generic version of thread safe singletons based on: https://csharpindepth.com/Articles/Singleton

singleton.cs

    // ---------------------------------------------------------------------------------------------
    //                               SOME UTILY CLASSES
    // ---------------------------------------------------------------------------------------------
    public abstract class ThreadSafeSingleton<T> where T : ThreadSafeSingleton<T>, new()
    {
        static ThreadSafeSingleton(){}
        protected ThreadSafeSingleton() { }
        public static T Instance { get; } = new T();
    }

    public abstract class LazyThreadSafeSingleton<T> where T : LazyThreadSafeSingleton<T>, new()
    {
        protected static readonly Lazy<T> lazy = new Lazy<T>(() => new T());
        protected LazyThreadSafeSingleton() { }
        public static T Instance => lazy.Value;
    }

    // ---------------------------------------------------------------------------------------------
    //                               USAGE EXAMPLE
    // ---------------------------------------------------------------------------------------------
    public sealed class STestClass1 : ThreadSafeSingleton<STestClass1>
    {
        public readonly int i = 3;
    }

    public sealed class STestClass2 : LazyThreadSafeSingleton<STestClass2>
    {
        public readonly int i = 4;
    }

    class SomeClass { 
        static void Main(string[] args)
        {
            Console.WriteLine(STestClass1.Instance.i);
            Console.WriteLine(STestClass2.Instance.i);
        }
    }
    // ---------------------------------------------------------------------------------------------