Add CORS middleware

- [Instructor] If you've decided that your API does need cors, adding it to the asp.net core pipeline is simple. In startup.cs, go down to the configure services method and we'll add services.AddCors. And we'll specify some options. We'll do options.AddPolicy to create a cors policy. Then we'll specify a string name, we'll just call this AllowMyApp. This can be any name you want. Then to build the policy, we'll say policy.WithOrigins and we'll specify the applications origin here.

This is the browser origin that'll be making requests. During development, sometime it can be useful to change this out for .AllowAnyOrigin just while you're doing testing. When you move to production, you should remove any of these any wildcard settings and lock down cors to just the origins that you need to use. Don't forget down in the configure method we also need to add it here. We need to say app.UseCors. And specify the name of that policy which we said was AllowMyApp.

It's important that UseCors is above any middle wear that generates a response such as UseNVC. If you need more help with cors there's plenty of documentation available on the asp.net cor documentation site at docs.asp.net.