REST in WCF: Varying response content type based on HTTP Request Headers

Damian's used the same WCF extensibility points I used to produce some boilerplate that varies the response content type from JSON to XML, based on the Accept or Content-Type header of the GET request.  He extends WebHttpBehavior to return an IDispatchMessageFormatter that does either JSON or XML.

 

class WebHttpBehavior2Ex : WebHttpBehavior { protected override IDispatchMessageFormatter GetReplyDispatchFormatter(OperationDescription operationDescription, ServiceEndpoint endpoint) { WebGetAttribute webGetAttribute = operationDescription.Behaviors.Find<WebGetAttribute>(); DynamicResponseTypeAttribute mapAcceptedContentTypeToResponseEncodingAttribute = operationDescription.Behaviors.Find<DynamicResponseTypeAttribute>(); if (webGetAttribute != null && mapAcceptedContentTypeToResponseEncodingAttribute != null) { // We need two formatters, since we don't know what type we will need until runtime webGetAttribute.ResponseFormat = WebMessageFormat.Json; IDispatchMessageFormatter jsonDispatchMessageFormatter = base.GetReplyDispatchFormatter(operationDescription, endpoint); webGetAttribute.ResponseFormat = WebMessageFormat.Xml; IDispatchMessageFormatter xmlDispatchMessageFormatter = base.GetReplyDispatchFormatter(operationDescription, endpoint); return new DynamicFormatter() { jsonDispatchMessageFormatter = jsonDispatchMessageFormatter, xmlDispatchMessageFormatter = xmlDispatchMessageFormatter }; } return base.GetReplyDispatchFormatter(operationDescription, endpoint); } }

 

And then in the DynamicFormatter code, he just picks the formatter as appropriate:

 

class DynamicFormatter : IDispatchMessageFormatter { public IDispatchMessageFormatter jsonDispatchMessageFormatter { get; set; } public IDispatchMessageFormatter xmlDispatchMessageFormatter { get; set; } public void DeserializeRequest(System.ServiceModel.Channels.Message message, object[] parameters) { throw new NotImplementedException(); } public System.ServiceModel.Channels.Message SerializeReply(MessageVersion messageVersion, object[] parameters, object result) { Message request = OperationContext.Current.RequestContext.RequestMessage; // This code is based on ContentTypeBasedDispatch example in WCF REST Starter Kit Samples // It calls either HttpRequestMessageProperty prop = (HttpRequestMessageProperty)request.Properties[HttpRequestMessageProperty.Name]; string accepts = prop.Headers[HttpRequestHeader.Accept]; if (accepts != null) { if (accepts.Contains("text/xml") || accepts.Contains("application/xml")) { return xmlDispatchMessageFormatter.SerializeReply(messageVersion, parameters, result); } else if (accepts.Contains("application/json")) { return jsonDispatchMessageFormatter.SerializeReply(messageVersion, parameters, result); } } else { string contentType = prop.Headers[HttpRequestHeader.ContentType]; if (contentType != null) { if (contentType.Contains("text/xml") || contentType.Contains("application/xml")) { return xmlDispatchMessageFormatter.SerializeReply(messageVersion, parameters, result); } else if (contentType.Contains("application/json")) { return jsonDispatchMessageFormatter.SerializeReply(messageVersion, parameters, result); } } } return xmlDispatchMessageFormatter.SerializeReply(messageVersion, parameters, result); } }

 

It's pretty nifty and it requires no changes to the app logic.  You need to use a custom ServiceHost and use a custom attribute on each Operation.  Because it uses the HTTP headers and not the URI itself to determine content-type of the response, I think it has some nice benefits over the approach I described in April.

Damian's got a full VS2008 solution will all the boilerplate code.

check it out.