Get a specific response header (e.g., Content-Disposition) in Angular from an ASP.NET Web API 2 response for a cross-origin http.get request

Question

I cannot get a specific header (Content-Disposition) when I'm accessing it via an Angular service. CORS is enabled and the Angular HTTPClient is set to retrieve ALL headers.

Startup.cs

 public void Configuration(IAppBuilder app)
        {

            HttpConfiguration config = new HttpConfiguration();
            WebApiConfig.Register(config);
            ConfigureOAuth(app, config);
            app.UseCors(Microsoft.Owin.Cors.CorsOptions.AllowAll);
            app.UseWebApi(config);
 } 

fileController.cs

[Route("file/{idFile}")]
        public HttpResponseMessage GetFile(string idFile)
        {
            string file;
            byte[] file;

            document = fileService.GetFile(idDFile, out fileName);

            var result = new HttpResponseMessage(HttpStatusCode.OK)
            {
                Content = new ByteArrayContent(file)
            };
            result.Content.Headers.ContentDisposition =
                new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment")
                {
                    FileName = nomeFile
                };
            result.Content.Headers.ContentType =
                new MediaTypeHeaderValue("application/octet-stream");

            return result;
        }

fileService.ts

getFile(fileId: number): Observable<Response> {
        const url = `${urlBackEnd}/file/${fileId}`;
        return this.http.get(url, { observe: 'response', responseType: 'blob' })
          .map(res => {;
            console.log(res.headers.keys()); // There's no CONTENT-DISPOSITION
            saveAs(<Blob>res.body);
            return res.body;
          })
          .catch(e => this.handleErrors(e));
}

Here's the header console.log:

[
  "content-type",
  "cache-control"
]

What am I missing? I just want to get the Content-Disposition header.

Answer 1

In the fileController.cs file, along with setting the Content-Type and Content-Disposition response headers, you need to set Access-Control-Expose-Headers:

result.Content.Headers.Add("Access-Control-Expose-Headers", "Content-Disposition");

Note that while the Fetch spec does actually allow "*" as the value of Access-Control-Expose-Headers (though that’s not very clear from reading the current spec text…) — browsers don’t yet conform to the spec on that; so instead you should explicitly list all response header names the browser should expose to your frontend JavaScript code — except for Cache-Control, Content-Language, Content-Type, Expires, Last-Modified, and Pragma, which are always exposed. For any response headers other than those six and the ones you explicitly list in the value of the Access-Control-Expose-Headers header, browsers block frontend code from accessing them.