Action Results in Web API 2

https://docs.microsoft.com/en-us/aspnet/web-api/overview/getting-started-with-aspnet-web-api/action-results

This topic describes how ASP.NET Web API converts the return value from a controller action into an HTTP response message.

A Web API controller action can return any of the following:

1. void
2. HttpResponseMessage
3. IHttpActionResult
4. Some other type

Depending on which of these is returned, Web API uses a different mechanism to create the HTTP response.

Return type	How Web API creates the response
void	Return empty 204 (No Content)
HttpResponseMessage	Convert directly to an HTTP response message.
IHttpActionResult	Call ExecuteAsync to create an HttpResponseMessage, then convert to an HTTP response message.
Other type	Write the serialized return value into the response body; return 200 (OK).

The rest of this topic describes each option in more detail.

void

If the return type is void, Web API simply returns an empty HTTP response with status code 204 (No Content).

Example controller:

  public class ValuesController : ApiController
    {
        [HttpGet]
        public void Post()
        {
        }
    }

request

GET http://localhost/Chuck_WebApi/api/values/post HTTP/1.1
User-Agent: Fiddler
Host: localhost
Content-Length: 0

response

HTTP/1.1 204 No Content
Cache-Control: no-cache
Pragma: no-cache
Expires: -1
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Thu, 17 Jan 2019 10:04:57 GMT

HttpResponseMessage

If the action returns an HttpResponseMessage, Web API converts the return value directly into an HTTP response message, using the properties of the HttpResponseMessage object to populate the response.

This option gives you a lot of control over the response message. For example, the following controller action sets the Cache-Control header.

需要把路由从默认的routeTemplate: "api/{controller}/{id}"调整为routeTemplate: "api/{controller}/{action}/{id}"

  public HttpResponseMessage Get()
        {
            HttpResponseMessage response = Request.CreateResponse(HttpStatusCode.OK, "value");
            response.Content = new StringContent("hello", Encoding.Unicode);
            response.Headers.CacheControl = new CacheControlHeaderValue()
            {
                MaxAge = TimeSpan.FromMinutes()
            };
            return response;
        }

request

GET http://localhost/Chuck_WebApi/api/values/get HTTP/1.1
User-Agent: Fiddler
Host: localhost
Content-Length: 0

response

HTTP/1.1 200 OK
Cache-Control: max-age=1200
Content-Length: 10
Content-Type: text/plain; charset=utf-16
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Thu, 17 Jan 2019 10:25:14 GMT

hello

If you pass a domain model to the CreateResponse method, Web API uses a media formatter to write the serialized model into the response body.

https://docs.microsoft.com/en-us/aspnet/web-api/overview/advanced/dependency-injection

https://autofaccn.readthedocs.io/en/latest/integration/webapi.html

此处代码的前提是，在项目启动的时候，使用了Autofac.WebApi2初始化GlobalConfiguration.Configuration.DependencyResolver

 public HttpResponseMessage Get2()
        {
            var dependencyResolver = GlobalConfiguration.Configuration.DependencyResolver;
            var controller = dependencyResolver.GetService(typeof(ProductsController));
            ProductsController productsController = controller as ProductsController;
            //productsController.ControllerContext //might need set value here

            // Get a list of products from a database.
            IEnumerable<Product> products = productsController.GetAllProducts();

            // Write the list to the response body.
            HttpResponseMessage response = Request.CreateResponse(HttpStatusCode.OK, products);
            return response;
        }

Web API uses the Accept header in the request to choose the formatter. For more information, see Content Negotiation.

request

GET http://localhost/Chuck_WebApi/api/values/get2 HTTP/1.1
User-Agent: Fiddler
Host: localhost
Content-Length: 0

response

HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Content-Type: application/json; charset=utf-8
Expires: -1
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Thu, 17 Jan 2019 11:13:15 GMT
Content-Length: 182

[{"Id":1,"Name":"Tomato Soup","Category":"Groceries","Price":1.0},{"Id":2,"Name":"Yo-yo","Category":"Toys","Price":3.75},{"Id":3,"Name":"Hammer","Category":"Hardware","Price":16.99}]

IHttpActionResult

The IHttpActionResult interface was introduced in Web API 2. Essentially, it defines an HttpResponseMessage factory. Here are some advantages of using the IHttpActionResult interface:

• Simplifies unit testing your controllers.
• Moves common logic for creating HTTP responses into separate classes.
• Makes the intent of the controller action clearer, by hiding the low-level details of constructing the response.

