Case Insensitive Compare TextBox in ASP.Net MVC

There is a Textbox in my Web Page. I want that when user types anything in that Textbox, it should not be case sensitive. i.e., user types "Add" or "ADD" or "AdD" any of them inside Textbox, it should do the Addition no matter how text is written inside it. I am using Asp.Net MVC Web Form. "Views" Code is:

<li>
     @Html.LabelFor(m => m.Action)
     @Html.TextBoxFor(m => m.Action)
     @Html.ValidationMessageFor(m => m.Action)
</li>

Please reply how to do this in Asp.Net MVC4.