我是Angular 5的新手，我正在ASP.NET Core 2.1应用程序中使用它。我试图通过外部服务使用其API来实现身份验证，但我被LinkedIn API卡住了。这与Google或Facebook的API完全不同，因为Linkedin改变了他的方法，用JSON取代了JS API。我使用的是Oauth 2.0 nuget包，提供Linkedin的Oauth。我有一个问题，与重定向到LinkedIn API与登录页面。我在没有Angular的情况下用ASP.NET Core应用程序测试了同样的方法，它工作正常，所以问题出在Angular方面，或者可能是我的项目没有正确配置为使用Angular。
我在点击LinkedIn按钮时添加了操作：

<button><img class="button-linkedIn" (click)="OnLinkedInLoad()" 
    src="../../assets/images/linkedIn-register2.png"></button>

它在login组件中处理：

@Component({
      moduleId: module.id,
      selector: 'app-login',
      templateUrl: './login.component.html',
      styleUrls: ['./login.component.css']
    })
    export class LoginComponent implements OnInit {

      ngOnInit() {
      }

      constructor(
        private userSrv: UsersService) { }

      OnLinkedInLoad() {
        this.userSrv.loginUserLinkedIn().subscribe()
      }
    }

登录组件使用users.service组件发送请求（GET）：

@Injectable()
    export class UsersService {
      constructor(private httpClient: HttpClient) { }

      addUser(user: User) {
        return this.httpClient.post('/api/Users', user);
      }

      loginUserLinkedIn(): Observable<any> {
        return this.httpClient.get<any>('/api/Users/signInLinkedIn');
      }
    }

最后在UsersController中：

[Route("api/[controller]")]
    [ApiController]
    public class UsersController : ControllerBase
    {
        public UsersController(LoginTestContext context)
        {
        }

        [HttpGet("signInLinkedIn")]
        public async Task LinkedInSignInAsync()
        {
            await HttpContext.ChallengeAsync("linkedin", new AuthenticationProperties() { RedirectUri = "/" });
        }
    }

下面是我在启动时的配置：

public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<LoginTestContext>(options =>
                options.UseNpgsql(Configuration.GetConnectionString("LoginTestContext")));

            services.AddAuthentication(options =>
            {
                options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
            })
            .AddCookie(
                )
            .AddLinkedIn("linkedin", options =>
            {
                options.ClientId = this.Configuration.GetValue<string>("linkedin:clientid");
                options.ClientSecret = this.Configuration.GetValue<string>("linkedin:clientSecret");
            });

            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

            services.AddSpaStaticFiles(configuration =>
            {
                configuration.RootPath = "ClientApp/dist";
            });
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseBrowserLink();
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("Users/Error");                
            }

            //app.UseHsts();
            app.UseSpaStaticFiles();
            app.UseStaticFiles();
            app.UseAuthentication();

            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Users}/{action=Index}/{id?}");
            });

            app.UseSpa(spa =>
            {
                // To learn more about options for serving an Angular SPA from ASP.NET Core,
                // see https://go.microsoft.com/fwlink/?linkid=864501

                spa.Options.SourcePath = "ClientApp";

                if (env.IsDevelopment())
                {
                    spa.UseAngularCliServer(npmScript: "start");
                }
            });
        }
    }

编辑：
在调试过程中，我注意到控制台中的一个错误，单击LinkedIn按钮后，出现错误日志：

Access to XMLHttpRequest at
'https://www.linkedin.com/oauth/v2/authorization?client_id=XXXXXXXXX&scope=r_liteprofile%20r_emailaddress&response_type=code&redirect_uri=http%3A%2F%2Flocalhost%3A44325%2Fsignin-linkedin&state=CfDJ8NjmD6pBjdpBuQvhUqQ7m6g41igeIumDrha6i0V2JZrOt1u6nweQ9cllyzBMFUj8F2dttqoYX7GYZs9wg-W4O2N8Y9XGPXUwkj5Ojl16sHlYHab93vT3jQEbSXuCQja-Fyths8Rw6YwId0-Ibg8sTeBK-IJSB6_VN16o7h9Nlw24M1Qo3ZRAR8Aq-Yp9DZSdLCVOfzz0yFkvkhy5cJ1OhC0'
(redirected from 'http://localhost:44325/api/Users/signInLinkedIn') from origin 'http://localhost:44325'
has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

我尝试了一些CORS策略的修复：
1)添加

services.AddCors();

配置到启动中的ConfigureServices方法，并

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

2)向HTTPContext响应添加标头：

this.ControllerContext.HttpContext.Response.Headers.Add("Access-Control-Allow-Origin", "*");

3)调用loginUserLinkedIn时添加HttpHeaders

loginUserLinkedIn(): Observable<any> {
const headers = new HttpHeaders().append('Access-Control-Allow-Origin', ['*']);

return this.httpClient.get<any>('/api/Users/signInLinkedIn', { headers });

但是在这些修复之后，错误仍然发生。
如何在使用Angular的ASP.NET Core应用程序中正确设置CORS策略？