Program to write a Singleton Pattern using Nested Class in C#

The below program will create a Singleton Pattern using Nested Class concept in C#

namespace ConsoleApplication1
{
    public sealed class Singleton
    {
        private static Singleton instance = null;
        private static readonly object lockObj = new object();

        Singleton(){} //private constructor

        public static Singleton Instance
        {
            get
            {
                lock (lockObj)
                {
                    if (System.Object.ReferenceEquals(instance, null))
                    {
                        instance = Inner.Instance;
                    }
                    return instance;
                }
            }
        }

        class Inner
        {            
            Inner(){ }//private constructor
           
            public static Singleton Instance
            {
                get
                {
                    return new Singleton();
                }
            }
        }
    }
}