How to use this Singleton Class in C#?

Question

I am struggling with using Singleton design pattern. I am trying to use it in this simple console application. I have a problem with it in the Main method in Program class. I want to define object from the Singleton class such as: var data = Singleton.Instance; but I don't know why I can't do that Also, I don't know why I am getting the following error message when I run the program:

Unhandled Exception: System.NullRefernceException: Object reference not 
  set to an instance of an object.

So how to fix that?
Singleton Class:

namespace Singleton
{
    class Singleton
    {
        //Variable
        private static Singleton instance;
        private List<string> Messages;
        //Constructor
        private Singleton() { }
        //Property
        public static Singleton Instance
        {
            get 
            {
                if (instance == null)
                {
                    instance = new Singleton();
                }
                return instance;
            }
        }
        //Methods
        public void Message(string message)
        {
            Messages.Add(message);
        }

        public bool HasMessage(string message)
        {
            return Messages.Contains(message);
        }
    }
}

Program Class:

namespace Singleton
{
    class Program
{
    static void Main(string[] args)
    {
        var data = Singleton.Instance;
        Singleton.Instance.Message("Hello World!!!");
        if(Singleton.Instance.HasMessage("12"))
            Console.WriteLine("NO STRING!!!");
        else
            Console.WriteLine("There is a match");

    }
}
}

UPDATE:

Guys, I really appreciate your help so far. The program is working now but the logic is not working. If you look at the main program, you will see that the list only has "Hello World!!!". However, when I used the HasMessage method doesn't work. Because the program keeps showing "There is a match". But it should show me "NO STRING!!!" as there is no match. So how to fix that?

Answer 1

Your field Messages is not initialized to anything. That is why you are getting the exception. In your class do:

 private List<string> Messages = new List<string>();

You may also look at Thread Safe Singleton implementation by Jon Skeet

EDIT:

Based on the updated question. Your Check and Message are opposite. It should be:

if (Singleton.Instance.HasMessage("12"))
    Console.WriteLine("There is a match");
else
    Console.WriteLine("NO STRING!!!");

Your method HasMessage returns true if the passed parameter is present in the list and false otherwise.

Answer 2

It looks like you're almost there. Consider rewriting your code like this:

class Singleton
{
    //Variable
    private static Singleton Instance;
    private List<string> Messages;
    //Constructor
    private Singleton() 
    {
     Messages = new List<string>(); //Make sure to instantiate instance types before use. 
    }
    //Property
    public static Singleton GetInstance()
    {
            if (Instance == null)
            {
                Instance = new Singleton();
            }
            return Instance;

    }
    //Methods
    public void Message(string message)
    {
        Messages.Add(message);
    }

    public bool HasMessage(string message)
    {
        return Messages.Contains(message);
    }
}

There are some helpful C# tutorials for design patterns on this site.