How to create custom filters in ASP.NET MVC?

You can create custom filter attributes by implementing an appropriate filter interface for which you want to create a custom filter and derive the FilterAttribute class to use that class as an attribute.

For example, implement IExceptionFilter and the FilterAttribute class to create a custom exception filter. In the same way, implement an IAuthorizatinFilter interface and FilterAttribute class to create a custom authorization filter.

class MyErrorHandler : FilterAttribute, IExceptionFilter
{
    public override void IExceptionFilter.OnException(ExceptionContext filterContext)
    {
        Log(filterContext.Exception);

        base.OnException(filterContext);
    }

    private void Log(Exception exception)
    {
        //log exception here..
 
    }
}

Alternatively, you can also derive a built-in filter class and override an appropriate method to extend built-in filters' functionality.

Let's create a custom exception filter to log every unhandled exception by deriving the built-in HandleErrorAttribute class and overriding the OnException method, as shown below.

class MyErrorHandler : HandleErrorAttribute
{
    public override void OnException(ExceptionContext filterContext)
    {
        Log(filterContext.Exception);

        base.OnException(filterContext);
    }

    private void Log(Exception exception)
    {
        //log exception here..
 
    }
}

You can now apply the MyErrorHandler attribute at the global level or controller or action method level, the same way we applied an HandleError attribute.

[MyErrorHandler]
public class HomeController : Controller
{
    public ActionResult Index()
    {
        return View();
    }

}