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A simple, lightweight, fast, easy to use, flexible and portable JSON library for .NET
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PetaJson is a simple but flexible JSON library implemented in a single C# file. Features include:

  • Standard JSON parsing and generation
  • Supports strongly typed serialization through reflection or custom code
  • Supports weakly typed serialization
  • Supports standard C# collection classes - no JSON specific classes (ie: no "JArray", "JObject" etc...)
  • Support for dynamic Expando (read) and anonymous types (write)
  • Choose from good performance + portable (System.Reflection), or high performance + less portable (using System.Reflection.Emit)
  • Custom formatting and parsing of any type
  • Support for serialization of polymorphic types
  • Utilities for cloning and reparsing objects into different types
  • Bonus IDictionary<string,object> extensions simplifies working with weakly typed JSON data.
  • Directly reads from TextReader and writes to TextWriter and any underlying stream
  • Simple set of custom attributes to control serialization
  • Optional non-strict parsing allows comments, non-quoted dictionary keys, trailing commas and hex literals (great for config files)
  • Optional pretty formatting
  • No dependencies, one file - PetaJson.cs
  • Works on .NET, Mono, Xamarin.Android, Xamarin.iOS.


Here goes, a 5 minute whirl-wind tour of using PetaJson...


  1. Add PetaJson.cs to your project
  2. That's it

Well almost, you'll probably want some using PetaJson; clauses and depending on your target platform you might need to define the following symbols in your project to disable some functionality:

  • PETAJSON_NO_DYNAMIC - disable support for System.Dynamic.ExpandoObject
  • PETAJSON_NO_EMIT - disable use of System.Reflection.Emit (slower, more portable)
  • PETAJSON_NO_DATACONTRACT - disable support for System.Runtime.Serialization.DataContract/Member

Generating JSON

To a string:

var o = new int[] { 1, 2, 3 };
var json = Json.Format(o);

or, write to a file

Json.WriteFile("MyData.json", o);

using objects

class Person
	string Name;
	string Address;

var p = new Person() { Name = "Joe Sixpack", Address = "Home" };
var json = Json.Format(p);

would yield:

	"name": "Joe Sixpack",
	"address": "Home"

Parsing JSON

From a string:

int o = Json.Parse<int>("23");

From string to a dynamic:

dynamic o = Json.Parse<object>("{\"apples\":\"red\", \"bananas\":\"yellow\" }");
string appleColor = o.apples;
string bananaColor = o.bananas;

Weakly typed dictionary:

var dict = Json.Parse<Dictionary<string, object>>("{\"apples\":\"red\", \"bananas\":\"yellow\" }");

Or an array:

int[] array = Json.Parse<int[]>("[1,2,3]");

Strongly typed object:

Person person = Json.Parse<Person>(jsonFromPersonExampleAbove);

From a file:

var person = Json.ParseFile<Person>("aboutme.json");

String into an existing instance:

Json.ParseInto<Person>(jsonFromPersonExampleAbove, person);

From file into an existing instance:

var person = new Person();
Json.ParseFileInto<Person>("aboutme.json", person);


PetaJson provides two attributes for decorating objects for serialization - [Json] and [JsonExclude].

The [Json] attribute when applied to a class or struct marks all public properties and fields for serialization:

class Person
	public string Name;				// Serialized as "name"
	public string Address;			// Serialized as "address"
	public string AlsoSerialized;	// Serialized as "alsoSerialized"
	private string NotSerialized;

When applied to one or more field or properties but not applied to the class itself, only the decorated members will be serialized:

class Person
	[Json] public string Name;	// Serialized as "name":
	public string Address;		// Not serialized

By default members are serialized using the name of the field or property with the first letter lowercased. To override the serialized name, include the name as a parameter to the [Json] attribute:

class Person
	[Json("PersonName")] public string Name; 	// Serialized as "PersonName"

Use the [JsonExclude] attribute to exclude public fields or properties from serialization

class Person
	public string Name;		// Serialized as "name"
	public string Address;	// Serialized as "address"

	[JsonExclude]			// Not serialized
	public int Age
		get { return calculateAge(); }

Sometimes you'll want sub-objects to be serialized into an existing object instance.


class MyApp
	public MyApp()
		// Settings object has an owner reference that needs to be maintained
		// across serialization
		CurrentSettings = new Settings(this);

	Settings CurrentSettings;

In this example the existing CurrentSettings object will be serialized into. If KeepInstance was set to false, PetaJson would instantiate a new Settings object, load it and then assign it to the CurrentSettings property.

