Can I optionally turn off the JsonIgnore attribute at runtime?

Question

I am creating a JSON file with Newtonsoft.Json from a set of classes. The file created is very large, so I have created JsonProperty\'s for the properties to re

Answer 1

Json support us to ignore property that don't want return. Example

class Foo
{
    public int Id { get; set; }
    public string Name { get; set; }
    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string AlternateName { get; set; }    
}

How to use it:

Foo foo = new Foo
{
    Id = 1,
    Name = "Thing 1",
    AlternateName = null,   
};

string json = JsonConvert.SerializeObject(foo);

Answer 2

Yes, this can be done using a custom ContractResolver.

You didn't show any code, so I'll just make up an example. Let's say I have a class Foo as shown below. I want the Id and Name properties in the serialization output, but I'm definitely not interested in the AlternateName and Color. I've marked those with [JsonIgnore]. I want the description to appear, but sometimes this can get really long, so I've used a custom JsonConverter to limit its length. I also want to use a shorter property name for the description, so I've marked it with [JsonProperty("Desc")].

class Foo
{
    public int Id { get; set; }
    public string Name { get; set; }
    [JsonIgnore]
    public string AlternateName { get; set; }
    [JsonProperty("Desc")]
    [JsonConverter(typeof(StringTruncatingConverter))]
    public string Description { get; set; }
    [JsonIgnore]
    public string Color { get; set; }
}

When I serialize an instance of the above...

Foo foo = new Foo
{
    Id = 1,
    Name = "Thing 1",
    AlternateName = "The First Thing",
    Description = "This is some lengthy text describing Thing 1 which you'll no doubt find very interesting and useful.",
    Color = "Yellow"
};

string json = JsonConvert.SerializeObject(foo, Formatting.Indented);

...I get this output:

{
  "Id": 1,
  "Name": "Thing 1",
  "Desc": "This is some lengthy text describing Thing 1 "
}

Now, let's say that I sometimes want to get the full JSON output, ignoring my customizations. I can use a custom ContractResolver to programmatically "unapply" the attributes from the class. Here's the code for the resolver:

class IgnoreJsonAttributesResolver : DefaultContractResolver
{
    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
    {
        IList<JsonProperty> props = base.CreateProperties(type, memberSerialization);
        foreach (var prop in props)
        {
            prop.Ignored = false;   // Ignore [JsonIgnore]
            prop.Converter = null;  // Ignore [JsonConverter]
            prop.PropertyName = prop.UnderlyingName;  // restore original property name
        }
        return props;
    }
}

To use the resolver, I add it to the JsonSerializerSettings and pass the settings to the serializer like this:

JsonSerializerSettings settings = new JsonSerializerSettings();
settings.ContractResolver = new IgnoreJsonAttributesResolver();
settings.Formatting = Formatting.Indented;

string json = JsonConvert.SerializeObject(foo, settings);

The output now includes the ignored properties, and the description is no longer truncated:

{
  "Id": 1,
  "Name": "Thing 1",
  "AlternateName": "The First Thing",
  "Description": "This is some lengthy text describing Thing 1 which you'll no doubt find very interesting and useful.",
  "Color": "Yellow"
}

Full demo here: https://dotnetfiddle.net/WZpeWt

Answer 3

If you are willing to use F# (or simply use an API not optimized for C#), the FSharp.JsonSkippable library contains a generic wrapper type that allows you to control in a simple and strongly typed manner whether to include a given property when serializing (and determine whether a property was included when deserializing), and moreover, to control/determine exclusion separately of nullability. (Full disclosure: I'm the author of the library.)