使用 JSON.NET 在 C# 中的 class 中使用缺少默认构造函数反序列化 json

Question

我正在尝试将一个字符串 deserialize 转换为一个对象。问题是我想使用默认构造函数反序列化，但 class 中不存在。 class 只有一个带参数的构造函数。而且我不允许更改 class。我的场景是这样的：

using System;
using System.Collections.Generic;
using Newtonsoft.Json;

public class ConnectionSummary
{   
    public ConnectionSummary(Connection connection)
    {
        this.ConnectionId = connection.Id;
        this.SystemId = connection.SystemId;
    }

    [JsonProperty(PropertyName = "connectionId", Required = Required.Always)]
    public string ConnectionId { get; set; }

    [JsonProperty(PropertyName = "systemId")]
    public string SystemId { get; set; }
}


public class Connection
{
    public Connection()
    {
        // Initialization of some properties
    }

    [JsonProperty(PropertyName = "systemId", Required = Required.Always)]
    public string SystemId { get; set; }

    [JsonProperty(PropertyName = "id", Required = Required.Always)]
    public string Id { get; set; }

    // Other properties
}


public class Program
{
    public static void Main()
    {
        var json = "{\"connectionId\":\"id\",\"systemId\":\"sId\"}";
        var deserial = JsonConvert.DeserializeObject<ConnectionSummary>(json); // getting error here.
        Console.WriteLine(deserial.ToString());
    }
}

堆栈跟踪：

Run-time exception (line 43): Exception has been thrown by the target of an invocation.

Stack Trace:

[System.NullReferenceException: Object reference not set to an instance of an object.]
   at ConnectionSummary..ctor(Connection connection) :line 9

[System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.]
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Newtonsoft.Json.Utilities.LateBoundReflectionDelegateFactory.<>c__DisplayClass3_0.<CreateParameterizedConstructor>b__0(Object[] a)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateObjectUsingCreatorWithParameters(JsonReader reader, JsonObjectContract contract, JsonProperty containerProperty, ObjectConstructor`1 creator, String id)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateNewObject(JsonReader reader, JsonObjectContract objectContract, JsonProperty containerMember, JsonProperty containerProperty, String id, Boolean& createdFromNonDefaultCreator)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateObject(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateValueInternal(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)
   at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)
   at Program.Main() :line 43

我发现如果我在 ConnectionSummary class 中添加一个 private 默认构造函数并在 JsonSerializerSettings 中添加 ConstructorHandling = ConstructorHandling.AllowNonPublicDefaultConstructor 可以解决问题但我不能那样做。我还能在这里做其他事情吗？ Fiddle Url

Answer 1

在这种情况下，您可以自己调用构造函数，并将实例提供给转换器：

var json = "{\"connectionId\":\"id\",\"systemId\":\"sId\"}";
var cs = new ConnectionSummary(new Connection());
Newtonsoft.Json.JsonConvert.PopulateObject(json, cs);

Answer 2

您可以通过为 ConnectionSummary class 定制 JsonConverter 来解决这个问题，如下所示：

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

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JObject jo = JObject.Load(reader);
        Connection conn = new Connection
        {
            Id = (string)jo["connectionId"],
            SystemId = (string)jo["systemId"]
        };
        return new ConnectionSummary(conn);
    }

    public override bool CanWrite 
    {
        get { return false; }
    }

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

然后像这样反序列化：

var deserial = JsonConvert.DeserializeObject<ConnectionSummary>(json, new ConnectionSummaryConverter());

Fiddle: https://dotnetfiddle.net/U4UR3o

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