DataContract and DataMember attributes

You can also use the system supplied DataContract and DataMember attributes. They'll only be used if there are no PetaJson attributes on the class or it's members. You must specify DataContract on the type and DataMember on all members that require serialization.

class Person
	[DataMember] public string Name;		// Serialized as "Name"
	[DataMember] public string Address;		// Serialized as "Address"
	public string Hot;						// Serialized as "Cool"
	public int Age {...}					// Not serialized

Note that the first letter of the member is left as upper case (unlike when using the Json attributes) and there's no need for an exclude attribute as only members marked DataMember are included in the first place.

Custom Formatting

Custom formatting can be used on any type. Say we have the following type:

struct Point
    public int X;
    public int Y;

and we want to serialize points as a comma separated string like this:

	"TopLeft": "10,20",
	"BottomRight: "30,40",

To do this, we can to register a formatter:

// Register custom formatter
Json.RegisterFormatter<Point>( (writer,point) => 
    writer.WriteStringLiteral(string.Format("{0},{1}", point.X, point.Y));

And a custom parser:

Json.RegisterParser<Point>( literal => {

    var parts = ((string)literal).Split(',');
    if (parts.Length!=2)
        throw new InvalidDataException("Badly formatted point");

    return new Point()
        X = int.Parse(parts[0], CultureInfo.InvariantCulture),
        Y = int.Parse(parts[0], CultureInfo.InvariantCulture),


We can now format and parse Points:

// Format a Point
var json = Json.Format(new Point() { X= 10, Y=20 });		// "10,20"

// Parse a Point
var point = Json.Parse<Point>("\"10,20\"");

Note that in this example we're formatting to a single string literal. We can do more complex custom serialization using the IJsonReader and IJsonWriter interfaces - see below.

Simple Formatting for Value Types

The problem with the above method is that it requires pre-registering the formatters and parsers which can be a pain. Another way to do custom formatting for value types is by implementing methods directly on the struct. This approach is more intrusive but also more self-contained.

The methods must have the following method names and signatures and can be public or not.

// One of these:
void FormatJson(IJsonWriter w);
string FormatJson();

// And one of these
static T ParseJson(IJsonReader r);
static T ParseJson(string literal);

For example, this is the equivalent of the above example:

struct Point
    public int X;
    public int Y;

	string FormatJson() 
		return string.Format("{0},{1}", X, Y);

	static Point ParseJson(string literal)
		var parts = literal.Split(',');
		if (parts.Length!=2)
			throw new InvalidDataException("Badly formatted point");

		return new Point()
			X = int.Parse(parts[0], CultureInfo.InvariantCulture),
			Y = int.Parse(parts[0], CultureInfo.InvariantCulture),

Note: this approach only works for structs (not classes)

Custom Factories and Polymorphic Types

Suppose we have a class heirarchy something like this:

abstract class Shape
	// Omitted

class Rectangle : Shape
	// Omitted

class Ellipse : Shape
	// Omitted

and we'd like to serialize a list of Shapes to JSON like this:

	{ "kind": "Rectangle", /* other rectangle properties omitted */ },
	{ "kind": "Shape", /* other ellipse properties omitted */ },
	// etc...

In other words a value in the JSON dictionary determines the type of object that needs to be instantiated for that element.

We can write out the shape kind by implementing the IJsonWriting interface which gets called before the other properties of the object are written:

abstract class Shape : IJsonWriting
    // Override OnJsonWriting to write out the derived class type
    void IJsonWriting.OnJsonWriting(IJsonWriter w)

For parsing, we need to register a callback function that creates the correct instances:

// Register a type factory that can instantiate Shape objects
Json.RegisterTypeFactory(typeof(Shape), (reader, key) =>
    // This method will be called back for each key in the json dictionary
    // until an object instance is returned

    // We wrote the object type using the key "kind", look for it
    if (key != "kind")
        return null;

    // Read the next literal and instantiate the correct object type
    return reader.ReadLiteral(literal =>
        switch ((string)literal)
            case "Rectangle": return new Rectangle();
            case "Ellipse": return new Ellipse();
                throw new InvalidDataException(string.Format("Unknown shape kind: '{0}'", literal));

When attempting to deserialize Shape objects, the registered callback will be called with each key in the dictionary until it returns an object instance. In this case we're looking for a key named "kind" and we use it's value to create either a Rectangle or Ellipse.

