How do I define a method in Razor?

It's very simple to define a function inside razor.

@functions {

    public static HtmlString OrderedList(IEnumerable<string> items)
    { }
}

So you can call a the function anywhere. Like

@Functions.OrderedList(new[] { "Blue", "Red", "Green" })

However, this same work can be done through helper too. As an example

@helper OrderedList(IEnumerable<string> items){
    <ol>
        @foreach(var item in items){
            <li>@item</li>
        }
    </ol>
}

So what is the difference?? According to this previous post both @helpers and @functions do share one thing in common - they make code reuse a possibility within Web Pages. They also share another thing in common - they look the same at first glance, which is what might cause a bit of confusion about their roles. However, they are not the same. In essence, a helper is a reusable snippet of Razor sytnax exposed as a method, and is intended for rendering HTML to the browser, whereas a function is static utility method that can be called from anywhere within your Web Pages application. The return type for a helper is always HelperResult, whereas the return type for a function is whatever you want it to be.

You could also do it with a Func like this

@{
    var getStyle = new Func<int, int, string>((width, margin) => string.Format("width: {0}px; margin: {1}px;", width, margin));
}

<div style="@getStyle(50, 2)"></div>

Leaving alone any debates over when (if ever) it should be done, @functions is how you do it.

@functions {

    // Add code here.

}

You mean inline helper?

@helper SayHello(string name)
{
    <div>Hello @name</div>
}

@SayHello("John")