Action filters are a powerful mechanism in web development frameworks, such as ASP.NET Core, used to intercept the execution pipeline of a request. Action filters contain logic that is executed before and after a controller action executes. This allows developers to inject custom processing logic at specific points during the request lifecycle, specifically around the execution of a controller method.
How Action Filters Work
An action filter essentially wraps the execution of a controller action. This provides two key points where custom code can run:
- Before the action executes: Logic can run before the controller method is invoked. This is useful for tasks like validating input, checking user permissions, or setting up data needed by the action.
- After the action executes: Logic can run after the controller method has finished executing but before the result is processed. This point is useful for modifying the outcome of the action. As mentioned in the reference, you can use an action filter, for instance, to modify the view data that a controller action returns. This means you could alter the data model being passed to a view or even change the type of result (e.g., return a different view or a redirect).
Most frameworks implement action filters using interfaces or attributes that define methods corresponding to these execution points. For example, you might have methods like OnActionExecuting (runs before the action) and OnActionExecuted (runs after the action).
Common Use Cases for Action Filters
Action filters are incredibly versatile and can be applied at the global level, controller level, or even on individual action methods. Here are some practical examples of their application:
- Input Validation: Check the validity of incoming model data before the action is processed, returning an error response if validation fails.
- Authorization and Authentication: Verify if a user is authenticated and authorized to access a specific action.
- Logging: Log information about the request, user, and action being executed, both before and after it runs.
- Response Caching: Implement caching logic based on action output.
- Modifying Output: As highlighted in the reference, modify the data model or result returned by the action.
- Error Handling: Wrap action execution in a try-catch block to handle exceptions gracefully.
Using filters helps keep controller actions focused purely on handling the request logic, delegating cross-cutting concerns to the filters.
Action Filters vs. Result Filters
While action filters operate around the controller action execution, other types of filters exist in the pipeline. For instance, the reference mentions: "Result filters contain logic that is executed before and after a view result is executed." This illustrates a key difference:
| Filter Type | Execution Point | Primary Focus |
|---|---|---|
| Action Filter | Before and after the controller action runs | Pre/post-processing action logic |
| Result Filter | Before and after the result runs | Pre/post-processing result execution |
Understanding these different filter types is crucial for placing your custom logic at the correct stage of the request processing pipeline. The information provided on 11-Jul-2022 clearly defines action filters by their execution point relative to the controller action.
Action filters are a fundamental tool for adding modular and reusable cross-cutting concerns to web applications structured around the Model-View-Controller (MVC) pattern or similar architectures.
