Usage of Extension Methods

instance.SomeExtensionMethod()

StaticExtensionMethodClass.SomeExtensionMethod(instance);

public class extended {
    public int sum() {
        return 7+3+2;
    }
}

public static class extending {
    public static float average(this extended extnd) {
        return extnd.sum() / 3;
    }
}

extended ex = new extended();

Console.WriteLine(ex.average());

public static void ThrowIfNull<T>(this T obj, string name) where T : class
{
    if(obj == null) throw new ArgumentNullException(name);
}

var foo = source.Where(predicate).OrderBy(selector);

var foo = Enumerable.OrderBy(Enumerable.Where(source,predicate),selector);

ExtendUnlimitedCredit(AddVIP(tblCustomer, "Bill Gates"))

tblCustomer.AddVIP("Bill Gates").ExtendUnlimitedCredit()

public class Stock {
   public Code { get; private set; }
   public Name { get; private set; }
}

public static class StockExtender {
    public static List <Quote> GetQuotesByDate(this Stock s, DateTime date)
    {...}
}