Why when I deserialize with JSON.NET ignores my default value?

Question

I'm using JSON.NET as my main serializer.

This is my model, look that I've setted some JSONProperties and a DefaultValue.

public class AssignmentContentItem
{
    [JsonProperty("Id")]
    public string Id { get; set; }
    [JsonProperty("Qty")]
    [DefaultValue(1)]
    public int Quantity { get; set; }
}

When I serialize a List<AssignmentContentItem>, it doing a good work:

private static JsonSerializerSettings s = new JsonSerializerSettings
{
    DefaultValueHandling = DefaultValueHandling.Ignore,
    NullValueHandling = NullValueHandling.Ignore
};

OUTPUT:

[{"Id":"Q0"},{"Id":"Q4"},{"Id":"Q7"}]

But when I'd like to deserialize this jsonContent, the property Qty is always 0 and is not set to the default value. I mean, when I deserialize that jsonContent, as DefaultValue for Quantity should be one instead of 0.

public static List<AssignmentContentItem> DeserializeAssignmentContent(string jsonContent)
{
    return JsonConvert.DeserializeObject<List<AssignmentContentItem>>(jsonContent, s);
}

What should I do

Solution 1

The DefaultValue attribute does not set the value of the property. See this question: .NET DefaultValue attribute

What you might be better off doing is setting the value in the constructor:

public class AssignmentContentItem
{
    [JsonProperty("Id")]
    public string Id { get; set; }
    [JsonProperty("Qty")]
    public int Quantity { get; set; }

    public AssignmentContentItem()
    {
        this.Quantity = 1;
    }
}

Where this line:

AssignmentContentItem item =
    JsonConvert.DeserializeObject<AssignmentContentItem>("{\"Id\":\"Q0\"}");

Results in an AssignmentContentItem with its Quantity set to 1.

Solution 2

You can use the the DefaultValueHandling.Populate setting so that Json.Net will populate the created object with the default value.

public static List<AssignmentContentItem> DeserializeAssignmentContent(string jsonContent)
{
    return JsonConvert.DeserializeObject<List<AssignmentContentItem>>(jsonContent, new JsonSerializerSettings
    { 
            DefaultValueHandling = DefaultValueHandling.Populate, 
            NullValueHandling = NullValueHandling.Ignore 
    });
}

http://www.newtonsoft.com/json/help/html/T_Newtonsoft_Json_DefaultValueHandling.htm

Comments

[Oscar Canek]

You can add [JsonProperty(NullValueHandling = NullValueHandling.Ignore)] if the json does not have the property and it's nullable so it will take the value set in the constructor otherwise it will set to null