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() //容許任何來源的主機訪問
                    builder
                    
                    .WithOrigins("http://*.*.*.*")//.SetIsOriginAllowedToAllowWildcardSubdomains()//設置容許訪問的域

                    .AllowAnyMethod()

                    .AllowAnyHeader()

                    .AllowCredentials();//

                });

            });
            services.AddControllers();
        }

而後新增 

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);
        }
    }

而後 使用中間件

 app.UseMiddleware<CorsMiddleware>();