ASP.NET Core Authorization: Understanding the Role of the Authorizer

In ASP.NET Core, an authorizer is a component that checks if a user has the necessary permissions to perform a specific action. It's like a gatekeeper for your application's resources.

When a user requests a resource or performs an action, the authorizer is called to check if the user has the appropriate permission to access that resource or perform that action. If the user doesn't have the necessary permissions, the authorizer will deny access and return an error.

For example, let's say you have a web API that allows users to create, read, update, and delete (CRUD) products. You might use an authorizer to check if the user has the appropriate permission to perform each of these actions. If the user doesn't have the necessary permissions, the authorizer will deny access and return an error.

Authorizers can be implemented in a variety of ways, such as using custom attributes, filters, or middleware. They can also be integrated with other authentication and authorization systems, such as OAuth or OpenID Connect.

In ASP.NET Core, the built-in authorizer is called the "DefaultAuthorizationPolicy" and it's based on the "AuthorizeAttribute" class. This attribute can be applied to controllers, actions, and even individual parameters to specify the required permissions for access.

For example, here's an example of how you might use the AuthorizeAttribute to require a user to have the "Admin" role to access a specific action:
```
[Authorize(Roles = "Admin")]
public IActionResult MyAction()
{
// This action is only accessible to users with the Admin role
}
```
In this example, the AuthorizeAttribute is applied to the "MyAction" method and requires that the user have the "Admin" role in order to access it. If the user doesn't have the "Admin" role, they will be denied access and an error will be returned.