I am using Json.NET Serialize the class to JSON.

I have classes like this:

class Test1
{
    [JsonProperty("id")]
    public string ID { get; set; }
    [JsonProperty("label")]
    public string Label { get; set; }
    [JsonProperty("url")]
    public string URL { get; set; }
    [JsonProperty("item")]
    public List<Test2> Test2List { get; set; }
}

I want to add the JsonIgnore() property to the Test2List property only if Test2List is null. If it's not null, then I want to include it in my json.

#1 building

An alternative solution to using the JsonProperty property:

[JsonProperty(NullValueHandling=NullValueHandling.Ignore)]
// or
[JsonProperty("property_name", NullValueHandling=NullValueHandling.Ignore)]

// or for all properties in a class
[JsonObject(ItemNullValueHandling = NullValueHandling.Ignore)]

Just as Online documents Seen.

#2 building

You can see it in this link on their website (http://james.newtonking.com/archive/2009/10/23/efficient-json-with-json-net-reducing-serialized-json-size.aspx). I support using [Default()] to specify the Default value

From link

   public class Invoice
{
  public string Company { get; set; }
  public decimal Amount { get; set; }

  // false is default value of bool
  public bool Paid { get; set; }
  // null is default value of nullable
  public DateTime? PaidDate { get; set; }

  // customize default values
  [DefaultValue(30)]
  public int FollowUpDays { get; set; }
  [DefaultValue("")]
  public string FollowUpEmailAddress { get; set; }
}


Invoice invoice = new Invoice
{
  Company = "Acme Ltd.",
  Amount = 50.0m,
  Paid = false,
  FollowUpDays = 30,
  FollowUpEmailAddress = string.Empty,
  PaidDate = null
};

string included = JsonConvert.SerializeObject(invoice,
  Formatting.Indented,
  new JsonSerializerSettings { });

// {
//   "Company": "Acme Ltd.",
//   "Amount": 50.0,
//   "Paid": false,
//   "PaidDate": null,
//   "FollowUpDays": 30,
//   "FollowUpEmailAddress": ""
// }

string ignored = JsonConvert.SerializeObject(invoice,
  Formatting.Indented,
  new JsonSerializerSettings { DefaultValueHandling = DefaultValueHandling.Ignore });

// {
//   "Company": "Acme Ltd.",
//   "Amount": 50.0
// }

#3 building

Similar to @ sirthomas' answer, JSON.NET also respects DataMemberAttribute The EmitDefaultValue property of :

[DataMember(Name="property_name", EmitDefaultValue=false)]

If you have used [DataContract] and [DataMember] in model types and do not want to add JSON.NET specific properties, you may need to do so.

#4 building

You can do this to ignore all empty values in the object you are serializing, and then no empty properties will appear in JSON

JsonSerializerSettings settings = new JsonSerializerSettings();
settings.NullValueHandling = NullValueHandling.Ignore;
var myJson = JsonConvert.SerializeObject(myObject, settings);

#5 building

Adapted to the / @ amit answer of @ mybrief, but suitable for people using VB

 Dim JSONOut As String = JsonConvert.SerializeObject(
           myContainerObject, 
           New JsonSerializerSettings With {
                 .NullValueHandling = NullValueHandling.Ignore
               }
  )

Please refer to: Object initializers: named and anonymous types (Visual Basic)

https://msdn.microsoft.com/en-us/library/bb385125.aspx