c#如何实现类型转换器

Question

我正在努力在 C# 中实现一个简单的类型转换器。 我按照本指南https://msdn.microsoft.com/en-us/library/ayybcxe5.aspx

这是我的课：

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()));，目前我只对"1231" -> new TestClass("", 1231)这样的案例感兴趣

这里是给我一个异常的代码；

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，并且缺少一些重要的东西。下面是一个将自定义类 CrazyClass 从和 string 转换的实现。

CrazyClassTypeConverter

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);
    }
}

CrazyClass

（注意这个类是用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。

你最好分班：

[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);
    }
}