How to enable CORS in ASP.net Core WebAPI

Because you have a very simple CORS policy (Allow all requests from XXX domain), you don't need to make it so complicated. Try doing the following first (A very basic implementation of CORS).

If you haven't already, install the CORS nuget package.

Install-Package Microsoft.AspNetCore.Cors

In the ConfigureServices method of your startup.cs, add the CORS services.

public void ConfigureServices(IServiceCollection services)
{
    services.AddCors(); // Make sure you call this previous to AddMvc
    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

Then in your Configure method of your startup.cs, add the following :

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    // Make sure you call this before calling app.UseMvc()
    app.UseCors(
        options => options.WithOrigins("http://example.com").AllowAnyMethod()
    );

    app.UseMvc();
}

Now give it a go. Policies are for when you want different policies for different actions (e.g. different hosts or different headers). For your simple example you really don't need it. Start with this simple example and tweak as you need to from there.

Further reading : http://dotnetcoretutorials.com/2017/01/03/enabling-cors-asp-net-core/

• In ConfigureServices add services.AddCors(); BEFORE services.AddMvc();

• Add UseCors in Configure

   app.UseCors(builder => builder
       .AllowAnyOrigin()
       .AllowAnyMethod()
       .AllowAnyHeader());   
   app.UseMvc();
  

Main point is that add app.UseCors, before app.UseMvc().

Make sure you declare the CORS functionality before MVC so the middleware fires before the MVC pipeline gets control and terminates the request.

After the above method works you can change it configure a specific ORIGIN to accept api calls and avoid leaving your API so open to anyone

public void ConfigureServices(IServiceCollection services)
{
    services.AddCors(options => options.AddPolicy("ApiCorsPolicy", builder =>
    {
        builder.WithOrigins("http://localhost:4200").AllowAnyMethod().AllowAnyHeader();
    }));

    services.AddMvc();
}

In the configure method tell CORS to use the policy you just created:

app.UseCors("ApiCorsPolicy");
app.UseMvc();

I just found this compact article on the subject - https://dzone.com/articles/cors-in-net-core-net-core-security-part-vi