WebApi - action paramer is null when using [FromBo

问题:

I have this controller and I can't figure out, why name parameter is null

public class DeviceController : ApiController
{
    [HttpPost]
    public void Select([FromBody]string name)
    {
        //problem: name is always null
    }
}

here is my route mapping:

public void Configuration(IAppBuilder appBuilder)
{
    HttpConfiguration config = new HttpConfiguration();
    config.Routes.MapHttpRoute(
        name: "ActionApi",
        routeTemplate: "api/{controller}/{action}"
    );

    appBuilder.UseWebApi(config);
}

and here is my request:

POST http://localhost:9000/api/device/Select HTTP/1.2
User-Agent: Fiddler
Host: localhost:9000
Content-Length: 16
Content-Type: application/json

{'name':'hello'}

I also tried to change body to plain string: hello.

POST http://localhost:9000/api/device/Select HTTP/1.2
User-Agent: Fiddler
Host: localhost:9000
Content-Length: 5
Content-Type: application/json

hello

The request returns 204 which is ok, but the parameter is never mapped to a posted value.

*I'm using self hosted owin service.

回答1:

In the first example You were using a complex object {'name':'hello'} when the [FromBody] attribute told the binder to look for a simple type.

In the second example your provided value in the body could not be interpreted as a simple type as it was missing quotation marks "hello"

Using [FromBody]

To force Web API to read a simple type from the request body, add the [FromBody] attribute to the parameter:

public HttpResponseMessage Post([FromBody] string name) { ... }

In this example, Web API will use a media-type formatter to read the value of name from the request body. Here is an example client request.

POST http://localhost:5076/api/values HTTP/1.1
User-Agent: Fiddler
Host: localhost:5076
Content-Type: application/json
Content-Length: 7

"Alice"

When a parameter has [FromBody], Web API uses the Content-Type header to select a formatter. In this example, the content type is "application/json" and the request body is a raw JSON string (not a JSON object).

At most one parameter is allowed to read from the message body. So this will not work:

// Caution: Will not work!    
public HttpResponseMessage Post([FromBody] int id, [FromBody] string name) { ... }

The reason for this rule is that the request body might be stored in a non-buffered stream that can only be read once.

Source: Parameter Binding in ASP.NET Web API