Deserialize json with missing default constructor in the class in C# using JSON.NET

Question

I am trying to deserialize a string into an object. The problem is I want to deserialize using the default constructor but that is not present in the class. The class has only one constructor with parameter. And I am not allowed to change the class. My scenario is something like this:

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());
    }
}

Stack trace:

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

I found that if I add a private default constructor in the ConnectionSummary class and adding ConstructorHandling = ConstructorHandling.AllowNonPublicDefaultConstructor in the JsonSerializerSettings, can resolve the issue but I can't do that. Is there any other thing I can do here? Fiddle Url

Why are you deserialising to ConnectionSummary when you should just be using Connection? — DavidG
– DavidG, Commented Mar 19, 2020 at 16:59
Sorry my bad, I should have explained. ConnectionSummary has other properties as well. — Sunil Kumar
– Sunil Kumar, Commented Mar 19, 2020 at 17:00
To give a correct answer here, the context is important. Which part of your process is supposed to call "Deserialize"? — Dejan Janjušević
– Dejan Janjušević, Commented Mar 19, 2020 at 17:00
But your JSON doesn't match your objects. You have shown us the child object here, Newtonsoft doesn't know how to make a Connection object unless you write a custom converter. — DavidG
– DavidG, Commented Mar 19, 2020 at 17:02
Context is something like this: A web API returns Task<ConnectionSummary>. I am writing the tests. I need to call this API and then deserialize the response.Content and then do further checks. — Sunil Kumar
– Sunil Kumar, Commented Mar 19, 2020 at 17:02

Brian Rogers · Accepted Answer · 2020-03-19 19:44:27Z

You can work around the issue by making a custom JsonConverter for the ConnectionSummary class like this:

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();
    }
}

Then deserialize like this:

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

Fiddle: https://dotnetfiddle.net/U4UR3o

Hari · Accepted Answer · 2020-03-19 18:13:56Z

0

In this case you can call the constructor yourself, and supply the instance to the converter:

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

answered Mar 19, 2020 at 18:13

Hari

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