How to write extension in .NET

Extension.cs

// Example of extensions for IWebElement
using System.Collections.ObjectModel;

namespace OpenQA.Selenium.Extensions
{
    public static class WebElement
    {
        public static void ClearAndSendKeys(this IWebElement webElement, string keys)
        {
            webElement.Clear();
            webElement.SendKeys(keys);
        }

        public static IWebElement FindElementByCss(this IWebElement we, string css)
            => we.FindElement(By.CssSelector(css));

        public static ReadOnlyCollection<IWebElement> FindElementsByCss(this IWebElement we, string css)
            => we.FindElements(By.CssSelector(css));

        public static void Mark(this IWebElement we) => MarkElement(we, true);
        public static void Unmark(this IWebElement we) => MarkElement(we, false);

        private static void MarkElement(IWebElement we, bool isChecked)
        {
            if (we.Selected != isChecked)
                we.Click();
        }

        public static void Mark(this ReadOnlyCollection<IWebElement> e) => MarkElements(e, true);
        public static void Unmark(this ReadOnlyCollection<IWebElement> e) => MarkElements(e, false);

        private static void MarkElements(ReadOnlyCollection<IWebElement> elements, bool isChecked)
        {
            foreach (var element in elements)
            {
                MarkElement(element, isChecked);
            }
        }
    }
}