TypeConverter 可以用于构造函数参数吗
Can a TypeConverter be used for constructor argument
我正在尝试编写这样的标记扩展:
[MarkupExtensionReturnType(typeof(Length))]
public class LengthExtension : MarkupExtension
{
// adding the attribute like this compiles but does nothing.
public LengthExtension([TypeConverter(typeof(LengthTypeConverter))]Length value)
{
this.Value = value;
}
[ConstructorArgument("value")]
public Length Value { get; set; }
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this.Value;
}
}
这样使用:
<Label Content="{units:Length 1 mm}" />
错误:
The TypeConverter for type "Length" does not support converting from string.
TypeConverter 工作如果我:
- 把它放在
Value
属性 上并有一个默认的 ctor。
- 用属性修饰
Length
类型。
虽然这可能 x/y 我不想要任何这些解决方案。
这是转换器的代码:
public class LengthTypeConverter : TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
if (sourceType == typeof(string))
{
return true;
}
return base.CanConvertFrom(context, sourceType);
}
public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
{
if (destinationType == typeof(InstanceDescriptor) || destinationType == typeof(string))
{
return true;
}
return base.CanConvertTo(context, destinationType);
}
public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
{
var text = value as string;
if (text != null)
{
return Length.Parse(text, culture);
}
return base.ConvertFrom(context, culture, value);
}
public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
{
if (value is Length && destinationType != null)
{
var length = (Length)value;
if (destinationType == typeof(string))
{
return length.ToString(culture);
}
else if (destinationType == typeof(InstanceDescriptor))
{
var factoryMethod = typeof(Length).GetMethod(nameof(Length.FromMetres), BindingFlags.Public | BindingFlags.Static, null, new Type[] { typeof(double) }, null);
if (factoryMethod != null)
{
var args = new object[] { length.metres };
return new InstanceDescriptor(factoryMethod, args);
}
}
}
return base.ConvertTo(context, culture, value, destinationType);
}
}
张贴批评:
[MarkupExtensionReturnType(typeof(Length))]
public class LengthExtension : MarkupExtension
{
public LengthExtension(string value)
{
this.Value = Length.Parse(value, CultureInfo.InvariantCulture);
}
public Length Value { get; private set; }
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this.Value;
}
}
这可行,但我不确定是否有任何缺点。
来自 MSDN, Applying the TypeConverterAttribute(强调我的):
In order for your custom type converter to be used as the acting type
converter for a custom class by a XAML processor, you must apply the
.NET Framework attribute TypeConverterAttribute to your class
definition...
和
You can also provide a type converter on a per-property basis. Instead
of applying a .NET Framework attribute TypeConverterAttribute to the
class definition, apply it to a property definition...
没有提及应用该属性的任何其他地方。所以你的问题的答案很有可能
No, a TypeConverter can not be used for a constructor argument.
我正在尝试编写这样的标记扩展:
[MarkupExtensionReturnType(typeof(Length))]
public class LengthExtension : MarkupExtension
{
// adding the attribute like this compiles but does nothing.
public LengthExtension([TypeConverter(typeof(LengthTypeConverter))]Length value)
{
this.Value = value;
}
[ConstructorArgument("value")]
public Length Value { get; set; }
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this.Value;
}
}
这样使用:
<Label Content="{units:Length 1 mm}" />
错误:
The TypeConverter for type "Length" does not support converting from string.
TypeConverter 工作如果我:
- 把它放在
Value
属性 上并有一个默认的 ctor。 - 用属性修饰
Length
类型。
虽然这可能 x/y 我不想要任何这些解决方案。
这是转换器的代码:
public class LengthTypeConverter : TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
if (sourceType == typeof(string))
{
return true;
}
return base.CanConvertFrom(context, sourceType);
}
public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
{
if (destinationType == typeof(InstanceDescriptor) || destinationType == typeof(string))
{
return true;
}
return base.CanConvertTo(context, destinationType);
}
public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
{
var text = value as string;
if (text != null)
{
return Length.Parse(text, culture);
}
return base.ConvertFrom(context, culture, value);
}
public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
{
if (value is Length && destinationType != null)
{
var length = (Length)value;
if (destinationType == typeof(string))
{
return length.ToString(culture);
}
else if (destinationType == typeof(InstanceDescriptor))
{
var factoryMethod = typeof(Length).GetMethod(nameof(Length.FromMetres), BindingFlags.Public | BindingFlags.Static, null, new Type[] { typeof(double) }, null);
if (factoryMethod != null)
{
var args = new object[] { length.metres };
return new InstanceDescriptor(factoryMethod, args);
}
}
}
return base.ConvertTo(context, culture, value, destinationType);
}
}
张贴批评:
[MarkupExtensionReturnType(typeof(Length))]
public class LengthExtension : MarkupExtension
{
public LengthExtension(string value)
{
this.Value = Length.Parse(value, CultureInfo.InvariantCulture);
}
public Length Value { get; private set; }
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this.Value;
}
}
这可行,但我不确定是否有任何缺点。
来自 MSDN, Applying the TypeConverterAttribute(强调我的):
In order for your custom type converter to be used as the acting type converter for a custom class by a XAML processor, you must apply the .NET Framework attribute TypeConverterAttribute to your class definition...
和
You can also provide a type converter on a per-property basis. Instead of applying a .NET Framework attribute TypeConverterAttribute to the class definition, apply it to a property definition...
没有提及应用该属性的任何其他地方。所以你的问题的答案很有可能
No, a TypeConverter can not be used for a constructor argument.