Authorization strategy depends upon the resource being accessed. Consider a document which has an author property. Only the author is allowed to update the document. Consequently, the document must be retrieved from the data store before authorization evaluation can occur.
Attribute evaluation occurs before data binding and before execution of the page handler or action which loads the document. For these reasons, declarative authorization with an `[Authorize]` attribute won't suffice. Instead, you can invoke a custom authorization method—a style known as imperative authorization.
Use the [sample apps](https://github.com/aspnet/Docs/tree/master/aspnetcore/security/authorization/resourcebased/samples) ([how to download](xref:tutorials/index#how-to-download-a-sample)) to explore the features described in this topic.
Authorization is implemented as an [IAuthorizationService](/dotnet/api/microsoft.aspnetcore.authorization.iauthorizationservice) service and is registered in the service collection within the `Startup` class. The service is made available via [dependency injection](xref:fundamentals/dependency-injection#fundamentals-dependency-injection) to page handlers or actions.
`IAuthorizationService` has two `AuthorizeAsync` method overloads: one accepting the resource and the policy name and the other accepting the resource and a list of requirements to evaluate.
In the following example, the resource to be secured is loaded into a custom `Document` object. An `AuthorizeAsync` overload is invoked to determine whether the current user is allowed to edit the provided document. A custom "EditPolicy" authorization policy is factored into the decision. See [Custom policy-based authorization](xref:security/authorization/policies) for more on creating authorization policies.
Writing a handler for resource-based authorization isn't much different than [writing a plain requirements handler](xref:security/authorization/policies#security-authorization-policies-based-authorization-handler). Create a custom requirement class, and implement a requirement handler class. The handler class specifies both the requirement and resource type. For example, a handler utilizing a `SameAuthorRequirement` requirement and a `Document` resource looks as follows:
If you're making decisions based on the outcomes of CRUD (**C**reate, **R**ead, **U**pdate, **D**elete) operations, use the [OperationAuthorizationRequirement](/dotnet/api/microsoft.aspnetcore.authorization.infrastructure.operationauthorizationrequirement) helper class. This class enables you to write a single handler instead of an individual class for each operation type. To use it, provide some operation names:
The preceding handler validates the operation using the resource, the user's identity, and the requirement's `Name` property.
To call an operational resource handler, specify the operation when invoking `AuthorizeAsync` in your page handler or action. The following example determines whether the authenticated user is permitted to view the provided document.
> [!NOTE]
> The following code samples assume authentication has run and set the `User` property.
If authorization succeeds, the page for viewing the document is returned. If authorization fails but the user is authenticated, returning `ForbidResult` informs any authentication middleware that authorization failed. A `ChallengeResult` is returned when authentication must be performed. For interactive browser clients, it may be appropriate to redirect the user to a login page.
If authorization succeeds, the view for the document is returned. If authorization fails, returning `ChallengeResult` informs any authentication middleware that authorization failed, and the middleware can take the appropriate response. An appropriate response could be returning a 401 or 403 status code. For interactive browser clients, it could mean redirecting the user to a login page.
