将Signalr与Web api2集成时出现Cors Ploicy错误

Question

我正在使用angualr 6将Signlar与Web api2集成。以下是我的文件：

Startup.cs（API）

// Branch the pipeline here for requests that start with "/signalr"
   app.Map("/NotificationHub", map =>
   {                       
     map.UseCors(CorsOptions.AllowAll);
     var hubConfiguration = new HubConfiguration
     {                            
       EnableDetailedErrors = true
     };                        
     map.RunSignalR(hubConfiguration);
   });

HubClient

public interface NotificationHubClient
{
    Task BroadcastMessage(string type, string payload);
}

NotificationHub

public class NotificationHub : Hub<NotificationHubClient>
{
    public void Hello()
    {
        Clients.All.BroadcastMessage("alert", "1");
    }
}

SignalRService（角度6）

import { Injectable } from '@angular/core';
import * as signalR from "@aspnet/signalr";
import { appConfig } from '../app.config';

 @Injectable()
 export class SignalRService {
 public notificationHubUrl = `${appConfig.ApiProjectUrl}NotificationHub`;    
 private hubConnection: signalR.HubConnection

 public startConnection = () => {
  this.hubConnection = new signalR.HubConnectionBuilder()
                        .withUrl(this.notificationHubUrl)
                        .build();

    this.hubConnection
     .start()
     .then(() => console.log('Connection started'))
     .catch(err => console.log('Error while starting connection: ' + err))
  }
}

在控制台窗口上出现以下错误：

Access to XMLHttpRequest at 'https://localhost/Project.API/api/NotificationHub/negotiate' from origin 'http://localhost:4301' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.