What are action filters and how do you create custom filters?

Action filters are attributes that add extra processing logic before or after specific stages in the request processing pipeline. They allow cross-cutting concerns like logging, caching, authorization, and exception handling.

Filter types and execution order:

1. Authorization filters - Run first, verify authorization
2. Resource filters - Run after authorization, good for caching
3. Action filters - Run before and after action method execution
4. Exception filters - Handle exceptions
5. Result filters - Run before and after result execution

Creating a custom action filter:

// Method 1: Inherit from ActionFilterAttribute
public class LogActivityFilter : ActionFilterAttribute
{
    private readonly ILogger _logger;

    public LogActivityFilter(ILogger logger)
    {
        _logger = logger;
    }

    public override void OnActionExecuting(ActionExecutingContext context)
    {
        _logger.LogInformation($"Executing action: {context.ActionDescriptor.DisplayName}");
        base.OnActionExecuting(context);
    }

    public override void OnActionExecuted(ActionExecutedContext context)
    {
        _logger.LogInformation($"Executed action: {context.ActionDescriptor.DisplayName}");
        base.OnActionExecuted(context);
    }
}

// Method 2: Implement IActionFilter interface
public class ValidateModelFilter : IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
            context.Result = new BadRequestObjectResult(context.ModelState);
        }
    }

    public void OnActionExecuted(ActionExecutedContext context)
    {
        // Logic after action execution
    }
}

// Async version
public class AsyncLoggingFilter : IAsyncActionFilter
{
    public async Task OnActionExecutionAsync(
        ActionExecutingContext context, 
        ActionExecutionDelegate next)
    {
        // Before action execution
        await next(); // Execute the action
        // After action execution
    }
}

Using filters:

// Apply to specific action
[LogActivityFilter]
public IActionResult GetUser(int id)
{
    return Ok();
}

// Apply to controller
[ValidateModelFilter]
public class UsersController : ControllerBase
{
}

// Register globally in Program.cs
builder.Services.AddControllers(options =>
{
    options.Filters.Add();
});