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Reducing Serialized JSON Size


One of the common problems encountered when serializing .NET objects to JSON is that the JSON ends up containing a lot of unwanted properties and values. This can be especially significant when returning JSON to the client. More JSON means more bandwidth and a slower website.

To solve the issue of unwanted JSON, Json.NET has a range of built-in options to fine-tune what gets written from a serialized object.

JsonIgnoreAttribute and DataMemberAttribute

By default Json.NET will include all of a class's public properties and fields in the JSON it creates. Adding the JsonIgnoreAttribute to a property tells the serializer to always skip writing it to the JSON result.

Opt-out Serialization Example
public class Car
    // included in JSON
    public string Model { get; set; }
    public DateTime Year { get; set; }
    public List<string> Features { get; set; }

    // ignored
    public DateTime LastModified { get; set; }

If a class has many properties and you only want to serialize a small subset of them, then adding JsonIgnore to all the others will be tedious and error prone. The way to tackle this scenario is to add the DataContractAttribute to the class and DataMemberAttribute to the properties to serialize. This is opt-in serialization - only the properties you mark up will be serialized, unlike opt-out serialization using JsonIgnoreAttribute.

Opt-in Serialization Example
public class Computer
    // included in JSON
    public string Name { get; set; }

    public decimal SalePrice { get; set; }

    // ignored
    public string Manufacture { get; set; }
    public int StockCount { get; set; }
    public decimal WholeSalePrice { get; set; }
    public DateTime NextShipmentDate { get; set; }

JSON written by the serializer with an option of Formatting set to Indented produces nicely formatted, easy-to-read JSON that is great for readability when you are developing. Formatting.None on the other hand keeps the JSON result small, skipping all unnecessary spaces and line breaks to produce the most compact and efficient JSON possible.


NullValueHandling is an option on the JsonSerializer and controls how the serializer handles properties with a null value. By setting a value of NullValueHandling.Ignore the JsonSerializer skips writing any properties that have a value of null.

NullValueHandling Class
public class Movie
    public string Name { get; set; }
    public string Description { get; set; }
    public string Classification { get; set; }
    public string Studio { get; set; }
    public DateTime? ReleaseDate { get; set; }
    public List<string> ReleaseCountries { get; set; }
NullValueHandling Ignore Example
Movie movie = new Movie();
movie.Name = "Bad Boys III";
movie.Description = "It's no Bad Boys";

string included = JsonConvert.SerializeObject(movie,
    new JsonSerializerSettings { });

// {
//   "Name": "Bad Boys III",
//   "Description": "It's no Bad Boys",
//   "Classification": null,
//   "Studio": null,
//   "ReleaseDate": null,
//   "ReleaseCountries": null
// }

string ignored = JsonConvert.SerializeObject(movie,
    new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore });

// {
//   "Name": "Bad Boys III",
//   "Description": "It's no Bad Boys"
// }

NullValueHandling can also be customized on individual properties using the JsonPropertyAttribute. The JsonPropertyAttribute value of NullValueHandling will override the setting on the JsonSerializer for that property.


DefaultValueHandling is an option on the JsonSerializer and controls how the serializer handles properties with a default value. Setting a value of DefaultValueHandling.Ignore will make the JsonSerializer skip writing any properties that have a default value to the JSON result. For object references this will be null. For value types like int and DateTime the serializer will skip the default uninitialized value for that value type.

Json.NET also allows you to customize what the default value of an individual property is using the DefaultValueAttribute. For example, if a string property called Department always returns an empty string in its default state and you don't want that empty string in your JSON, then placing the DefaultValueAttribute on Department with that value will mean Department is no longer written to JSON unless it has a value.

DefaultValueHandling Class
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
    public int FollowUpDays { get; set; }

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

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

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

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

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

DefaultValueHandling can also be customized on individual properties using the JsonPropertyAttribute. The JsonPropertyAttribute value of DefaultValueHandling will override the setting on the JsonSerializer for that property.


For more flexibility, the IContractResolver provides an interface to customize almost every aspect of how a .NET object gets serialized to JSON, including changing serialization behavior at runtime.

IContractResolver Class
public class DynamicContractResolver : DefaultContractResolver
    private readonly char _startingWithChar;

    public DynamicContractResolver(char startingWithChar)
        _startingWithChar = startingWithChar;

    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
        IList<JsonProperty> properties = base.CreateProperties(type, memberSerialization);

        // only serializer properties that start with the specified character
        properties =
            properties.Where(p => p.PropertyName.StartsWith(_startingWithChar.ToString())).ToList();

        return properties;

public class Book
    public string BookName { get; set; }
    public decimal BookPrice { get; set; }
    public string AuthorName { get; set; }
    public int AuthorAge { get; set; }
    public string AuthorCountry { get; set; }
IContractResolver Example
Book book = new Book
    BookName = "The Gathering Storm",
    BookPrice = 16.19m,
    AuthorName = "Brandon Sanderson",
    AuthorAge = 34,
    AuthorCountry = "United States of America"

string startingWithA = JsonConvert.SerializeObject(book, Formatting.Indented,
    new JsonSerializerSettings { ContractResolver = new DynamicContractResolver('A') });

// {
//   "AuthorName": "Brandon Sanderson",
//   "AuthorAge": 34,
//   "AuthorCountry": "United States of America"
// }

string startingWithB = JsonConvert.SerializeObject(book, Formatting.Indented,
    new JsonSerializerSettings { ContractResolver = new DynamicContractResolver('B') });

// {
//   "BookName": "The Gathering Storm",
//   "BookPrice": 16.19
// }
See Also