How do I serialize a C# anonymous type to a JSON string?

I'm attempting to use the following code to serialize an anonymous type to JSON:

var serializer = new DataContractJsonSerializer(thing.GetType());
var ms = new MemoryStream();
serializer.WriteObject(ms, thing);
var json = Encoding.Default.GetString(ms.ToArray()); 

However, I get the following exception when this is executed:

> Type > '<>f__AnonymousType1`3[System.Int32,System.Int32,System.Object[]]' > cannot be serialized. Consider marking > it with the DataContractAttribute > attribute, and marking all of its > members you want serialized with the > DataMemberAttribute attribute. See > the Microsoft .NET Framework > documentation for other supported > types.

I can't apply attributes to an anonymous type (as far as I know). Is there another way to do this serialization or am I missing something?

Try the JavaScriptSerializer instead of the DataContractJsonSerializer

JavaScriptSerializer serializer = new JavaScriptSerializer();
var output = serializer.Serialize(your_anon_object);

As others have mentioned, Newtonsoft JSON.NET is a good option. Here is a specific example for simple JSON serialization:

return JsonConvert.SerializeObject(
    new
    {
       DataElement1,
       SomethingElse
    });

I have found it to be a very flexible, versatile library.

You can try my ServiceStack JsonSerializer it's the fastest .NET JSON serializer at the moment. It supports serializing DataContract's, Any POCO Type, Interfaces, Late-bound objects including anonymous types, etc.

Basic Example

var customer = new Customer { Name="Joe Bloggs", Age=31 };
var json = customer.ToJson();
var fromJson = json.FromJson<Customer>(); 

Note: Only use Microsofts JavaScriptSerializer if performance is not important to you as I've had to leave it out of my benchmarks since its up to 40x-100x slower than the other JSON serializers.

Please note this is from 2008. Today I would argue that the serializer should be built in and that you can probably use swagger + attributes to inform consumers about your endpoint and return data.

---

Iwould argue that you shouldn't be serializing an anonymous type. I know the temptation here; you want to quickly generate some throw-away types that are just going to be used in a loosely type environment aka Javascript in the browser. Still, I would create an actual type and decorate it as Serializable. Then you can strongly type your web methods. While this doesn't matter one iota for Javascript, it does add some self-documentation to the method. Any reasonably experienced programmer will be able to look at the function signature and say, "Oh, this is type Foo! I know how that should look in JSON."

Having said that, you might try JSON.Net to do the serialization. I have no idea if it will work

The fastest way I found was this:

var obj = new {Id = thing.Id, Name = thing.Name, Age = 30};
JavaScriptSerializer serializer = new JavaScriptSerializer();
string json = serializer.Serialize(obj);

Namespace: System.Web.Script.Serialization.JavaScriptSerializer

For those checking this around the year 2020:

Microsoft's System.Text.Json namespace is the new king in town. In terms of performance, it is the best as far as I can tell:

var model = new Model
{
    Name = "Test Name",
    Age = 5
};

string json = JsonSerializer.Serialize(model);

As some others have mentioned, NewtonSoft.Json is a very nice library as well.

You could use Newtonsoft.Json.

var warningJSON = JsonConvert.SerializeObject(new {
               warningMessage = "You have been warned..."
            });

A faster alternative with Microsofts' new library on System.Text.Json

var warningJSON = JsonSerializer.Serialize(new {
               warningMessage = "You have been warned..."
            });

Assuming you are using this for a web service, you can just apply the following attribute to the class:

[System.Web.Script.Services.ScriptService]

Then the following attribute to each method that should return Json:

[ScriptMethod(ResponseFormat = ResponseFormat.Json)]

And set the return type for the methods to be "object"

public static class JsonSerializer
{
    public static string Serialize<T>(this T data)
    {
        try
        {
            DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(T));
            var stream = new MemoryStream();
            serializer.WriteObject(stream, data);
            string jsonData = Encoding.UTF8.GetString(stream.ToArray(), 0, (int)stream.Length);
            stream.Close();
            return jsonData;
        }
        catch
        {
            return "";
        }
    }
    public static T Deserialize<T>(this string jsonData)
    {
        try
        {
            DataContractJsonSerializer slzr = new DataContractJsonSerializer(typeof(T));
            var stream = new MemoryStream(Encoding.UTF8.GetBytes(jsonData));
            T data = (T)slzr.ReadObject(stream);
            stream.Close();
            return data;
        }
        catch
        {
            return default(T);
        }
    }
}