net core 3.1 跨域 Cors 找不到 “Access-Control-Allow-Origin”

首先在configureservices添加

public void configureservices(iservicecollection services)
        {
            services.addcors(options =>
            {
                options.addpolicy("any", builder =>
                {
                    //builder.allowanyorigin() //允许任何来源的主机访问
                    builder
                    
                    .withorigins("http://*.*.*.*")//.setisoriginallowedtoallowwildcardsubdomains()//设置允许访问的域

                    .allowanymethod()

                    .allowanyheader()

                    .allowcredentials();//

                });

            });
            services.addcontrollers();
        }

然后新增 

public class corsmiddleware
    {
        private readonly requestdelegate _next;
        public corsmiddleware(requestdelegate next)
        {
            _next = next;
        }

        public async task invoke(httpcontext context)
        {
            if (!context.response.headers.containskey("access-control-allow-origin"))
            {
                context.response.headers.add("access-control-allow-origin", "*");
            }
            await _next(context);
        }
    }

然后 使用中间件

 app.usemiddleware<corsmiddleware>();