owin cors or web api cors - asp.net-web-api

Question

there are 100s of question on CORS on web-api, and on how to enable CORS, there is a different answer each one provides. I am so confused and dont know which answer is correct. And the problem is none of the answers actually explains it point wise, what each line of code does, so that I can understand and solve my problem rather than copy-pasting the code.
anyways, the question is: I am using asp.net web api 2 using owin. And i need to enable CORS. how do I do it? There is cors settings for OWIN
application.UseCors(CorsOptions.AllowAll);
and there is cors settings for asp.net web api
var cors = new EnableCorsAttribute("*", "*", "*", "*");
config.EnableCors(cors);
which one should I use given I am not using OAUTH (I am specifying this because answers on SO differ on when we use OAUTH v/s when we dont use it).
Do i need to enable CORS for both OWIN & WEB-API or only for one of them. There is issue if both are enabled, read here
It would be really helpful if someone can explain me the difference between
OWIN CORS
WEB API CORS
CORS with OAUTH using OWIN/WEBAPI
Also there are answers for self-hosted web api against owin hosted web-api, which further adds to the confution :(, sorry for the rant

Answer 1

You are supposed to use Web API's CORS if you need CORS applied to your API Controllers. For everything else (like a token service) you're stuck with having to use Owin.Cors.
If you end up using both, you'll need to make sure they don't overlap and apply CORS twice to the same request.
Web API 2.2 makes it easy to enable CORS by providing the EnableCorsAttribute.
Basic Usage
[EnableCors("*", "*", "*")]
public class ResourcesController : ApiController
{
...
Attribute definition
[AttributeUsageAttribute(AttributeTargets.Class|AttributeTargets.Method, AllowMultiple = false)]
public EnableCorsAttribute(
string origins,
string headers,
string methods
)
To enable CORS globally use
public static class WebApiConfig
{
public static void Register(HttpConfiguration config)
{
var cors = new EnableCorsAttribute("www.example.com", "*", "*");
config.EnableCors(cors);
// ...
}
}
You will also need to install the CORS package from nuget
Install-Package Microsoft.AspNet.WebApi.Cors

Answer 2

There is a way to fix this. Since OWIN and ASP.NET.CORS libraries are working simultaneously. Owin token or authentication method needs to be configured to enable CORS separately from all other API controllers.
Fist thing first, don't use cors with Owin in Startup.cs :
public void Configuration(IAppBuilder app)
{
//app.UseCors(CorsOptions.AllowAll);
Find GrantResourceOwnerCredentials method and add Access-Control-Allow-Origin to context so when it returns a call after authentication is completed that browser finds the header and accepts it.
public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
{
context.OwinContext.Response.Headers.Add("Access-Control-Allow-Origin", new[] { "http://localhost" });
Now install Microsoft.AspNet.WebApi.Cors package from Nuget to your webapi project, and add this to Register method
public static void Register(HttpConfiguration config)
{
var cors = new EnableCorsAttribute("http://localhost, ", "accept,accesstoken,authorization,cache-control,pragma,content-type,origin", "GET,PUT,POST,DELETE,TRACE,HEAD,OPTIONS");
config.EnableCors(cors);
Worked for me.