}
通过以上步骤，发现界面上的文本展示了苹果。
类型转换器是如何起作用的？
通过在MyEnumConverter的构造函数和ConvertTo方法处打断点。我看到了调用堆栈。
构造函数调用堆栈
WpfApp1.exe!WpfApp1.MyEnumConverter.MyEnumConverter(System.Type type)
System.dll!System.SecurityUtils.SecureConstructorInvoke(System.Type type, System.Type[] argTypes, object[] args, bool allowNonPublic, System.Reflection.BindingFlags extraFlags)
System.dll!System.SecurityUtils.SecureConstructorInvoke(System.Type type, System.Type[] argTypes, object[] args, bool allowNonPublic)
System.dll!System.ComponentModel.ReflectTypeDescriptionProvider.CreateInstance(System.Type objectType, System.Type callingType)
System.dll!System.ComponentModel.ReflectTypeDescriptionProvider.ReflectedTypeData.GetConverter(object instance)
System.dll!System.ComponentModel.ReflectTypeDescriptionProvider.GetConverter(System.Type type, object instance)
System.dll!System.ComponentModel.TypeDescriptor.TypeDescriptionNode.DefaultTypeDescriptor.System.ComponentModel.ICustomTypeDescriptor.GetConverter()
System.dll!System.ComponentModel.TypeDescriptor.GetConverter(System.Type type)
PresentationFramework.dll!MS.Internal.Data.DefaultValueConverter.GetConverter(System.Type type)
PresentationFramework.dll!MS.Internal.Data.DefaultValueConverter.Create(System.Type sourceType, System.Type targetType, bool targetToSource, MS.Internal.Data.DataBindEngine engine)
PresentationFramework.dll!MS.Internal.Data.DataBindEngine.GetDefaultValueConverter(System.Type sourceType, System.Type targetType, bool targetToSource)
PresentationFramework.dll!System.Windows.Data.BindingExpression.SetupDefaultValueConverter(System.Type type)
PresentationFramework.dll!MS.Internal.Data.ClrBindingWorker.SetupDefaultValueConverter(System.Type type)
......此处省略很多行
PresentationFramework.dll!MS.Internal.Data.DataBindEngine.Task.Run(bool lastChance)
PresentationFramework.dll!MS.Internal.Data.DataBindEngine.Run(object arg)
PresentationFramework.dll!MS.Internal.Data.DataBindEngine.OnLayoutUpdated(object sender, System.EventArgs e)
原来控件加载时绑定引擎开始工作，通过绑定表达式和反射，最后获取到了MyEnum类型上的MyEnumConverter
ConvertTo方法调用堆栈
WpfApp1.exe!WpfApp1.MyEnumConverter.ConvertTo(System.ComponentModel.ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, System.Type destinationType)
PresentationFramework.dll!MS.Internal.Data.DefaultValueConverter.ConvertHelper(object o, System.Type destinationType, System.Windows.DependencyObject targetElement, System.Globalization.CultureInfo culture, bool isForward)
PresentationFramework.dll!MS.Internal.Data.DefaultValueConverter.ConvertTo(object o, System.Type destinationType, System.Windows.DependencyObject targetElement, System.Globalization.CultureInfo culture)
PresentationFramework.dll!MS.Internal.Data.SourceDefaultValueConverter.Convert(object o, System.Type type, object parameter, System.Globalization.CultureInfo culture)
..........此处省略很多行
PresentationFramework.dll!MS.Internal.Data.DataBindEngine.Run(object arg)
PresentationFramework.dll!MS.Internal.Data.DataBindEngine.OnLayoutUpdated(object sender, System.EventArgs e)
PresentationCore.dll!System.Windows.ContextLayoutManager.fireLayoutUpdateEvent()
PresentationCore.dll!System.Windows.ContextLayoutManager.UpdateLayout()
PresentationCore.dll!System.Windows.UIElement.UpdateLayout()
通过观察ConvertTo方法调用堆栈，我们了得知：也是因为绑定引擎，Apple才可以转换成苹果。