To define a method within a razor (cshtml) file. Very useful for recursive rendering of data trees.

@model ListOfItemsViewModel

@if (Model.Items.Any())
{
    <ul>
    @foreach(var item in Model.Items)
    {
        @RenderItem(item);
    }
    </ul>
}

@helper RenderItem(Item item)
{
    @MvcHtmlString.Create(string.Format"<li>{0}", item.Name)
    if (item.Children.Any())
    {
        @MvcHtmlString.Create("<ul>")
        foreach(var childItem in itemChildren)
        {
            @RenderItem(childItem);
        }
        @MvcHtmlString.Create("</ul>")
    }
    @MvcHtmlString.Create("</li>");
}