c# - The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8' -

for months following web api 2 (simplified example) responded correctly to

http://localhost/testapi2/api/tests this.  [{"id":1,"name":"naples"},{"id":2,"name":"paris"}]      //using newtonsoft.json;     namespace testapi2.controllers     {         public class personscontroller : apicontroller         {             [httpget]             [route("api/tests")]             public httpresponsemessage getall()             {                 //  dbcontext generated databasefirst ef generation                 using (var cleandbcontext = new cleandbentities())                 {                     var tests = cleandbcontext.testtables;                     // return                     return request.createresponse(httpstatuscode.ok, tests, configuration.formatters.jsonformatter);                 }             }         }     } 

then updated nugets on project same call responded with:

<exceptionmessage> 'objectcontent`1' type failed serialize response body content type 'application/json; charset=utf-8'. </exceptionmessage> <exceptiontype>system.invalidoperationexception</exceptiontype> <stacktrace/> 

apparently in nuget update, perhaps newtonsoft, formatter, cannot serialize list implied in "var list =". list ok because can foreach through examine contents.

after lot of agony, , experimentation, changed query explicitly generate list while keeping anonymous var target.

var tests = cleandbcontext.testtables.tolist(); 

modifying query explicitly generate list fixed issue. i'll send info newtonsoft. it's great product.