The Accept request header lists the media types a client is willing to receive. A client can give them relative preferences with q values: 1.0 is the default, while */* means it will accept any format. The header expresses a preference, not a guarantee about the response. section 20.4.2
In ASP.NET Core MVC, the available response formatters matter as much as the request. With only the default JSON formatting configured, a request for application/xml still receives JSON. Adding XML formatters with AddXmlDataContractSerializerFormatters() makes XML available for an object response. Negotiation can also be configured to return 406 Not Acceptable when no requested format is available. section 20.4.2
Prefer XML, allow JSON
Send Accept: application/xml,application/json;q=0.8 to GET /api/content/object. XML is preferred, but JSON is acceptable. If GetObject has [Produces("application/json")], that attribute rules out XML for the action, so MVC returns JSON. section 20.4.3
Listing 20.20 Requesting data
Invoke-WebRequest http://localhost:5000/api/content/object -Headers
@{Accept="application/xml,application/json;q=0.8"} | select
@{n='Content-Type';e={ $_.Headers."Content-Type" }}, Content
This PowerShell request tests format selection and displays both the response Content-Type and body. section 20.4.3
The same header can be read as action data rather than used only for response selection. An action parameter declared as [FromHeader] string accept receives the request’s Accept value through model binding. section 28.6.2
