Json.NET, Unable to de-serialize nullable type

Question

I'm trying to convert JSON to C# object using Json.NET. The object looks like this in C#:

public class MyObject 
{
   public int? integerValue {get;set;}
   public DateTime? dateTimeValue {get;set;}
}

But when I run JsonConvert.DeserializeObject() on the incoming JSON, I get the following Exception:

Unable to find a constructor to use for type System.Nullable`1[System.Int32]. A class should either have a default constructor or only one constructor with arguments.

--- EDIT ----

Well it turns out that after doing many tests, the problem boils down to that my input for my JSON was like this:

{integerValue:{}, dateTimeValue: {} }

instead of:

{integerValue: null, dateTimeValue: null}

It turns out that the {} is a valid way of representing a null object in JSON but the JSON.Net parser did not know to treat {} tokens the same way as 'null' when de-serializing.

Thanks everyone for your input!

Answer 1

The error is telling you that it cant find aa constructor that it can use for the deserialization.

Try adding a default constructor to the class:

public class MyObject
{
    public int? integerValue { get; set; }
    public DateTime? dateTimeValue { get; set; }

    public MyObject(){}
} 

Patrick.

--EDIT--

So I've just created a simple console app using your MyObject , with and without a default constructor and I'm getting no errors. Here is my example:

class Program
{
    static void Main(string[] args)
    {
        var mo = new MyObject { integerValue = null, dateTimeValue = null };
        var ser = Newtonsoft.Json.JsonConvert.SerializeObject(mo);
        var deser = Newtonsoft.Json.JsonConvert.DeserializeObject(ser, typeof(MyObject));
    }
}

public class MyObject
{
    public int? integerValue { get; set; }
    public DateTime? dateTimeValue { get; set; }        
}  

I get no exceptions...

Can you show an example of the JSON that you are trying to deserialize?

Answer 2

The solution for me was to create Converter according to this answer

public class BoolConverter : JsonConverter
{
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteValue(((bool)value) ? 1 : 0);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.Value == null || reader.Value.ToString() == "False")
        {
            return false;
        }
        return true;
    }

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(bool);
    }
}

And than specify in model

[JsonConverter(typeof(BoolConverter))]
public Boolean bold;

Answer 3

I dont know is it right answer or not, but at least You can create custom converter for Nullable<>, it helps me a lot with DataRow serializing/deserializing it also does not have default constructor. Here is sample

Answer 4

A completed version of the @Patrick answer:

static void Main(string[] args)
{
    var mo = new MyObject ();
    var ser = Newtonsoft.Json.JsonConvert.SerializeObject(mo);
    var myStr = "{}";
    var myStr1 = "{tITi: 10}";
    var myStr2 = "{integerValue: 10}";
    var deser0 = Newtonsoft.Json.JsonConvert.DeserializeObject(ser, typeof(MyObject));
    var deser1 = Newtonsoft.Json.JsonConvert.DeserializeObject(myStr, typeof(MyObject));
    var deser2 = Newtonsoft.Json.JsonConvert.DeserializeObject(myStr1, typeof(MyObject));
    var deser3 = Newtonsoft.Json.JsonConvert.DeserializeObject(myStr2, typeof(MyObject));
}

public class MyObject
{
    public int? integerValue { get; set; }
    public DateTime? dateTimeValue { get; set; }
    public int toto { get; set;  } = 5;
    public int Titi;
}

Output:

?deser0
{ConsoleApplication1.MyObject}
    Titi: 0
    dateTimeValue: null
    integerValue: null
    toto: 5
?deser1
{ConsoleApplication1.MyObject}
    Titi: 0
    dateTimeValue: null
    integerValue: null
    toto: 5
?deser2
{ConsoleApplication1.MyObject}
    Titi: 10
    dateTimeValue: null
    integerValue: null
    toto: 5
?deser3
{ConsoleApplication1.MyObject}
    Titi: 0
    dateTimeValue: null
    integerValue: 10
    toto: 5

Answer 5

还要确保您的属性具有公共设置器，以便反序列化工作。