JSON serialization – DataContractJsonSerializer vs JavaScriptSerializer

In C#, when you have to work with JSON data you’re usually represented with two choices – DataContractJsonSerializer or JavaScriptSerializer. There are other popular third-party libraries out there, such as the popular Json.Net project, but for the rest of this blog let’s just focus on the two built-in JSON serializers and see how they differ.

DataContracts

Without doubt the biggest difference between the two is that DataContractJsonSerializer only works with types decorated with the DataContract attribute, but JavaScriptSerializer can work with any object.

This makes perfect sense as the primary purpose of the DataContractJsonSerializer is to be used with WCF, and just as its name suggests, it’s a JSON serializer for DataContract types.

Anonymous Types

One interesting side effect of the above is that you can’t use DataContractJsonSerializer with anonymous types.

On the other hand, with the JavaScriptSerializer at least you can easily serialize an instance of an anonymous type:

   1: var jsSerializer = new JavaScriptSerializer();

   2:

   3: // define an anonymous type

   4: var anonymous = new { Id = Guid.NewGuid(), Name = "Yan", Age = 29 };

   5:

   6: // this writes:

   7: // {"Id":"2edebefb-2585-438c-bbc3-939e7688f630","Name":"Yan","Age":29}

   8: Console.WriteLine(jsSerializer.Serialize(anonymous));

Unfortunately you won’t be able to deserialize the JSON string back into an anonymous type as all the available deserialize methods requires a type which has a parameterless constructor.

Dictionaries

Another main functional difference between these two serializers is how they deal with dictionaries.

Take this simple dictionary for instance:

   1: var dictionary = new Dictionary<string, object>();

   2: dictionary.Add("Id", Guid.NewGuid());

   3: dictionary.Add("Name", "Yan");

   4: dictionary.Add("Age", 29);

The DataContractJsonSerializer serializes dictionaries as an array of KeyValuePair objects:

[{“Key”:”Id”,”Value”:”35285943-32d0-45d9-ada6-eb570f757d85″},{“Key”:”Name”,”Value”:”Yan”},{“Key”:”Age”,”Value”:29}]

whereas the JavaScriptSerializer serializes dictionaries in a much more ‘JSON’ way:

{“Id”:”35285943-32d0-45d9-ada6-eb570f757d85″,”Name”:”Yan”,”Age”:29}

This subtle different might seem trivial at first, but it can make a huge difference when you’re dealing with data that are originated from a non-.Net language. A perfect example came couple of days ago when we were testing out the Facebook graph API and one of the JSON responses we came across was a dictionary whose keys were numeric IDs like this:

{“1652438520054”:{“id”:”1652438520054″, …}}

In cases like this, you simply won’t be able to deserialize this JSON using the DataContractJsonSerializer!

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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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