How to Create Custom External Helper Methods in ASP.Net MVC

My article How to Create Custom Inline Helper Methods in ASP.NET MVC explained what helper methods are and how to create and use inline helper methods in MVC applications. (Note: if you have not read the article regarding custom inline helpers in MVC applications, I recommend reading it first since it is related to this article). Before getting started with how to create custom external helper methods, let`s just explain external helper methods and when we should use them.

External Helper Method

Helper methods are of two types, inline helper methods and external helper methods. Inline helper methods are convenient, since they are declared in the view and the complexity of the code is low. But if the code is becoming complex, then it will be difficult to read and understand the code in our view. The alternative for such a situation is to create an external helper method. External helper methods are expressed as C# extension methods. Now let`s get started with an example.

Getting Started

We will use the same example that we used in my article How to Create Custom Inline Helper Methods in ASP.NET MVC of displaying the list of fruits and flowers (for simplicity purposes I am using the same example). Let`s recall our example. We added the following code to the index action of the home Controller.

  1. public class HomeController : Controller  
  2. {  
  3.     public ActionResult Index()  
  4.     {  
  5.         ViewBag.Fruits = new string[] {"Apple""Mango""Lemon"};  
  6.         ViewBag.Flowers = new string[] { "Rose""Jasmine""Sunflower" };  
  7.         return View();  
  8.     }  
  10.     public ActionResult About()  
  11.     {  
  12.         ViewBag.Message = "Your application description page.";  
  14.         return View();  
  15.     }  
  17.     public ActionResult Contact()  
  18.     {  
  19.         ViewBag.Message = "Your contact page.";  
  21.         return View();  
  22.     }  
  23. }  
The content of the index view after using the inline helper method is as shown below.
  1. @{  
  2.     ViewBag.Title = "Home Page";  
  3. }  
  5. @helper ListItems(string[] items)  
  6. {  
  7.     <ul>  
  8.         @foreach (var item in items)  
  9.         {  
  10.             <li>@item</li>  
  11.         }  
  12.     </ul>  
  13. }  
  15. <div class="row">  
  16.     <div class="col-md-6">  
  17.         <h2>Fruits</h2>  
  18.         @ListItems(ViewBag.Fruits)  
  19.     </div>  
  20.     <div class="col-md-6">  
  21.         <h2>Flowers</h2>  
  22.         @ListItems(ViewBag.Flowers)  
  23.     </div>  
  24. </div>  
We will use the same example project that we created previously for the inline helper. Now to display the list of fruits and flowers using the custom external helper method, create a new folder called "Helpers" and add a new file called "ExternalHelper.cs" within it. Add the following code to that file.
  1. namespace CustomInlineHelper.Helpers  
  2. {  
  3.     public static class ExternalHelper  
  4.     {  
  5.         public static MvcHtmlString ListItems(this HtmlHelper htmlHelper, string[] items)  
  6.         {  
  7.             var ulElement=new TagBuilder("ul");  
  8.             foreach (var item in items)  
  9.             {  
  10.                 var liElement=new TagBuilder("li");  
  11.                 liElement.SetInnerText(item);  
  12.                 ulElement.InnerHtml += liElement.ToString();  
  13.             }  
  14.             return new MvcHtmlString(ulElement.ToString());  
  15.         }  
  16.     }  
  17. }  
In the preceding code, we have created a custom external helper method called "ListItems". It takes the array of strings as a parameter and generates the HTML of ul element containing the li element for each string in the array. It has two parameters, the first one is a HtmlHelper object and the second one is an array of strings. The first HtmlHelper object parameter with this keyword specifies that this is an extension method for the HtmlHelper object.

In our helper method, we use the TagBuilder class to generate the HTML elements. This class is available in the System.Web.Mvc namespace. To generate the HTML element, we need to create an instance of the TagBuilder class and pass the name of the HTML element that we want to construct as a constructor parameter. For example, we can create the ul element using the TagBuilder class as shown below.
  1. var ulElement=new TagBuilder("ul");  
The following arew useful methods/properties of the TagBuilder class:


  • AddCssClass(string): Add a CSS class to the HTML Element.
  • GenerateId(string): Generates an id for the HTML element. The string parameter specifies the id for the HTML element. This method replaces the periods in the id.
  • MergeAttribute(String, String, Boolean): Adds an attribute to the HTML element. The first parameter specifies the name of the attribute and the second parameter specifies the value of the attribute. The third boolean parameter specifies whether it should be replaced if the same attribute already exists.
  • SetInnerText(string): Set the text content of the HTML element.
  • InnerHtml: This property allows us to nest the HTML elements. (In our example, we have used this property to add the li element under the ul element).

The result of our helper method is the MvcHtmlString object, the content of which are directly written to the response. It takes the HTML markup that we generated using the TagBuilder class as a parameter and encodes that string so that it is safe to display. Now let`s update the index view to use our external helper method. The content of the updated index view is as shown below.

  1. @using CustomInlineHelper.Helpers  
  2. @{  
  3.     ViewBag.Title = "Home Page";  
  4. }  
  7. <!--Custom Inline Helper Method-->  
  8. <div class="row">  
  9.     <div class="col-md-6">  
  10.         <h2>Fruits</h2>  
  11.         @GlobalHelper.ListItems(ViewBag.Fruits)  
  12.     </div>  
  13.     <div class="col-md-6">  
  14.         <h2>Flowers</h2>  
  15.         @GlobalHelper.ListItems(ViewBag.Flowers)  
  16.     </div>  
  17. </div>  
  19. <!--Custom External Helper Method-->  
  20. <div class="row">  
  21.     <div class="col-md-6">  
  22.         <h2>Fruits</h2>  
  23.         @Html.ListItems((string[])ViewBag.Fruits)  
  24.     </div>  
  25.     <div class="col-md-6">  
  26.         <h2>Flowers</h2>  
  27.         @Html.ListItems((string[])ViewBag.Flowers)  
  28.     </div>  
  29. </div>  
There are the following two important points to note in the preceding content of the index view.


  • We need to add the namespace for the class containing our helper method at the top of the view to use the helper method (in our case, the namespace is CustomInlineHelper.Helpers).
  • We need to cast the ViewBag dynamic property parameter to the type we have defined in our external helper method. (in our case, the type of parameter is an array of strings).

There is an alternative solution for adding the namespace for our helper method. We can add that namespace to the Views/web.config file. So that our helper method will always be available for use and we don`t need to add the using statement in each and every view. The content of the Views/web.config file after adding the namespace is as shown below (as in Figure 1).

Add namespace
Add the namespace to the Views/web.config file (as in Figure 1).


We can encapsulate the complex code in an external helper method (extension method) and use it in multiple views. This also makes our view simpler and more readable. We can also create a class library project and include these external helper methods in that project. So that next time if we need the same functionality we just need to add the reference to our class library project. I hope this helps you.
I hope you enjoyed reading the article.

Happy Coding!