In ASP.NET MVC 3, what is filterContext.IsChildAction?

Question

From the sounds of it, it literally is a boolean value of whether or not the action is a child action.

I see this bit of code quite often:

protected override void OnActionExecuting(ActionExecutingContext filterContext) {     if (filterContext.IsChildAction) return;     ... } 

It appears to be there to "throttle" unnecessary code execution... but what does filterContext.IsChildAction actually mean?

Answer 1

In view pages, you may often need to inject output of another action into current page - for example, injecting menus. Menu generation may involve lots of business logic (determining rights or users, choosing selected item, etc), so it is not done in the partial view, but in controller.

public class MenuController : Controller {    [ChildActionOnly]    public ActionResult Menu()    {       MenuViewModel model = GenerateMenu();       return View(model);    } } 

This type of action is called ChildAction, as it cannot(and is not supposed to) be called from outside world(by visiting url). This may only be called by application itself, generally from within the view page.

@Html.Action("Menu", "Menu") 

And if you wish(or do not wish) to do some specific stuff when the action being executed is a child action, you inspect filterContext.IsChildAction property.

Answer 2

Maybe it's too late to point out but the accepted answer is slightly misleading, in the sense that: action marked with ChildActionOnlyAttribute absolutely cannot be run as standalone actions and so it is pointless to test with IsChildAction.

On the other hand if your action is called in two ways

1. as a regular action

2. as a child action from within another action

It can be useful to check for IsChildAction so that you can execute additional logic based on the value.