Singleton for MonoBehaviour

SingletonPattern.cs

public abstract class SingletonPattern<T> : MonoBehaviour where T : SingletonPattern<T>
{
    public static T Instance;

    protected virtual void Awake()
    {
        if (Instance != null)
        {
            Destroy(this);
        }
        else
        {
            Instance = this as T;
        }
    }
}

// then in some singleton/manager class

public class UIManager : SingletonPattern<UIManager>
{
    protected override void Awake()
    {
        base.Awake();
    }
}