asp.net core 3.1 的跨域问题，如果沿用2.2版本的方法是行不通的。3.1版本对跨域问题要“严格”很多。
微软官方给我的解释请如下网址：
不能 同时打开
AllowAnyOrigin() .AllowAnyMethod() .AllowAnyHeader() .AllowCredentials());
否则会抛异常。
// 会抛下面这个异常：
System.InvalidOperationException: Endpoint AnXin.DigitalFirePlatform.WebApi.Controllers.StaticPersonController.Get (AnXin.DigitalFirePlatform.WebApi) contains CORS metadata, but a middleware was not found that supports CORS.
Configure your application startup by adding app.UseCors() inside the call to Configure(..) in the application startup code. The call to app.UseAuthorization() must appear between app.UseRouting() and app.UseEndpoints(...).
at Microsoft.AspNetCore.Routing.EndpointMiddleware.ThrowMissingCorsMiddlewareException(Endpoint endpoint)
at Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
那么我们就只开其中的1,2个就行了。以下是我的代码，亲测可用：
1、Startup类里先定义一个全局变量：
readonly string MyAllowSpecificOrigins = "_myAllowSpecificOrigins";//名字随便起,zyiz.net2、ConfigureServices方法里写如下代码：
//找一找教程网原创文章
services.AddCors(options =>
{
options.AddPolicy(MyAllowSpecificOrigins,
builder => builder.AllowAnyOrigin()
.WithMethods("GET", "POST", "HEAD", "PUT", "DELETE", "OPTIONS")
);
});
3、Configure方法里添加中间件：
app.UseCors(MyAllowSpecificOrigins);
CORS 中间件必须配置为在对 UseRouting 和 UseEndpoints的调用之间执行。 配置不正确将导致中间件停止正常运行。
写个ajax测试下：
<script type="text/javascript">
$(function () {
$.get("https://webapi-dev.zyiz.net/api/Health/POk", function (result) {
$("#mycontent").html(result);
});
});
</script>效果如下：
