Web API控制器中的Action方法有如下幾種返回類型：
- void
- HttpResponseMessage
- IHttpActionResult
- 其它類型
| 返回類型 | Web API創建HTTP響應消息的機制 |
|---|---|
| void | 返回HTTP狀態碼204(無內容) |
| HttpResponseMessage | 直接轉換成HTTP響應消息 |
| IHttpActionResult | 調用接口的ExecuteAsync方法創建一個HttpResponseMessage對象，然后轉換成HTTP響應消息 |
| 其它類型 | 把序列化后的返回值寫入響應正文，並且返回HTTP狀態碼200(OK) |
下面詳細介紹幾種返回類型
void
如果返回類型是void， Web API 會返回一個HTTP狀態碼204(無內容)的空HTTP響應消息
示例代碼：
public class ReturnValueDemoController : ApiController { //返回類型為void public void Get() { } }
HTTP響應消息：
HTTP/1.1 204 No Content Cache-Control: no-cache Pragma: no-cache Expires: -1 Server: Microsoft-IIS/8.0 X-AspNet-Version: 4.0.30319 X-SourceFiles: =?UTF-8?B?RTpcV29ya1NwYWNlXERvdE5ldFxNdmNEZW1vXE12Y0RlbW8uV2ViVUlcYXBpXFJldHVyblZhbHVlRGVtbw==?= X-Powered-By: ASP.NET Date: Wed, 04 Mar 2015 14:34:09 GMT
HttpResponseMessage
如果返回類型是HttpResponseMessage ，Web API直接把返回值轉換成HTTP響應消息，HttpResponseMessage對象的屬性對應響應消息的屬性。你可以靈活的設置響應標頭和響應正文，這就給了你對響應消息的更大的控制權。如下例：
public class ReturnValueDemoController : ApiController { //返回類型為HttpResponseMessage public HttpResponseMessage Get() { HttpResponseMessage response = Request.CreateResponse(HttpStatusCode .OK, "hello"); response.Content = new StringContent ( "hello wep api", Encoding .Unicode); response.Headers.CacheControl = new CacheControlHeaderValue { MaxAge = TimeSpan .FromSeconds(600) }; return response; } }
響應：
HTTP/1.1 200 OK Cache-Control: max-age=600 Content-Type: text/plain; charset=utf-16 Vary: Accept-Encoding Server: Microsoft-IIS/8.0 X-AspNet-Version: 4.0.30319 X-SourceFiles: =?UTF-8?B?RTpcV29ya1NwYWNlXERvdE5ldFxNdmNEZW1vXE12Y0RlbW8uV2ViVUlcYXBpXFJldHVyblZhbHVlRGVtbw==?= X-Powered-By: ASP.NET Date: Wed, 04 Mar 2015 15:14:05 GMT Content-Length: 26 hello wep api
如果你給CreateResponse方法傳入了一個領域模型， Web API就會把序列化后的領域模型寫入響應正文。
示例：
public HttpResponseMessage Get() { IQueryable <Product > products = repository.GetProducts(); HttpResponseMessage response = Request.CreateResponse(HttpStatusCode .OK, products); return response; }
響應：
HTTP/1.1 200 OK Cache-Control: no-cache Pragma: no-cache Content-Type: application/json; charset=utf-8 Expires: -1 Server: Microsoft-IIS/8.0 X-AspNet-Version: 4.0.30319 X-SourceFiles: =?UTF-8?B?RTpcV29ya1NwYWNlXERvdE5ldFxNdmNEZW1vXE12Y0RlbW8uV2ViVUlcYXBpXFJldHVyblZhbHVlRGVtbw==?= X-Powered-By: ASP.NET Date: Wed, 04 Mar 2015 15:37:51 GMT Content-Length: 264 [{"ProductId":1,"Name":" 蘋果","Description":null,"Price":1.0,"Category":"水果"},{"ProductId":2,"Name":"鼠標","Description":null,"Price":50.0,"Category":"電腦配件"},{"ProductId":3,"Name":"洗發水","Description":null,"Price":20.0,"Category":"日用品"}]
IHttpActionResult
IHttpActionResult接口是在Web API 2才引入進來的，本質上，它定義了一個HttpResponseMessage工廠類。IHttpActionResult接口只定義了一個ExecuteAsync方法，該方法異步方式創建一個HttpResponseMessage實例。
public interface IHttpActionResult { Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken); }
如果控制器的Action返回類型是IHttpActionResult，Web API就會調用接口的ExecuteAsync方法創建一個HttpResponseMessage對象，然后轉換成HTTP響應消息。以下是一個IHttpActionResult接口的實現，它創建了一個普通文本響應消息。
