UnityElementInitialization.cs

// UnityElementInitialization.cs
//
using System;
using UnityEngine;

// In Unity, new struct or class elements in a serialized list do not
// get a chance to run static initializers or constructors.  All members
// are given their default(T) values instead.  This is an example of how
// to use the OnValidate() method to support initialization.

public class MyBehaviour: MonoBehaviour
{
    public List<Thingy> thingies = new();

    [Serializable]
    public class Thingy
    {
        public string name;
        public float threshold;
        [HideInInspector] public bool __init = false;
        public void OnValidate()
        {
            // fix invalid values
            if (threshold < 0f)
                threshold = 0f;
            // defaults set once
            if (__init) return; __init = true;
            if (threshold == 0f)
                threshold = 0.5f;
        }
    }

    public void OnValidate()
    {
        if (thingies != null)
            foreach (var thingy in thingies)
                thingy?.OnValidate();
    }
}