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

JC_Grubbs_4541 0
Viewed 137502

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?

json-serialization datacontractjsonserializer anonymous-types json c#
9 Answers

Try the JavaScriptSerializer instead of the DataContractJsonSerializer

JavaScriptSerializer serializer = new JavaScriptSerializer();
var output = serializer.Serialize(your_anon_object);
user_170

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

Jason_Jackson_131030

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..."
            });
user_21156900

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.

user_86062890

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"

Paul_413010
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