What is an Action filter and what functions does it…

Action filters in ASP.NET Core are a specific type of filter that intercepts the execution of a controller action method.

Main functions:

Before the action executes: They run code before the controller action method is called (OnActionExecuting).
After the action executes: They run code after the controller action method has been called, but before the result is formatted (OnActionExecuted).
Modifying the context: They can modify the execution context (ActionExecutingContext) or the post-execution context (ActionExecutedContext), for example, adding data to HttpContext.Items, changing routing, or setting the action result.
Exception handling: They do not directly handle exceptions but can be used to set an error result if an exception occurs during OnActionExecuting.

Usage examples:

Validating request data before calling the action.
Adding headers or modifying the response body.
Logging request and response information.
Caching action results.
Performing authorization and authentication (although specialized Authorization and Authentication filters are more commonly used for this).

Implementation: They are created by inheriting from ActionFilterAttribute or implementing the interfaces IActionFilter (synchronous) or IAsyncActionFilter (asynchronous).

// Example of a simple synchronous Action filter
public class CustomActionFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        // Logic before action execution
        Console.WriteLine("Before action execution");
        base.OnActionExecuting(context);
    }

    public override void OnActionExecuted(ActionExecutedContext context)
    {
        // Logic after action execution
        if (context.Exception == null && context.Result is ObjectResult objectResult)
        {
            // Modifying the result if there is no exception and the result is ObjectResult
            Console.WriteLine($"Action result status code: {objectResult.StatusCode}");
        }
        Console.WriteLine("After action execution");
        base.OnActionExecuted(context);
    }
}

They are applied to controllers or individual action methods using attributes:

[CustomActionFilter] // Applying to a controller
public class HomeController : Controller
{
    [HttpGet]
    [CustomActionFilter] // Applying to an action method
    public IActionResult Index()
    {
        return Ok("Hello from Index!");
    }
}

What is an Action filter and what functions does it perform?