Question

我有一个转换器，我只想在反序列化时使用。所以我将CanWrite设置为false，这很好，所有序列化都很好。然后，Json字符串包含一个对象图，其中有一个带有SantaClause项目数组的SantaClauseCollection和一个$ type表示它们是具体类型的SantaClause。

然而，当它在反序列化时遇到SantaClaus的集合时，它从不调用CanConvert（我有一个断点并看到SantaClausCollection，点击F5继续，然后当遇到集合中的项目时应再次达到断点） SantaClaus，但它没有。当它到达SantaClaus项目时，它不会尝试调用CanConvert。甚至没有为该项调用CanConvert来检查我的转换器是否会处理它，而是尝试反序列化它本身，这不会起作用，因为该类没有默认构造函数，也没有具有属性名匹配约定的构造函数：

无法找到用于类型SantaClaus的构造函数。一类应该有一个默认的构造函数，一个构造函数参数或用JsonConstructor属性标记的构造函数。

我理解为什么会出现这个错误，但问题是它表明Json.net试图反序列化该对象，而不是调用CanConvert检查并查看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只是从SantaClaus继承，添加一个带有重命名参数的构造函数来匹配属性：

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无法对SantaClaus进行反序列化，但它可以反序列化SantaClauseEx。

我在任何地方都使用了SantaClauseEx类，它运行得很好，但我想制作一个转换器来自动执行此操作。

这就是Json对于该系列的看法：

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

Answer 1

我有一个类似的问题反序列化从基类继承的对象（类似于你需要反序列化SantaClauseEx对象，但它们都被定义为SantaClause对象）。问题在于JSon.Net无法识别子类型。

请参阅stackoverflow.com/questions/8030538/how-to-implement-custom-jsonconverter-in-json-net-to-deserialize-a-list-of-base

Answer 2

我想您已将转换器添加到设置对象中的Converters集合中。

我用转换器编写简单的测试工作

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.
        }
    }

Answer 3

一旦我得到Rudus的答案，我就用我最初的尝试确定了这个问题。当你有一个没有默认构造函数的类型时，他很棒，但可以将属性值映射到其他构造函数之一，对我的特定情况来说肯定更容易。

如果出于某种原因，你确实需要像我最初尝试做的那样，在反序列化时创建一个不同类型的东西，我能够让它工作。

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)
    {
        //temporarily switch off name handling so it ignores "SantaClaus" type when
        //explicitely deserialize as SantaClausEx
        //This could cause issues with nested types however in a more complicated object graph
        var temp = serializer.TypeNameHandling;
        serializer.TypeNameHandling = TypeNameHandling.None;
        var desr = serializer.Deserialize<SantaClausEx>(reader);
        serializer.TypeNameHandling = temp;//restore previous setting

        return desr;
    }

    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 { false; } } //only for reading

}

JSON.Net没有为收集项调用CanConvert？

3 个答案: