ASP.NET Web API 2：Action的返回类型

绝地灬酷狼 2021-06-26 16:06 334阅读 0赞

Web API控制器中的`Action`方法有如下几种返回类型：

1.  `void`
2.  `HttpResponseMessage`
3.  `IHttpActionResult`
4.  `其它类型`

基于上面几种不同的返回类型，Web API创建`HTTP`响应消息的机制也不同。  
![在这里插入图片描述][2019050822545546.png]  
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;
            }

[2019050822545546.png]: /images/20210626/1359780b9d3d458ca66289426eff6880.png