Action Filter

What is an Action Filter ?

In ASP.NET Core, an action filter is a type of filter that allows you to add behavior to an action method before and after its execution. Action filters are executed as part of the filter pipeline and provide a way to implement cross-cutting concerns that apply specifically to an action method.

Action filters are applied to individual action methods or to the entire controller containing the action methods. They can perform various tasks, such as modifying the arguments of the action method, inspecting or modifying the action’s result, executing code before and after the action method, or handling exceptions that occur during the action’s execution.

Action filters in ASP.NET Core are implemented as attributes. You can create a custom action filter attribute by deriving from one of the provided filter attribute classes or by implementing the IActionFilter or IAsyncActionFilter interface.

Here’s an example of a custom action filter attribute that implements IActionFilter:

public class LogActionFilterAttribute : Attribute, IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext context)
    {
        // Code to be executed before the action method
        // For example, logging request details
    }

    public void OnActionExecuted(ActionExecutedContext context)
    {
        // Code to be executed after the action method
        // For example, logging response details
    }
}

You can apply the LogActionFilterAttribute to an action method or a controller to enable the defined behavior:

[LogActionFilter]
public IActionResult MyAction()
{
    // Code for the action method
}

In the above example, the LogActionFilterAttribute is applied to the MyAction method. When the action is invoked, the OnActionExecuting method of the action filter will be called before the action method executes, and the OnActionExecuted method will be called after the action method has executed.

By using action filters, you can add reusable and configurable behavior to specific action methods, such as logging, authorization checks, performance monitoring, and more. Action filters allow you to separate cross-cutting concerns from your core action method logic and apply consistent behavior across multiple actions in ASP.NET Core.