Part 71 - childactiononly attribute in mvc

Suggested Videos 
Part 68 - What is the use of NonAction attribute
Part 69 - Action filters
Part 70 - Authorize and AllowAnonymous action filters

In this video, we will discuss childactiononly attribute in asp.net mvc. Let us understand this with an example. 



Step 1: Create a blank asp.net mvc 4 application

Step 2: Add HomeController. Copy and paste the following code.
public class HomeController : Controller
{
    // Public action method that can be invoked using a URL request
    public ActionResult Index()
    {
        return View();
    }

    // This method is accessible only by a child request. A runtime 
    // exception will be thrown if a URL request is made to this method
    [ChildActionOnly]
    public ActionResult Countries(List<String> countryData)
    {
        return View(countryData);
    }
}



Step 3: Right click on the "Countries()" action method and add "Countries" view. This view will render the given list of strings as an un-ordered list.

@model List<string>
@foreach (string country in Model)
{
    <ul>
        <li>
            <b>
                @country
            </b>
        </li>
    </ul>
}

Step 4: Right click on the "Index()" action method and add "Index" view.  Copy and paste the following code. Notice that, to invoke childaction, we are using Action() HTML Helper.

<h2>Countries List</h2>
@Html.Action("Countries", new { countryData = new List<string>() { "US", "UK", "India" } })

Please Note: Child actions can also be invoked using "RenderAction()" HTMl helper as shown below.
@{
    Html.RenderAction("Countries", new { countryData = new List<string>() { "US", "UK", "India" } });
}

Points to remember about "ChildActionOnly" attribute
1. Any action method that is decorated with [ChildActionOnly] attribute is a child action method.

2. Child action methods will not respond to URL requests. If an attempt is made, a runtime error will be thrown stating - Child action is accessible only by a child request.

3. Child action methods can be invoked by making child request from a view using "Action()" and "RenderAction()" html helpers.

4. An action method doesn’t need to have [ChildActionOnly] attribute to be used as a child action, but use this attribute to prevent if you want to prevent the action method from being invoked as a result of a user request.

5. Child actions are typically associated with partial views, although this is not compulsory.

6. Child action methods are different from NonAction methods, in that NonAction methods cannot be invoked using Action() or RenderAction() helpers. We discussed NonAction methods in Part 70 of ASP.NET MVC tutorial series.

7. Using child action methods, it is possible to cache portions of a view. This is the main advantage of child action methods. We will cover this when we discuss [OutputCache] attribute.