Error when configuring swagger netcore3.1

Error Description: according netcore2.X configuration, after re-configuration swagger, / swagger / index.html open. . .

Solution: Configure method in Startup.cs file

             // allow cross-domain 
            app.UseCors ( " the any " );
             // Enable middleware services generated Swagger as JSON endpoint 
            app.UseSwagger ();
             // Enable middleware services to swagger-ui, designated Swagger JSON endpoint 
            app. UseSwaggerUI (C => 
            { 
                c.SwaggerEndpoint ( " /swagger/v1/swagger.json " , " My Vl the API " );
                 c.RoutePrefix = String .Empty; 
            });        

The standard yellow background of this removed it. Or to a particular route, to replace the default Swagger, such as 

c.RoutePrefix = "doc";

The swagger of access address becomes "/doc/index.html"

Reference Address: https://docs.microsoft.com/zh-cn/aspnet/core/tutorials/getting-started-with-swashbuckle?view=aspnetcore-3.1&tabs=visual-studio

To apply the root (HTTP: // provide Swagger UI, the property set to an empty string RoutePrefix <port> /: localhost):

It might be set to the default address conflict and procedures, but do not know why 2.X before the problem does not exist. . .