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

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.