Web API控制器中的Action方法有如下几种返回类型：
- void
- HttpResponseMessage
- IHttpActionResult
- 其它类型
| 返回类型 | Web API创建HTTP响应消息的机制 |
|---|---|
| void | 返回HTTP状态码204(无内容) |
| HttpResponseMessage | 直接转换成HTTP响应消息 |
| IHttpActionResult | 调用接口的ExecuteAsync方法创建一个HttpResponseMessage对象，然后转换成HTTP响应消息 |
| 其它类型 | 把序列化后的返回值写入响应正文，并且返回HTTP状态码200(OK) |
下面详细介绍几种返回类型
void
如果返回类型是void， Web API 会返回一个HTTP状态码204(无内容)的空HTTP响应消息
示例代码：
public class ReturnValueDemoController : ApiController { //返回类型为void public void Get() { } }
HTTP响应消息：
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
如果返回类型是HttpResponseMessage ，Web API直接把返回值转换成HTTP响应消息，HttpResponseMessage对象的属性对应响应消息的属性。你可以灵活的设置响应标头和响应正文，这就给了你对响应消息的更大的控制权。如下例：
public class ReturnValueDemoController : ApiController { //返回类型为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; } }
响应：
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
如果你给CreateResponse方法传入了一个领域模型， Web API就会把序列化后的领域模型写入响应正文。
示例：
public HttpResponseMessage Get() { IQueryable <Product > products = repository.GetProducts(); HttpResponseMessage response = Request.CreateResponse(HttpStatusCode .OK, products); return response; }
响应：
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":" 苹果","Description":null,"Price":1.0,"Category":"水果"},{"ProductId":2,"Name":"鼠标","Description":null,"Price":50.0,"Category":"电脑配件"},{"ProductId":3,"Name":"洗发水","Description":null,"Price":20.0,"Category":"日用品"}]
IHttpActionResult
IHttpActionResult接口是在Web API 2才引入进来的，本质上，它定义了一个HttpResponseMessage工厂类。IHttpActionResult接口只定义了一个ExecuteAsync方法，该方法异步方式创建一个HttpResponseMessage实例。
public interface IHttpActionResult { Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken); }
如果控制器的Action返回类型是IHttpActionResult，Web API就会调用接口的ExecuteAsync方法创建一个HttpResponseMessage对象，然后转换成HTTP响应消息。以下是一个IHttpActionResult接口的实现，它创建了一个普通文本响应消息。
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代码：
//返回类型是IHttpActionResult public IHttpActionResult Get() { return new PlainTextResult( "plain text result" ,Request); }
响应：
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
通常情况下，你无须自己去实现IHttpActionResult接口， 在System.Web.Http.Results命名空间下已经包含了很多实现该接口的Action返回类型。ApiContoller类也定义了很多方法获取这些内置的Action返回类型。
在下面的例子里，如果没有找到某个产品，控制器调用ApiController.NotFound方法创建一个404 (Not Found)的响应消息。
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，控制器调用ApiController.OK方法创建一个HTTP状态码200(OK)响应正文是产品信息的响应消息。
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":" 苹果","Description":null,"Price":1.0,"Category":"水果"}
其它返回类型
对于其它的返回类型，Web API会对返回值序列化，然后把序列化后的返回值写入到响应正文里，并且响应状态码是200(OK)。
public IQueryable < Product> GetpProducts() { return repository.GetProducts(); }
响应：
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":" 苹果","Description":null,"Price":1.0,"Category":"水果"},{"ProductId":2,"Name":"鼠标","Description":null,"Price":50.0,"Category":"电脑配件"},{"ProductId":3,"Name":"洗发水","Description":null,"Price":20.0,"Category":"日用品"}]
使用这种返回类型的一个缺点就是你不能直接返回一个错误代码，例如404。不过，你可以通过抛出HttpResponseException异常来解决这个问题。
public Product GetProductById( int id) { Product product = repository.GetProductById(id); if (product==null ) throw new HttpResponseException ( HttpStatusCode.NotFound); return product; }