Several months ago, Microsoft decided to change up the HttpResponseMessage class. Before, you could simply pass a data type into the constructor, and then return the message
The easiest single-line solution is to use
return new HttpResponseMessage( HttpStatusCode.OK ) {Content = new StringContent( "Your message here" ) };
For serialized JSON content:
return new HttpResponseMessage( HttpStatusCode.OK ) {Content = new StringContent( SerializedString, System.Text.Encoding.UTF8, "application/json" ) };