Top Level Static - What is It

I've tried the method you supplied in the comments of SA's solution, and apart from the incorrect variable (I assume "name" is a typo and should be "classname", or vice versa) it compiles fine as an extension method.

It sounds like you're just trying to put it in the wrong place.

I would strongly suggest that you shouldn't put this class in the same file as a designer based class. Indeed, good practice is to define only one class per file - so add another file and put this class in it - e.g. Extended.cs (same name as your class, although I think you should come up with a better name for the class). Although it doesn't have to be in a namespace to compile, it should be - just put it in your application's namespace.

Regards,
Ian.

Updated Example:

HtmlDocumentExtensions.cs

using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    static class HtmlDocumentExtensions
    {
        public static HtmlElement GetElementByName(this HtmlDocument document, string classname)
        {
            foreach (HtmlElement element in document.All)
            {
                if (element.GetAttribute("name").Contains(classname))
                {
                    return element;
                }
            }
            return null;
        }
    }
}

Somewhere else in the code, in this case in my test form, Form1.cs

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            HtmlDocument doc = null; // Do whatever you need to do to load the document...
            //...
            HtmlElement element = doc.GetElementByName("The name");
        }
    }
}