这两种方式都是以HttpResponseMessage的形式返回，
方式一：以字符串的形式
var content = new StringContent("{\"FileName\": \"" + fileName + "\"}");
HttpResponseMessage response = new HttpResponseMessage()
{
Content = content
};
response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
方式二：以对象（这里用的是字典）的方式
var fileNames = new Dictionary<string, string>();
fileNames.Add("FileName", fileName);
var content = new ObjectContent<Dictionary<string, string>>(fileNames, new JsonMediaTypeFormatter(), "application/json");
HttpResponseMessage response = Request.CreateResponse(HttpStatusCode.OK, content);
原文：http://www.cnblogs.com/JasonBie/p/4186770.html