问题描述
我有一个泛型类型，它包装了一个原始类型以赋予它值相等语义
I have a generic type that wraps a single primitive type to give it value equality semantics
public class ValueObject<T>
{
public T Value { get; }
public ValueObject(T value) => Value = value;
// various other equality members etc...
}
它的用法如下:
public class CustomerId : ValueObject<Guid>
{
public CustomerId(Guid value) : base(value) { }
}
public class EmailAddress : ValueObject<string>
{
public EmailAddress(string value) : base(value) { }
}
问题是在序列化如下类型时:
The issue is when serializing a type like:
public class Customer
{
public CustomerId Id { get; }
public EmailAddress Email { get; }
public Customer(CustomerId id, EmailAddress email)
{
Id = id;
Email = email;
}
}
从 ValueObject<T> 继承的每个对象都包装在 Value 属性中(如预期的那样).例如
Each object the inherits from ValueObject<T> is wrapped in a Value property (as expected). For example
var customerId = new CustomerId(Guid.NewGuid());
var emailAddress = new EmailAddress("some@email.com");
var customer = new Customer(customerId, emailAddress);
var customerAsJson = JsonConvert.SerializeObject(customer, Formatting.Indented, new JsonSerializerSettings
{
ContractResolver = new CamelCasePropertyNamesContractResolver()
})
结果
{
"id": {
"value": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c"
},
"email": {
"value": "some@email.com"
}
}
有没有办法编写自定义 JsonConverter 以便将 Value 属性排除在子类化 ValueObject 的类型中，以便上面的例子会输出
Is there a way to write a custom JsonConverter so the the Value property is excluded for types subclassing ValueObject<T> so that the above example would output
{
"id": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c",
"email": "some@email.com"
}
我希望有一个可以处理所有 ValueObjectJsonConverter 而不必为每个单独的 JsonConverterValueObject 子类
I would prefer to have a single JsonConverter that can handle all ValueObject<T> rather than having to define a separate JsonConverter for each ValueObject<T> subclass
我的第一次尝试是
public class ValueObjectOfTConverter : JsonConverter
{
private static readonly Type ValueObjectGenericType = typeof(ValueObject<>);
private static readonly string ValuePropertyName = nameof(ValueObject<object>.Value);
public override bool CanConvert(Type objectType) =>
IsSubclassOfGenericType(objectType, ValueObjectGenericType);
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
// converts "f5ce21a5-a0d1-4888-8d22-6f484794ac7c" => "value": "f5ce21a5-a0d1-4888-8d22-6f484794ac7c"
var existingJsonWrappedInValueProperty = new JObject(new JProperty(ValuePropertyName, JToken.Load(reader)));
return existingJsonWrappedInValueProperty.ToObject(objectType, serializer);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
// to implement
}
private static bool IsSubclassOfGenericType(Type typeToCheck, Type openGenericType)
{
while (typeToCheck != null && typeToCheck != typeof(object))
{
var cur = typeToCheck.IsGenericType ? typeToCheck.GetGenericTypeDefinition() : typeToCheck;
if (openGenericType == cur) return true;
typeToCheck = typeToCheck.BaseType;
}
return false;
}
}
推荐答案
您可以使用 自定义 JsonConverter 类似于 Json.Net 中显示的:将属性序列化/反序列化为值，而不是对象.但是，由于 ValueObject<T> 没有非泛型方法来获取和设置 Value 为对象，因此您需要使用反射.
You can do this with a custom JsonConverter similar to the ones shown in Json.Net: Serialize/Deserialize property as a value, not as an object. However, since ValueObject<T> does not have a non-generic method to get and set the Value as an object, you will need to use reflection.
这是一种方法:
class ValueConverter : JsonConverter
{
static Type GetValueType(Type objectType)
{
return objectType
.BaseTypesAndSelf()
.Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(ValueObject<>))
.Select(t => t.GetGenericArguments()[0])
.FirstOrDefault();
}
public override bool CanConvert(Type objectType)
{
return GetValueType(objectType) != null;
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
// You need to decide whether a null JSON token results in a null ValueObject<T> or
// an allocated ValueObject<T> with a null Value.
if (reader.SkipComments().TokenType == JsonToken.Null)
return null;
var valueType = GetValueType(objectType);
var value = serializer.Deserialize(reader, valueType);
// Here we assume that every subclass of ValueObject<T> has a constructor with a single argument, of type T.
