Content Negotiation in WebAPI

Content negotiation defined as "the process of selecting the best representation for a given response when there are multiple representations available."

The primary mechanism for content negotiation in HTTP are these request headers:

Accept: Which media types are acceptable for the response, such as "application/json," "application/xml," or a custom media type such as "application/vnd.example+xml"
Accept-Charset: Which character sets are acceptable, such as UTF-8 or ISO 8859-1.
Accept-Encoding: Which content encodings are acceptable, such as gzip.
Accept-Language: The preferred natural language, such as "en-us".

Serialization
If a Web API controller returns a resource as CLR type, the pipeline serializes the return value and writes it into the HTTP response body.

For example, consider the following controller action:

C#

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public Product GetProduct(int id)
{
    var item = _products.FirstOrDefault(p => p.ID == id);
    if (item == null)
    {
        throw new HttpResponseException(HttpStatusCode.NotFound);
    }
    return item; 
}
A client might send this HTTP request:

console

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GET http://localhost.:21069/api/products/1 HTTP/1.1
Host: localhost.:21069
Accept: application/json, text/javascript, */*; q=0.01
In response, the server might send:

console

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HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 57
Connection: Close

{"Id":1,"Name":"Gizmo","Category":"Widgets","Price":1.99}
In this example, the client requested either JSON, Javascript, or "anything" (*/*). The server responsed with a JSON representation of the Product object. Notice that the Content-Type header in the response is set to "application/json".

The object that serializes the resource is called a media formatter. Media formatters derive from the MediaTypeFormatter class. Web API provides media formatters for XML and JSON, and you can create custom formatters to support other media types.


https://docs.microsoft.com/en-us/aspnet/web-api/overview/formats-and-model-binding/content-negotiation