Using-JSONNET-for-dynamic-JSON-parsing

原文 https://weblog.west-wind.com/posts/2012/Aug/30/Using-JSONNET-for-dynamic-JSON-parsing

With the release of ASP.NET Web API as part of .NET 4.5 and MVC 4.0, JSON.NET has effectively pushed out the .NET native serializers to become the default serializer for Web API. JSON.NET is vastly more flexible than the built in DataContractJsonSerializer or the older JavaScript serializer. The DataContractSerializer in particular has been very problematic in the past because it can't deal with untyped objects for serialization - like values of type object, or anonymous types which are quite common these days. The JavaScript Serializer that came before it actually does support non-typed objects for serialization but it can't do anything with untyped data coming in from JavaScript and it's overall model of extensibility was pretty limited (JavaScript Serializer is what MVC uses for JSON responses).

JSON.NET provides a robust JSON serializer that has both high level and low level components, supports binary JSON, JSON contracts, Xml to JSON conversion, LINQ to JSON and many, many more features than either of the built in serializers. ASP.NET Web API now uses JSON.NET as its default serializer and is now pulled in as a NuGet dependency into Web API projects, which is great.

Dynamic JSON Parsing

One of the features that I think is getting ever more important is the ability to serialize and deserialize arbitrary JSON content dynamically - that is without mapping the JSON captured directly into a .NET type as DataContractSerializer or the JavaScript Serializers do. Sometimes it isn't possible to map types due to the differences in languages (think collections, dictionaries etc), and other times you simply don't have the structures in place or don't want to create them to actually import the data.

If this topic sounds familiar - you're right! I wrote about dynamic JSON parsing a few months back before JSON.NET was added to Web API and when Web API and the System.Net HttpClient libraries included the System.Json classes like JsonObject and JsonArray. With the inclusion of JSON.NET in Web API these classes are now obsolete and didn't ship with Web API or the client libraries. I re-linked my original post to this one. In this post I'll discus JToken, JObject and JArray which are the dynamic JSON objects that make it very easy to create and retrieve JSON content on the fly without underlying types.

Why Dynamic JSON?

So, why Dynamic JSON parsing rather than strongly typed parsing? Since applications are interacting more and more with third party services it becomes ever more important to have easy access to those services with easy JSON parsing. Sometimes it just makes lot of sense to pull just a small amount of data out of large JSON document received from a service, because the third party service isn't directly related to your application's logic most of the time - and it makes little sense to map the entire service structure in your application.

For example, recently I worked with the Google Maps Places API to return information about businesses close to me (or rather the app's) location. The Google API returns a ton of information that my application had no interest in - all I needed was few values out of the data. Dynamic JSON parsing makes it possible to map this data, without having to map the entire API to a C# data structure. Instead I could pull out the three or four values I needed from the API and directly store it on my business entities that needed to receive the data - no need to map the entire Maps API structure.

Getting JSON.NET

The easiest way to use JSON.NET is to grab it via NuGet and add it as a reference to your project. You can add it to your project with:

PM> Install-Package Newtonsoft.Json

From the Package Manager Console or by using Manage NuGet Packages in your project References. As mentioned if you're using ASP.NET Web API or MVC 4 JSON.NET will be automatically added to your project.

Alternately you can also go to the CodePlex site and download the latest version including source code:

http://json.codeplex.com/

Creating JSON on the fly with JObject and JArray

Let's start with creating some JSON on the fly. It's super easy to create a dynamic object structure with any of the JToken derived JSON.NET objects. The most common JToken derived classes you are likely to use are JObject and JArray.

JToken implements IDynamicMetaProvider and so uses the dynamic  keyword extensively to make it intuitive to create object structures and turn them into JSON via dynamic object syntax. Here's an example of creating a music album structure with child songs using JObject for the base object and songs and JArray for the actual collection of songs:

[TestMethod]
public void JObjectOutputTest()
{
    // strong typed instance
    var jsonObject = new JObject();

    // you can explicitly add values here using class interface
    jsonObject.Add("Entered", DateTime.Now);

    // or cast to dynamic to dynamically add/read properties    dynamic album = jsonObject;

    album.AlbumName = "Dirty Deeds Done Dirt Cheap";
    album.Artist = "AC/DC";
    album.YearReleased = 1976;

    album.Songs = new JArray() as dynamic;

    dynamic song = new JObject();
    song.SongName = "Dirty Deeds Done Dirt Cheap";
    song.SongLength = "4:11";
    album.Songs.Add(song);

    song = new JObject();
    song.SongName = "Love at First Feel";
    song.SongLength = "3:10";
    album.Songs.Add(song);

    Console.WriteLine(album.ToString());
}

This produces a complete JSON structure:

{
  "Entered": "2012-08-18T13:26:37.7137482-10:00",
  "AlbumName": "Dirty Deeds Done Dirt Cheap",
  "Artist": "AC/DC",
  "YearReleased": 1976,
  "Songs": [
    {
      "SongName": "Dirty Deeds Done Dirt Cheap",
      "SongLength": "4:11"
    },
    {
      "SongName": "Love at First Feel",
      "SongLength": "3:10"
    }
  ]
}

Notice that JSON.NET does a nice job formatting the JSON, so it's easy to read and paste into blog posts