Note that the field used to hold the type (ie: "kind") does not need to be the first field in the in the dictionary being parsed. After instantiating the object, the input stream is re-wound to the start of the dictionary and then re-parsed into the instantiated object. Note too that the underlying stream doesn't need to support seeking - the rewind mechanism is implemented in PetaJson.

Serialization Events

An object can receive notifications of various events during the serialization/deserialization process by implementing one or more of the following interfaces:

// Called before loading via reflection
public interface IJsonLoading
    void OnJsonLoading(IJsonReader r);

// Called after loading via reflection
public interface IJsonLoaded
    void OnJsonLoaded(IJsonReader r);

// Called for each field while loading from reflection
// Return true if handled
public interface IJsonLoadField
    bool OnJsonField(IJsonReader r, string key);

// Called when about to write using reflection
public interface IJsonWriting
    void OnJsonWriting(IJsonWriter w);

// Called after written using reflection
public interface IJsonWritten
    void OnJsonWritten(IJsonWriter w);

For example, it's often necessary to wire up ownership references on loaded sub-objects:

class Drawing : IJsonLoaded
	public List<Shape> Shapes;

	void IJsonLoaded.OnJsonLoaded()
		// Shapes have been loaded, set owner references
		Shapes.ForEach(x => x.Owner = this);

The IJsonLoadField interface can be used to "fix up" incorrect incoming JSON data. For example, imagine a situation where a numeric ID field was incorrectly provided by a server as a string (enclosed in quotes) instead of a plain number.

class MyRecord : IJsonLoadField
	[Json] long id;				// Note: numeric (not string) field
	[Json] string description;

	// Override OnJsonField to intercept the bad server data
	bool IJsonLoadField.OnJsonField(IJsonReader r, string key)
		// id provided as string? Eg: "id": "1234"
		if (key=="id" && r.GetLiteralKind()==LiteralKind.String)
			// Parse the string
			id = long.Parse(r.GetLiteralString());

			// Skip the string literal now that we've handled it

			// Return true to suppress default processing 
			return true;

		// Other keys and non-quoted id field values processed as normal
		return false;			

Note: although these event methods could have been implemented using reflection rather than interfaces, the use of interfaces is more discoverable through Intellisense/Autocomplete.

Cloning and Re-parsing Objects

PetaJson includes a couple of helper functions for cloning objects by saving to them to JSON and then reloading:

var person1 = new Person() { Name = "Mr Json Bourne"; }
var person2 = Json.Clone(person1);

You can also clone into an existing instance

var person3 = new Person();
Json.CloneInto(person3, person1);		// Copies from person1 to person3

Similar to cloning is re-parsing. While cloning copies from one object to another of the same type, reparsing allows converting from one object type to another. For example you can convert a dictionary of values into a person:

IDictionary<string,object> dictionary = getDictionaryFromSomewhere();
var person = Json.Reparse<Person>(dictionary);

You can also go the other way:

var dictionary = Json.Reparse<IDictionary<string,object>>(person);

Bonus Dictionary Helpers

PetaJson includes some super handy extensions to IDictionary<string,object> that make working with weakly typed JSON data easier. Some of these methods are particularly handy when an app is using JSON to store configuration options or settings.

Suppose we have the following JSON:


and we parse all this into a weakly typed dictionary:

var data = Json.ParseFile<IDictionary<string,object>>("settings.json");

We can get a setting like this:

bool firstRun = data.GetPath<bool>("settings.appSettings.firstRun", true);

Or set it like this:

data.SetPath("settings.appSettings.firstRun", false);

SetPath creates the path using a set of Dictionary<string,object> if necessary:

var data = new Dictionary<String, object>();
data.SetPath("settings.appSettings.serverUrl", "");

GetPath can reparse if necessary to satify the requested type:

var userSettings = data.GetPath<UserSettings>("settings.userSettings", null);

You can check if a path exists like this:

if (data.PathExists("settings.appSettings"))
	// yep

And finally, there's T GetObjectAtPath<T>(string path) which does a few things:

  1. Makes sure the path exists, and creates it if it doesn't
  2. If the path does exist, reparses whatever it finds there into type T.
  3. If the path doesn't exists, creates a new T
  4. Stores the T instance back into the dictionary at that path.

So now we can work with parts of a weakly typed JSON dictionary with strong types.


var userSettings = data.GetObjectAtPath<UserSettings>("settings.userSettings");

and saving data, will get the changes:

// Make a change = "";

// It sticks...
var json = Json.Format(data);

Note: GetObjectAtPath only works with reference types, not structs.


