Csharp 为值对象编写自定义JsonConverter

与自定义JsonConverter不同，您可以编写一个custom contract resolver，它在从ValueObject继承的对象中查找静态Create()方法，并将调用它的委托分配给JsonObjectContract.OverrideCreator.这样做之后，静态方法将被调用，而不是普通构造函数.

为此，首先创建以下合同解析器:

public class ValueObjectResolver : DefaultContractResolver
{
    const string CreateMethodName = "Create";

    protected override JsonObjectContract CreateObjectContract(Type objectType)
    {
        var contract = base.CreateObjectContract(objectType);
        if (typeof(ValueObject).IsAssignableFrom(objectType))
        {
            try
            {
                var createMethod = objectType.GetMethod(CreateMethodName, BindingFlags.Static | BindingFlags.Public | BindingFlags.DeclaredOnly);
                if (createMethod != null)
                {
                    contract.OverrideCreator = (args) => createMethod.Invoke(null, args) ?? throw new InvalidOperationException($"null created object {objectType.Name}");
                    contract.CreatorParameters.Clear();
                    foreach (var param in CreateStaticConstructorParameters(createMethod, contract.Properties))
                        contract.CreatorParameters.Add(param);
                }
            }
            catch (AmbiguousMatchException ex)
            {
                // TODO: Handle multiple Create() methods with dfferent argument lists however you want.
                Debug.WriteLine(ex);
            }
        }

        return contract;
    }

    JsonPropertyCollection CreateStaticConstructorParameters(MethodInfo createMethod, JsonPropertyCollection memberProperties)
    {
        // Adapted from https://github.com/JamesNK/Newtonsoft.Json/blob/master/Src/Newtonsoft.Json/Serialization/DefaultContractResolver.cs#L743
        // By https://github.com/JamesNK/
        // License: https://github.com/JamesNK/Newtonsoft.Json/blob/master/LICENSE.md
        var parameterCollection = new JsonPropertyCollection(createMethod.DeclaringType!);
        foreach (var parameterInfo in createMethod.GetParameters().Where(i => i.Name != null))
        {
            var matchingMemberProperty = MatchProperty(memberProperties, parameterInfo.Name, parameterInfo.ParameterType);
            if (CreatePropertyFromConstructorParameter(matchingMemberProperty, parameterInfo) is {} property)
                parameterCollection.AddProperty(property);
        }
        return parameterCollection;
    }

    static JsonProperty? MatchProperty(JsonPropertyCollection properties, string? name, Type type)
    {
        if (name == null)
            return null;
        var property = properties.GetClosestMatchProperty(name);
        // must match type as well as name
        if (property?.PropertyType == type)
            return property;
        return null;
    }       
}

要使用它，请按照docs的建议将实例静态缓存到某个位置:

public static DefaultContractResolver Resolver { get; } = new ValueObjectResolver()
{
    NamingStrategy = new CamelCaseNamingStrategy(),
};

现在，您将能够往返包含ValueObject个数据模型，如下所示:

Individual individual1 = new()
{
    Name = PersonName.Create("John", "", "Wayne"),
    Citizenship = "American",
};

JsonSerializerSettings settings = new()
{
    ContractResolver = Resolver,
    // Add other settings as required.
    // TODO, CAUTION -- If you use TypeNameHandling you should write a custom serialization binder for security reasons.
    // See https://www.newtonsoft.com/json/help/html/t_newtonsoft_json_typenamehandling.htm
    TypeNameHandling = TypeNameHandling.Objects,
};

var json = JsonConvert.SerializeObject(individual1, Formatting.Indented, settings);

var individual2 = JsonConvert.DeserializeObject<Individual>(json, settings);

备注:

• 为简单起见，我直接拨打MethodInfo.Invoke().如果性能成为一个问题，您可以使用反射来制造一个委托.

• C#record types也支持value-based equality.你可能会考虑是否可以使用它们，而不是发明类似的东西.

• 我注意到你用的是TypeNameHandling.请注意，此设置存在安全风险.正如docs中所解释的那样:

  当您的应用程序从外部源反序列化JSON时，应谨慎使用TypeNameHandling.当使用None以外的值进行反序列化时，传入类型应使用自定义SerializationBinder进行验证.

  详细信息请参见100和101.

• 解析器假定每个ValueObject子类型只有一个Create()方法.如果您曾经引入具有多个Create()个方法的值对象，则必须相应地更新CreateObjectContract()，例如，通过 Select 具有最多参数的Create()个方法.

• 有关何时使用自定义转换器与何时使用自定义解析器的指导，请参见100.在实践中，自定义解析器往往在TypeNameHandling和引用保留方面发挥得更好.

演示小提琴here.