public class PlainTextResult : IHttpActionResult { private string text; private HttpRequestMessage request; public PlainTextResult(string text, HttpRequestMessage request) { this .text = text; this .request = request; } public Task < HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken) { HttpResponseMessage response=new HttpResponseMessage { Content = new StringContent (text, Encoding.Unicode), RequestMessage = request }; return Task .FromResult(response); } }
控制器Action代碼：
//返回類型是IHttpActionResult public IHttpActionResult Get() { return new PlainTextResult( "plain text result" ,Request); }
響應：
HTTP/1.1 200 OK Cache-Control: no-cache Pragma: no-cache Content-Type: text/plain; charset=utf-16 Expires: -1 Vary: Accept-Encoding Server: Microsoft-IIS/8.0 X-AspNet-Version: 4.0.30319 X-SourceFiles: =?UTF-8?B?RTpcV29ya1NwYWNlXERvdE5ldFxNdmNEZW1vXE12Y0RlbW8uV2ViVUlcYXBpXFJldHVyblZhbHVlRGVtbw==?= X-Powered-By: ASP.NET Date: Wed, 04 Mar 2015 16:17:30 GMT Content-Length: 34 plain text result
通常情況下，你無須自己去實現IHttpActionResult接口， 在System.Web.Http.Results命名空間下已經包含了很多實現該接口的Action返回類型。ApiContoller類也定義了很多方法獲取這些內置的Action返回類型。
在下面的例子里，如果沒有找到某個產品，控制器調用ApiController.NotFound方法創建一個404 (Not Found)的響應消息。
HTTP/1.1 404 Not Found Cache-Control: no-cache Pragma: no-cache Expires: -1 Server: Microsoft-IIS/8.0 X-AspNet-Version: 4.0.30319 X-SourceFiles: =?UTF-8?B?RTpcV29ya1NwYWNlXERvdE5ldFxNdmNEZW1vXE12Y0RlbW8uV2ViVUlcYXBpXFJldHVyblZhbHVlRGVtb1w1?= X-Powered-By: ASP.NET Date: Wed, 04 Mar 2015 16:54:21 GMT Content-Length: 0
如果找到了，控制器調用ApiController.OK方法創建一個HTTP狀態碼200(OK)響應正文是產品信息的響應消息。
HTTP/1.1 200 OK Cache-Control: no-cache Pragma: no-cache Content-Type: application/json; charset=utf-8 Expires: -1 Server: Microsoft-IIS/8.0 X-AspNet-Version: 4.0.30319 X-SourceFiles: =?UTF-8?B?RTpcV29ya1NwYWNlXERvdE5ldFxNdmNEZW1vXE12Y0RlbW8uV2ViVUlcYXBpXFJldHVyblZhbHVlRGVtb1wx?= X-Powered-By: ASP.NET Date: Wed, 04 Mar 2015 16:53:16 GMT Content-Length: 82 {"ProductId":1,"Name":" 蘋果","Description":null,"Price":1.0,"Category":"水果"}
其它返回類型
對於其它的返回類型，Web API會對返回值序列化，然后把序列化后的返回值寫入到響應正文里，並且響應狀態碼是200(OK)。
public IQueryable < Product> GetpProducts() { return repository.GetProducts(); }
響應：
HTTP/1.1 200 OK Cache-Control: no-cache Pragma: no-cache Content-Type: application/json; charset=utf-8 Expires: -1 Server: Microsoft-IIS/8.0 X-AspNet-Version: 4.0.30319 X-SourceFiles: =?UTF-8?B?RTpcV29ya1NwYWNlXERvdE5ldFxNdmNEZW1vXE12Y0RlbW8uV2ViVUlcYXBpXFJldHVyblZhbHVlRGVtb1w=?= X-Powered-By: ASP.NET Date: Wed, 04 Mar 2015 17:03:13 GMT Content-Length: 264 [{"ProductId":1,"Name":" 蘋果","Description":null,"Price":1.0,"Category":"水果"},{"ProductId":2,"Name":"鼠標","Description":null,"Price":50.0,"Category":"電腦配件"},{"ProductId":3,"Name":"洗發水","Description":null,"Price":20.0,"Category":"日用品"}]
使用這種返回類型的一個缺點就是你不能直接返回一個錯誤代碼，例如404。不過，你可以通過拋出HttpResponseException異常來解決這個問題。
public Product GetProductById( int id) { Product product = repository.GetProductById(id); if (product==null ) throw new HttpResponseException ( HttpStatusCode.NotFound); return product; }