Defining options in Startup.ConfigureServices() vs Configure() methods

Question

User1160092068 posted

I am new to .net core framework. I found that defining options can be done interchangeably in ConfigureServices() and Configure() methods. For e.g. to enable CORS we can do either of the following :

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddCors(options => {
                options.AddDefaultPolicy(builder =>
                {
                    builder.AllowAnyOrigin();
                });
            });
            ...
        }
          
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            ...
            app.UseCors();
            ...
        }          

We can also achieve the same thing with this:

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddCors();
            ...
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            ...
            app.UseCors(x => x.AllowAnyOrigin());
            ...
        }        

I want to understand the difference between these two types of implementation.

Answer 1

User475983607 posted

ConfigureServices registers services for dependency injection.  Configure is used to setup middleware.

See the docs; https://docs.microsoft.com/en-us/aspnet/core/fundamentals/startup?view=aspnetcore-5.0

Answer 2

User1160092068 posted

Thanks for the explanation. However I don't get why there is option to configure CORS in both ConfigureServices() as well as Configure(). Is this something by design. Any example where it would be useful to configure CORS in one vs the other?

Answer 3

User475983607 posted

arnab_das

Thanks for the explanation. However I don't get why there is option to configure CORS in both ConfigureServices() as well as Configure(). Is this something by design. Any example where it would be useful to configure CORS in one vs the other?

You're making assumptions.  The CORS service is registered and configured in ConfigurServices() then implemented as middleware in Configure().  Middleware.   Most all the built-in ASP.NET Core Middleware components follow this pattern.

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware/?view=aspnetcore-5.0#built-in-middleware

Middleware reference

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware/write?view=aspnetcore-5.0

Keep in mind, this is a just pattern.  It possible to have one method in startup that handles all the configuration or all the configuration could be in the program.cs.  This information is in the first link I shared...