Set default response type in WCF Web Api

Question

I've a set of services hosted with WCF Web Api and I communicate with them in JSON from javascript. In most cases I'm okay modifying the accepts bit of the header to require a JSON response but there are some cases arising where I can't do this. This is due the the javascript framework that I'm using (Ext JS). For some things it only lets me specify a URL and not the proxy defaults such as headers.

This isn't an Ext JS question however. Web Api seems to default to returning XML, and I'd like to know whether it's possible to change this default so that it can return JSON instead. Thanks in advance!

What is the default Accept header value set to when you can't change it? — hskan, Jul 22 '11 at 08:11

score 4 · Answer 1 · answered Jan 25 '12 at 11:40

A bit of experimentation seems to indicate that the order of the configured formatters matter (which is quite intuitive).

By default, when you create an instance of HttpConfiguration, its Formatters collection contains these formatters:

XmlMediaTypeFormatter
JsonValueMediaTypeFormatter
JsonMediaTypeFormatter
FormUrlEncodedMediaTypeFormatter

The reason why XML is the default formatting is because it's the first formatter. To make JSON the default value, you can reorder the collection to look like this:

JsonValueMediaTypeFormatter
JsonMediaTypeFormatter
XmlMediaTypeFormatter
FormUrlEncodedMediaTypeFormatter

Given an instance config of HttpConfiguration, here's one way to reorder the collection:

var jsonIndex = Math.Max(
    config.Formatters.IndexOf(config.Formatters.JsonFormatter),
    config.Formatters.IndexOf(config.Formatters.JsonValueFormatter));
var xmlIndex = config.Formatters.IndexOf(
    config.Formatters.XmlFormatter);

config.Formatters.Insert(jsonIndex + 1, config.Formatters.XmlFormatter);
config.Formatters.RemoveAt(xmlIndex);

Whether or not this is supported I don't know, but it seems to work on WebApi 0.6.0.

score 3 · Answer 2 · answered Apr 17 '12 at 19:08

I actually found a simple way of dealing with this. First make sure that the default JSON formatter is first. And then set its type to text/html. This will insure that the browser gets JSON even if it does not set the header. Nice aspect of the below is that you never have to remember to set the accept header in client code. It just works and always default to JSON.

var jsonformatter = config.Formatters.Where(t => t.GetType() == typeof(JsonMediaTypeFormatter)).FirstOrDefault());
config.Formatters.Remove(jsonformatter );
config.Formatters.Insert(0, jsonformatter); 
config.Formatters[0].SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/html"));

score 2 · Answer 3 · answered Jul 26 '11 at 20:13

You could use the a delegating channel as described here http://blog.alexonasp.net/post/2011/07/26/Look-Ma-I-can-handle-JSONP-(aka-Cross-Domain-JSON)-with-WCF-Web-API-and-jQuery!.aspx which maps URIs like http://myserver/myresource/1/json to http://myserver/myresource/1 and sets accept header to application/json.

The delegating channel is part of the ContactManager_Advanced sample when you're downloading WCF Web API from http://wcf.codeplex.com.

It is contained in the UriFormatExtensionMessageChannel.cs file.

Look at the global.asax.cs of the sample on how to get it running.

A more concise example of an delegating channel and the solution of the above asked question is provided by Glenn Block himself: [http://codepaste.net/wehh49](http://codepaste.net/wehh49) — Joachim Rosskopf, Aug 09 '11 at 05:04

score 1 · Answer 4 · answered Jul 22 '11 at 08:42

According to the code the WCF Web API will always default to the XmlFormatter if it is in the collection of usable formatters. If it isn't the JsonFormatter is used instead if this is present. There is also a DefaultFormatter property but that is internal so you can't set that. Maybe a useful feature request to add?

Set default response type in WCF Web Api

4 Answers4