ASP MVC Filters, where implement interface or not?

Question

Just wondering, googling around how to use filters in asp mvc 4. I have found some people defines them like this:

public class CustomFilter : ActionFilterAttribute

And some like this:

public class CustomFilter : ActionFilterAttribute, IActionFilter

ActionFilterAttribute already has all operations to override, why should i implement the interface as well?

Also for example in the next code, at the end the filter is called again, why is this?

public class 

CustomActionFilter : ActionFilterAttribute, IActionFilter
{
    void IActionFilter.OnActionExecuting(ActionExecutingContext filterContext)


    {
        // TODO: Add your acction filter's tasks here

        // Log Action Filter Call
        MusicStoreEntities storeDB = new MusicStoreEntities();

        ActionLog log = new ActionLog()
        {
            Controller = filterContext.ActionDescriptor.ControllerDescriptor.ControllerName,
            Action = filterContext.ActionDescriptor.ActionName + " (Logged By: Custom 

Action Filter)",
            IP = filterContext.HttpContext.Request.UserHostAddress,
            DateTime = filterContext.HttpContext.Timestamp
        };

        storeDB.ActionLogs.Add(log);
        storeDB.SaveChanges();

        this.OnActionExecuting(filterContext);
    }
}

Answer 1

If you check the MSDN definition of ActionFilterAttribute you will see that it is an abstract class that inherits interfaces IActionFilter AND IResultFilter.

In effect inheriting from ActionFilterAttribute,

public class CustomFilter : ActionFilterAttribute

is equivalent to inheriting class FilterAttribute and interfaces IActionFilter and IResultFilter

public class CustomFilter : FilterAttribute, IActionFilter, IResultFilter

and is no different than

public class CustomFilter : ActionFilterAttribute, IActionFilter, IResultFilter

So inheriting from ActionFilterAttribute is no different than inheriting from ActionFilterAttribute and IActionFilter.