CSharp - Extension method usage

/*

Author: Jiangong SUN

*/

Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. 
Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type.

The most common extension methods are in LINQ; Linq provides two classes: Enumerable and Queryable. And their extensions methods operate on IEnumerable<T> and IQueryable<T>.

Extension methods are static methods who reside in static utility classes.

For example:

        public static bool IsNull(this object x)
        {
            return x == null;
        }
        public static bool IsNullOrEmpty(this object text)
        {
            return text == null || (string)text == "";
        }
        public static IEnumerable<int> GetOddMembers(this List<int> list)
        {
            return list.Where(x => x%2 != 0);
        }

            object y = null;
            Console.WriteLine(y.IsNull()); //True
            y = new object();
            Console.WriteLine(y.IsNull()); //False

            object s = null;
            Console.WriteLine(s.IsNullOrEmpty()); //True
            s = string.Empty;
            Console.WriteLine(s.IsNullOrEmpty()); //True
            s = "hello";
            Console.WriteLine(s.IsNullOrEmpty()); //False
            
            Console.WriteLine(IsObjectNull.IsNullOrEmpty(null)); //True

            List<int> list = new List<int>() {1, 2, 5, 7, 12};
            var oddMembers = list.GetOddMembers();
            foreach (var oddMember in oddMembers)
            {
                Console.WriteLine(oddMember); //1 5 7
            }

I Hope this can do help to you! Enjoy coding!