Check ModelState in action filter

Can I check ModelState.IsValid in my custom action filter in OnActionExecuting method?

Yes. ModelState is part ViewData. So you can get it using:

filterContext.Controller.ViewData.ModelState

For example, if you wanted to inject some code after the action executes, but only if ModelState.IsValid == true, you can do:

public override void OnActionExecuted(ActionExecutedContext filterContext)
{
    if (!filterContext.Controller.ViewData.ModelState.IsValid) return;
    // do something
}