PetaJson has a couple of formatting/parsing options. These can be set as global defaults:

Json.WriteWhitespaceDefault = true;		// Pretty formatting
Json.StrictParserDefault = true;		// Enable strict parsing

or, provided on a case by case basis:

Json.Format(person, JsonOption.DontWriteWhitespace);		// Force pretty formatting off
Json.Format(person, JsonOption.WriteWhitespace);			// Force pretty formatting on
Json.Parse<object>(jsonData, JsonOption.StrictParser);		// Force strict parsing
Json.Parse<object>(jsonData, JsonOption.NonStrictParser);	// Disable strict parsing

Non-strict mode relaxes the parser to allow:

  • Inline C /* */ or C++ // style comments
  • Trailing commas in arrays and dictionaries
  • Non-quoted dictionary keys
  • Hex number literals

eg: the non-strict parser will allow this:

	/* This is a C-style comment */
	"quotedKey": "allowed",
	nonQuotedKey: "also allowed",
	"arrayWithTrailingComma": [1,2,3,],	
	"hexAllowed": 0x1234,
	"trailing commas": "allowed ->",	// <- see the comma, not normally allowed

IJsonReader and IJsonWriter

These interfaces only need to be used when writing custom formatters and parsers. They are the low level interfaces used to read and write the JSON stream and are passed to the callbacks for custom parsers and formatters.


The IJsonReader interface reads from the JSON input stream.

public interface IJsonReader
    object ReadLiteral(Func<object, object> converter);
    void ReadDictionary(Action<string> callback);
    void ReadArray(Action callback);
    object Parse(Type type);
    T Parse<T>();
    LiteralKind GetLiteralKind();
    string GetLiteralString();
    void NextToken();

ReadLiteral - reads a single literal value from the input stream. Throws an exception if the next token isn't a literal value. You should provide a callback that converts the raw literal to the required value, which will then be returned as the return value from ReadLiteral.

Wherever possible, conversion should be done in the callback to ensure that errors in the conversion report the error location just before the bad literal, instead of after it.

ReadDictionary - reads a JSON dictionary, calling the callback for each key encountered. The callback routine should read the key's value using the IJsonReader interface. If nothing is read by the callback, PetaJson will skip the value and move onto the next key.

ReadArray - reads a JSON array, calling the callback at each element position. The callback routine must read each value from the IJsonReader before returning.

Parse - parses a typed value from the input stream.

GetLiteralKind, GetLiteralString and NextToken provide ability to read literals without boxing the value into an Object. Used by the "Reflection.Emit" type parsers, these are much faster than ReadLiteral, but less convenient to use.


The IJsonWriter interface writes to the JSON output stream:

public interface IJsonWriter
    void WriteStringLiteral(string str);
    void WriteRaw(string str);
    void WriteArray(Action callback);
    void WriteDictionary(Action callback);
    void WriteValue(object value);
    void WriteElement();
    void WriteKey(string key);

WriteStringLiteral - writes a string literal to the output stream, including the surrounding quotes and escaping the content as required.

WriteRaw - writes directly to the output stream. Use for comments, or self generated JSON data.

WriteArray - writes an array to the output stream. The callback should write each element.

WriteDictionary - writes a dictionary to the output stream. The callback should write each element.

WriteValue - formats and writes any object value.

WriteElement - call from the callback of WriteArray to indicate that the next element is about to be written. Causes PetaJson to write separating commas and whitespace.

WriteKey - call from the callback of WriteDictionary to write the key part of the next element. Writes whitespace, separating commas, the key and it's quotes, the colon.

eg: to write a dictionary:

writer.WriteDictionary(() =>

eg: to write an array:

	for (int i=0; i<10; i++)


Wondering about performance? When Reflection.Emit is enabled, PetaJson is right up there with the best of them. Some simple benchmarks serializing a long list of objects with a mix of different primitive types yielded this: (smaller tick value = quicker, better)

PetaJson     format: 491865 ticks
Json.NET     format: 757618 ticks x1.54
ServiceStack format: 615091 ticks x1.25

PetaJson      parse: 1011818 ticks
Json.NET      parse: 1204574 ticks x1.19
ServiceStack  parse: 1177895 ticks x1.16

Although this test shows PetaJson to be quicker, different data types may yield different results. In otherwords: I tested enough to make sure it wasn't ridiculously slow, but haven't done extensive benchmarks.


Copyright (C) 2014 Topten Software ( All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this product except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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