c# - JSON.Net 没有为收集项调用 CanConvert？

Question

我有一个转换器，我只想在反序列化时使用它。所以我将 CanWrite 设置为 false，它工作正常并且一切都很好地序列化。然后，Json 字符串包含一个对象图，其中有一个 SantaClauseCollection，其中包含一个 SantaClause 项目数组和一个 $type，表明它们是具体类型 SantaClause。

但是，当它在反序列化时遇到圣诞老人的集合时，它永远不会调用 CanConvert(我有一个断点并看到 SantaClausCollection，按 F5 继续，然后在遇到集合中的项目时应该再次打断点圣诞老人，但事实并非如此)。当它到达 SantaClaus 项目时，它不会尝试调用 CanConvert。它甚至没有为该项目调用 CanConvert 来检查我的转换器是否会处理它，而是尝试自行反序列化它，这不会起作用，因为该类没有默认构造函数，也没有具有属性名称匹配约定的构造函数:

Unable to find a constructor to use for type SantaClaus. A class should either have a default constructor, one constructor with arguments or a constructor marked with the JsonConstructor attribute.

我明白为什么会收到此错误，但问题是它表明 Json.net 尝试反序列化对象，而不是调用 CanConvert 来检查我的转换器是否想要处理反序列化。

为什么没有为集合中的每个项目调用 CanConvert？

我的转换器:

class SantaClaus2JsonConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(SantaClaus);          
    }

    /// <summary>
    /// Deserializes a SantaClaus as a SantaClausEx which has a matching constructor that allows it to deserialize naturally.
    /// </summary>       
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        return serializer.Deserialize<SantaClausEx>(reader);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        serializer.Serialize(writer, value);
    }


    public override bool CanRead
    {
        get
        {
            return true;
        }
    }

    public override bool CanWrite
    {
        get
        {
            return false;//We only need this converter when reading.
        }
    }

}

SantaClausEx 只是继承自SantaClaus 添加了一个构造函数，参数重命名以匹配属性:

class SantaClaus //a third party class I can't modify
{
    string Name {get;set;}
    public SantaClaus(string santaClauseName) { this.Name = santaClauseName }
}

class SantaClausEx:SantaClaus 
{
    //provide a constructor with param names matching property names
    public SantaClausEx(string name) : base(name)
}

Json.net 无法反序列化 SantaClaus，但它可以反序列化 SantaClauseEx。

我到处都使用 SantaClauseEx 类，它工作得很好，但我想制作一个转换器来自动执行此操作。

这是集合的 Json 的样子:

SantaClausCollection: [
{
  $type: "SantaClaus, XMasClasses.NET20"
  Name: "St. Bob"
},
{
  $type: "SantaClaus, XMasClasses.NET20"
  Name: "St. Jim"
}
]

Answer 1

我想您已将转换器添加到设置对象中的 Converters 集合。

我用转换器编写了简单的测试

public class SantaClausJsonTest
{
    public SantaClausJsonTest()
    {
        Settings = new JsonSerializerSettings();
        Settings.TypeNameHandling = TypeNameHandling.Objects;
        Settings.Converters.Add(new SantaClaus2JsonConverter());
    }

    private JsonSerializerSettings Settings;

    [Fact]
    public void SerializeAndDeserialize()
    {
        var collection = new []
            {
                new SantaClaus("St. Bob"),
                new SantaClaus("St. Jim"),
            };

        var serialized = JsonConvert.SerializeObject(collection, Settings);

        Console.WriteLine(serialized);
        Assert.False(string.IsNullOrEmpty(serialized));

        var deserialized = JsonConvert.DeserializeObject<SantaClaus[]>(serialized, Settings);

        Console.WriteLine(deserialized.GetType().ToString());
        Assert.NotNull(deserialized);
        Assert.True(deserialized.Any(a => a.Name == "St. Bob"));
        Assert.True(deserialized.Any(a => a.Name == "St. Jim"));
    }
}

public class SantaClaus
{
    public SantaClaus(string santaClauseName)
    {
        Name = santaClauseName;
    }

    public string Name { get; private set; }
}

public class SantaClaus2JsonConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(SantaClaus);
    }

    /// <summary>
    /// Deserializes a SantaClaus as a SantaClausEx which has a matching constructor that allows it to deserialize naturally.
    /// </summary>       
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var name = string.Empty;

        while (reader.Read())
        {
            if (reader.TokenType == JsonToken.String && reader.Path.EndsWith("Name"))
            {
                name = reader.Value as string;
            }
            if (reader.TokenType == JsonToken.EndObject)
            {
                break;
            }
        }

        return Activator.CreateInstance(objectType, name);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotSupportedException();
    }


    public override bool CanRead
    {
        get
        {
            return true;
        }
    }

    public override bool CanWrite
    {
        get
        {
            return false;//We only need this converter when reading.
        }
    }