Tuesday, March 8, 2016

Enabling CORS in ASP.NET Web API

When testing some code I noticed that a specific AJAX call kept returning a 405 Method not allowed response. Although I was calling $.ajax({type: "POST"}) behind the scenes the browser changed the request to an OPTIONS request.

What was going on? What I didn’t notice at first sight was that the AJAX request was send to another domain, meaning I’m making a cross-origin HTTP request. For security reasons, browsers restrict cross-origin HTTP requests initiated from within scripts. Before you can do a cross-origin request, the browser will initiate a preflight CORS(Cross-Origin Resource Sharing) check. This explains the OPTION request going out.

It is up to the called API to handle this preflight check and return headers describing what’s allowed and not.

Permission/Feature Request Header Response Header
Origin Origin Access-Control-Allow-Origin
HTTP method Access-Control-Request-Method Access-Control-Allow-Method
Request headers Access-Control-Request-Headers Access-Control-Allow-Headers
Response headers   Access-Control-Expose-Headers
Credentials   Access-Control-Allow-Credentials
Cache preflight response   Access-Control-Max-Age

Enabling this for an ASP.NET Web API is easy:

  • Download and install the Microsoft.ASPNET Web API CORS NuGet package:
    • Install-Package Microsoft.AspNet.WebApi.Cors
  • Add the following code to your WebApiConfig.Register method:
  • This is all you need to do.
    • Remark: In this case I globally enabled CORS support, but it is also possible to do this for a specific controller

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