using System;

namespace DTS.Common.Classes
{
    /// <summary>
    /// This is the generic for singletons.
    /// </summary>
    /// <typeparam name="T">Type of derived Singleton object (i.e. class MySingletone: Singleton&lt;MySingleton&gt;).</typeparam>
    public class Singleton<T> where T : new()
    {
        /// <summary>
        /// This is a protected constructor.
        /// </summary>
        protected Singleton()
        {
            if (!ReferenceEquals(Instance, null))
                throw new InvalidOperationException("You have tried to create a new singleton class where you should have instanced it. Replace your \"new class()\" with \"class.Instance\"");
        }

        /// <summary>
        /// Gets the singleton instance of type T.
        /// </summary>
        public static T Instance
        {
            get
            {
                if (SingletonCreator.exception != null)
                    throw SingletonCreator.exception;

                return SingletonCreator.instance;
            }
        }

        /// <summary>
        /// Creates the singleton instance of type T.
        /// </summary>
        // ReSharper disable once ClassNeverInstantiated.Local
        private class SingletonCreator
        {
            internal static readonly T instance;
            internal static readonly Exception exception;

            /// <summary>
            /// Creates the singleton instance of type T.
            /// </summary>
            static SingletonCreator()
            {
                try
                {
                    instance = new T();
                }
                catch (Exception ex)
                {
                    exception = ex.InnerException ?? ex;
                    Console.WriteLine(@"Singleton: {0}", exception);
                }
            }
        }
    }
}