C# 如何实现类型转换器

Question

我正在努力用 C# 实现一个简单的类型转换器。 我遵循了本指南 https://msdn.microsoft.com/en-us/library/ayybcxe5.aspx

这是我的班级：

public class TestClass: TypeConverter
{
public string Property1{ get; set; }
public int Property2 { get; set; }
public TestClass(string p1, int p2)
{
Property1= p1;
Property2 = p2;
}
public override bool CanConvertFrom(ITypeDescriptorContext context,
Type sourceType)
{
if (sourceType == typeof(string)) {
return true;
}
return base.CanConvertFrom(context, sourceType);
}
public override object ConvertFrom(ITypeDescriptorContext context,
CultureInfo culture, object value)
{
if (value is string) {
return new TestClass ("", Int32.Parse(value.ToString()));
}
return base.ConvertFrom(context, culture, value);
}
public override object ConvertTo(ITypeDescriptorContext context,
CultureInfo culture, object value, Type destinationType)
{
if (destinationType == typeof(string)) {
return "___"
}
return base.ConvertTo(context, culture, value, destinationType);
}
}

我做以下TestClass ("", Int32.Parse(value.ToString()));因为现在我只对"1231" -> new TestClass("", 1231)这样的情况感兴趣

这是给我一个例外的代码;

TypeConverter converter=TypeDescriptor.GetConverter( typeof(TestClass));
Object lalala = converter.ConvertFromString("234");

这段代码抛出NotSupportedException但我不明白为什么

Answer 1

提供的代码有点混乱，缺少一些重要的东西。下面是一个将自定义类从CrazyClass转换为string的实现。

疯狂类类型转换器

public class CrazyClassTypeConverter : TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
return sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
}
public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
{
var casted = value as string;
return casted != null
? new CrazyClass(casted.ToCharArray())
: base.ConvertFrom(context, culture, value);
}
public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
{
var casted = value as CrazyClass;
return destinationType == typeof (string) && casted != null
? String.Join("", casted.Charray)
: base.ConvertTo(context, culture, value, destinationType);
}
}

疯狂课堂

(注意，类是用TypeConverterAttribute装饰的)

[TypeConverter(typeof(CrazyClassTypeConverter))]
public class CrazyClass
{
public char[] Charray { get; }
public CrazyClass(char[] charray)
{
Charray = charray;
}
}

用法

var crazyClass = new CrazyClass(new [] {'T', 'e', 's', 't'});
var converter = TypeDescriptor.GetConverter(typeof(CrazyClass));
//this should provide you the string "Test"        
var crazyClassToString = converter.ConvertToString(crazyClass); 
//provides you an instance of CrazyClass with property Charray set to {'W', 'h', 'a', 't' } 
var stringToCrazyClass = converter.ConvertFrom("What"); 

Answer 2

您必须将此转换器附加到具有TypeConverter属性的类。
TypeDescriptor.GetConverter获取该类的附加转换器。

你最好拆分类：

[TypeConverter(typeof (TestClassConverter))]
public class TestClass
{
public string Property1 { get; set; }
public int Property2 { get; set; }
public TestClass(string p1, int p2)
{
Property1 = p1;
Property2 = p2;
}
}
[TypeConverter(typeof (TestClassConverter))]
public class TestClassConverter : TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context,
Type sourceType)
{
if (sourceType == typeof(string))
{
return true;
}
return base.CanConvertFrom(context, sourceType);
}
public override object ConvertFrom(ITypeDescriptorContext context,
CultureInfo culture, object value)
{
if (value is string)
{
return new TestClass("", Int32.Parse(value.ToString()));
}
return base.ConvertFrom(context, culture, value);
}
public override object ConvertTo(ITypeDescriptorContext context,
CultureInfo culture, object value, Type destinationType)
{
if (destinationType == typeof(string)) { return "___"; }
return base.ConvertTo(context, culture, value, destinationType);
}
}