return Activator.CreateInstance(objectType, value);
}
const string ValuePropertyName = nameof(ValueObject<object>.Value);
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var contract = (JsonObjectContract)serializer.ContractResolver.ResolveContract(value.GetType());
var valueProperty = contract.Properties.Where(p => p.UnderlyingName == ValuePropertyName).Single();
// You can simplify this to .Single() if ValueObject<T> has no other properties:
// var valueProperty = contract.Properties.Single();
serializer.Serialize(writer, valueProperty.ValueProvider.GetValue(value));
}
}
public static partial class JsonExtensions
{
public static JsonReader SkipComments(this JsonReader reader)
{
while (reader.TokenType == JsonToken.Comment && reader.Read())
;
return reader;
}
}
public static class TypeExtensions
{
public static IEnumerable<Type> BaseTypesAndSelf(this Type type)
{
while (type != null)
{
yield return type;
type = type.BaseType;
}
}
}
然后您可以将转换器直接应用于 ValueType<T>，如下所示:
You could then apply the converter directly to ValueType<T> like so:
[JsonConverter(typeof(ValueConverter))]
public class ValueObject<T>
{
// Remainder unchanged
}
或者在设置中应用它:
var settings = new JsonSerializerSettings
{
Converters = { new ValueConverter() },
ContractResolver = new CamelCasePropertyNamesContractResolver()
};
var customerAsJson = JsonConvert.SerializeObject(customer, Formatting.Indented, settings);
工作示例 .Net fiddle #1 这里.
Working sample .Net fiddle #1 here.
或者，您可以考虑添加一个非泛型方法以将值作为 object 访问，例如像这样:
Alternatively, you might consider adding a non-generic method to access the value as an object, e.g. like so:
public interface IHasValue
{
object GetValue(); // A method rather than a property to ensure the non-generic value is never serialized directly.
}
public class ValueObject<T> : IHasValue
{
public T Value { get; }
public ValueObject(T value) => Value = value;
// various other equality members etc...
#region IHasValue Members
object IHasValue.GetValue() => Value;
#endregion
}
有了这个添加，WriteJson() 变得更加简单:
With this addition, WriteJson() becomes much simpler:
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
serializer.Serialize(writer, ((IHasValue)value).GetValue());
}
工作示例 .Net fiddle #2 这里.
Working sample .Net fiddle #2 here.
注意事项:
ReadJson()假定Value<T>的每个子类都有一个公共构造函数，该构造函数采用T类型的单个参数.
ReadJson()assumes that every subclass ofValue<T>has a public constructor taking a single argument of typeT.
使用 ValueType<T>"noreferrer">[JsonConverter(typeof(ValueConverter))] 的性能会稍好一些，因为 CanConvert 永远不需要被调用.请参阅性能提示:JsonConverters详情.
Applying the converter directly to ValueType<T> using [JsonConverter(typeof(ValueConverter))] will have slightly better performance, since CanConvert need never get called. See Performance Tips: JsonConverters for details.
您需要决定如何处理 null JSON 令牌.它应该导致一个空的 ValueType<T>，还是一个分配的 ValueTypeValue?
You need to decide how to handle a null JSON token. Should it result in a null ValueType<T>, or an allocated ValueType<T> with a null Value?
在 ValueType<T> 的第二个版本中，我明确实现了 IHasValue.GetValue() 以阻止在 实例的情况下使用它ValueType<T> 用于静态类型代码.
In the second version of ValueType<T> I implemented IHasValue.GetValue() explicitly to discourage its use in cases where an instance of ValueType<T> is used in statically typed code.
如果您真的只想将转换器应用于 类型 子类化 ValueObject<T> 而不是 ValueObject<T> 本身，在 GetValueType(Type objectType) 添加对 .Skip(1) 的调用:
If you really only want to apply the converter to types subclassing ValueObject<T> and not ValueObject<T> itself, in GetValueType(Type objectType) add a call to .Skip(1):
static Type GetValueType(Type objectType)
{
return objectType
.BaseTypesAndSelf()
.Skip(1) // Do not apply the converter to ValueObject<T> when not subclassed
.Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(ValueObject<>))
.Select(t => t.GetGenericArguments()[0])
.FirstOrDefault();
}
然后将转换器应用到 JsonSerializerSettings.Converters 而不是直接应用到 ValueObject<T>.
And then apply the converter in JsonSerializerSettings.Converters rather than directly to ValueObject<T>.
这篇关于Json.NET - 序列化没有属性名称的泛型类型包装器的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！