Unique instance using Singleton pattern

For my recent project which my colleague was in charge, I faced a question on How to restrict a class from creating multiple instances.
After five minutes of R&D I came across a word "Singleton". I referred a bulky book from shelf and rolled down until this word signalled my brain "Singleton Pattern".

1) What is a singleton pattern? 
2) Why is it used for? 
3) How to implement?

1) What is singleton pattern?
A: In software engineering this is a design pattern used to restrict instantiation of a class to one object. In another point of view, we make one and only one instance of a class during runtime. The same instance will be resused throughout the life cycle untile exclusively destroyed.

2) Why is it used for?
A: Depends on the requirement.

In my application the singleton was used to keep only one instance until EOD(End Of Day). For which I used a static (C#.Net)/shared(VB.Net) method instead of giving access to constructor.

eg: If you want all the users to use the only instance of a class for all purposes(transactions in my case), this is the right choice.

3) How to Implement?

A: Remember the following points 

1. Declare the default and all overridden constructors with private access specifier.
2. Create a static(C#.Net)/shared(VB.Net) function which will take care of instantiation of a new object.
3. You use only the static method to create a new instance, use of keyword 'new' will be restricted in this case.

SOURCE CODE AND DESIGN

In the windows form there are two buttons

a) Get : Calls the unique instance method.

b) Destroy : Destroy the unique instance created for a new creation.

My Singleton class

public class SingletonClass

    {

        private static SingletonClass MySingletonClass;

        

        string _szValue = "";

        //use this if you have any custom constructor

        private SingletonClass(string szValue)

        {

            _szValue = szValue;

        }

        //This is how you restrict an instance is created outside this class using keyword NEW

        private SingletonClass()

        {

        }

        public string szDate

        {

            get;

            set;

        }

        ///The function handles creation of a new instance of this class.

        ///A new instance is created only if there is no previous instances created.

        public static SingletonClass UniqueInstance(string szValue)

        {

            if (MySingletonClass == null)

            {

                MySingletonClass = new SingletonClass(szValue);

                MySingletonClass.szDate = DateTime.Now.ToString();

            }

            return MySingletonClass;

        }

        public static void DestroyInstance()

        {

            MySingletonClass = null;

        }

    }

Form1 code behind

        ///Button click event of Get.

        ///Returns date at which instance was initially created. 

        private void btnGet_Click(object sender, EventArgs e)

        {

            ///The most important point is this.

            ///Instead of declaring it using keyword new, you call the function.

            ///ie, we are avoiding SingletonClass mysingleton = new SingletonClass() 

            SingletonClass mysingleton = SingletonClass.UniqueInstance("hello");

            MessageBox.Show(mysingleton.szDate);

        }

        private void btnDestroy_Click(object sender, EventArgs e)

        {

            SingletonClass.DestroyInstance();

        }