singleton pattern C# example

Program.cs

class Singleton
{
    private static Singleton instance;
    private int value;

    private Singleton()
    {
        value = 50;
    }

    public static Singleton GetInstance()
    {
        return instance ?? (instance = new Singleton());
    }

    public int GetValue()
    {
        return value;
    }

    public void SetValue(int value)
    {
        this.value = value;
    }
}

class Program
{
    static void Main()
    {
        var instance = Singleton.GetInstance();
        Console.WriteLine(instance.GetValue());

        instance.SetValue(75);
        Console.WriteLine(instance.GetValue());
    }
}