Generic singleton class initialization for Non MonoBehaviours

Singleton.cs

using System;

/// <summary>
/// Non Monobehaviour Singleton Instance
/// </summary>
/// <typeparam name="T">Class to be Singleton</typeparam>
public class Singleton<T> where T : new()
{

    private static Lazy<T> lazyInstance =
            new Lazy<T>(() => new T());

    public static T Instance
    {
        get
        {
            return lazyInstance.Value;
        }
    }
}

For MonoSingleton check this: https://gist.github.com/brenocogu/41079ee0d093c3ad65dd67859a548e9f