IHttpActionResult contains a single method, ExecuteAsync, which asynchronously creates an HttpResponseMessage instance.

public interface IHttpActionResult
{
    Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken);
}

If a controller action returns an IHttpActionResult, Web API calls the ExecuteAsync method to create an HttpResponseMessage. Then it converts the HttpResponseMessage into an HTTP response message.

Here is a simple implementaton of IHttpActionResult that creates a plain text response:

public class TextResult : IHttpActionResult
    {
        string _value;
        HttpRequestMessage _request;

        public TextResult(string value, HttpRequestMessage request)
        {
            _value = value;
            _request = request;
        }

        public Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
        {
            var response = new HttpResponseMessage()
            {
                Content = new StringContent(_value),
                RequestMessage = _request
            };
            return Task.FromResult(response);
        }
    }

Example controller action:

 public IHttpActionResult Get3()
        {
            return new TextResult("hello", Request);
        }

request

GET http://localhost/Chuck_WebApi/api/values/get3 HTTP/1.1
User-Agent: Fiddler
Host: localhost
Content-Length: 0

response

HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Content-Length: 5
Content-Type: text/plain; charset=utf-8
Expires: -1
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Fri, 18 Jan 2019 02:19:02 GMT

hello

More often, you will use the IHttpActionResult implementations defined in the System.Web.Http.Results namespace. The ApiController class defines helper methods that return these built-in action results.

https://docs.microsoft.com/en-us/previous-versions/aspnet/dn314678(v=vs.118)    系统内定的一些IHttpActionResult 的实现

ApiController 本身也内置了一部分的方法，来返回对应的实现

BadRequestErrorMessageResult

UnauthorizedResult

In the following example, if the request does not match an existing product ID, the controller calls ApiController.NotFound to create a 404 (Not Found) response.

Otherwise, the controller calls ApiController.OK, which creates a 200 (OK) response that contains the product.

public IHttpActionResult Get4(int id)
        {
            var product = products.FirstOrDefault((p) => p.Id == id);
            if (product == null)
            {
                return NotFound(); // Returns a NotFoundResult
            }
            return Ok(product);  // Returns an OkNegotiatedContentResult
        }

request1

GET http://localhost/Chuck_WebApi/api/values/get4/3 HTTP/1.1
User-Agent: Fiddler
Host: localhost
Content-Length: 0

response1

HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Content-Type: application/json; charset=utf-8
Expires: -1
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Fri, 18 Jan 2019 02:36:31 GMT
Content-Length: 60

{"Id":3,"Name":"Hammer","Category":"Hardware","Price":16.99}

request2

GET http://localhost/Chuck_WebApi/api/values/get4/4 HTTP/1.1
User-Agent: Fiddler
Host: localhost
Content-Length: 0

response2

HTTP/1.1 404 Not Found
Cache-Control: no-cache
Pragma: no-cache
Expires: -1
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Fri, 18 Jan 2019 02:50:06 GMT
Content-Length: 0

Other Return Types

For all other return types, Web API uses a media formatter to serialize the return value.

Web API writes the serialized value into the response body.

The response status code is 200 (OK).

public IEnumerable<Product> GetAllProducts()
        {
            return products;
        }

A disadvantage of this approach is that you cannot directly return an error code, such as 404.

However, you can throw an HttpResponseException for error codes.

For more information, see Exception Handling in ASP.NET Web API.

Web API uses the Accept header in the request to choose the formatter. For more information, see Content Negotiation.

request

GET http://localhost/Chuck_WebApi/api/products/getallproducts HTTP/1.1
User-Agent: Fiddler
Host: localhost
Content-Length: 0

response

HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Content-Type: application/json; charset=utf-8
Expires: -1
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Fri, 18 Jan 2019 03:10:56 GMT
Content-Length: 182

[{"Id":1,"Name":"Tomato Soup","Category":"Groceries","Price":1.0},{"Id":2,"Name":"Yo-yo","Category":"Toys","Price":3.75},{"Id":3,"Name":"Hammer","Category":"Hardware","Price":16.99}]