Newtonsoft.Json: error on deserialize objects with generic fields

Question

I have a question: Does Json.NET correctly work with generics? I have the next code:

[TestClass]
public class TestClass1_Test
{
    [TestMethod]
    public void ToJson()
    {
        var mot = new TestClass1(1, "title");
        var result = mot.ToJson();
        Assert.IsNotNull(result);

        var pobject = TestClass1.FromJson(result);

        Assert.AreEqual(pobject.Id, mot.Id);
    }
}

public class TestClass1
{
    public TestClass1(int id, string name)
    {
        Id = new Field<int>(id);
        Name = new Field<string>(name);
    }

    public Field<int> Id { get; set; }
    public Field<string> Name { get; set; }

    public string ToJson()
    {
        var jobject = JObject.FromObject(this);
        return jobject.ToString();
    }

    public static TestClass1 FromJson(string json)
    {
        var obj = JObject.Parse(json).ToObject<TestClass1>();
        return obj;
    }
}

public class Field<T>
{
    public Field(T val)
    {
        Value = default(T);
    }
    public T Value { get; set; }
}

But when I call var obj = JObject.Parse(json).ToObject<TestClass1>() I get next error:

Newtonsoft.Json.JsonReaderException: Error reading integer. Unexpected token: StartObject. Path 'Id', line 2, position 10.

Where is my mistake? Or Json.NET does not work with generics?

Out of interest, why wouldn't you just set the types on the class public int Id { get; set;}? Whats the usage for Field<T> ? I Would assume its because Field<Int> != Int, i.e. Newtonsoft doesn't know what a Field<Int> is. Maybe you can write a custom converter for Field<T> ? — Mike
– Mike, Commented Nov 28, 2012 at 21:03
@Mike, it just example. I whant to undirstend ho Json.NET work with generic. In real live Field more dificult. — Sergey Shulik
– Sergey Shulik, Commented Nov 29, 2012 at 6:23

nick_w · Accepted Answer · 2012-11-28 21:15:48Z

2

Json.NET does indeed work with generics - I was able to serialize and deserialize one of your Field<int> objects just fine.

The error message I get with the above code (using Json.NET 4.5 r10) is:

Error reading integer. Unexpected token: StartObject. Path 'Id', line 2, position 10

where the stack trace implied it was trying to deserialize an integer when it ran into a {, which was the beginning of the Id object. I think this could well be a bug.

Yet this seems to work as expected when using Json.NET 3.5 r8. I did have to swap JsonConvert.DeserializeObject<TestClass1>(json) for JObject.Parse(json).ToObject<TestClass1>() as the latter isn't in this version.

The answer therefore is to try a different version of Json.NET.

There is also a bug in the Field constructor.

Value = default(T);

should be:

Value = val;

answered Nov 28, 2012 at 21:15

nick_w

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Comments

79E09796 · Accepted Answer · 2013-06-19 09:39:21Z

1

For reference; this error can also come about if you are deserializing an object that contains a nested JSON object as a string.

If you forget to stringify it, the parser throws up this error.

answered Jun 19, 2013 at 9:39

79E09796

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