Unity: Static reference disappears after domain reload (editor reload)

Question

I have a C# script that boils down to this:

public class SingletonTest : MonoBehaviour
{
    public static SingletonTest Singleton = null;
    public int Value = 42;

    void Start() {
        Singleton = this;
    }
}

This runs fine at first, but my issue is that when I edit a script and then click back into the Unity editor/IDE, I get a whole string of NullReferenceExceptions for the line SingletonTest.Singleton.Value in some other class.

I googled around a bit and this editor reload seems to trigger a process called Domain Reloading (see also). Apparently domain reloading also resets all static fields which explains my error message. I tried a few workarounds suggested on that page but none of them work:

using UnityEngine;

public class SingletonTest : MonoBehaviour
{
    public static SingletonTest Singleton = null;
    public int Value = 42;

    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.SubsystemRegistration)] 
    static void StaticStart()
    {
        var arr = FindObjectsOfType<SingletonTest>();
        Debug.Log($"Len arr: {arr.Length}");  // This is 0! :(
        if (arr.Length > 0)
            Singleton = arr[0];
    }

    void Start()
    {
        Singleton = this;
    }

    void OnAfterDeserialize()
    {
        Singleton = this;
    }
}

I am able to kind-of get it to work by putting Singleton = this in the Update() function but that solution is ugly, and still gives me a NullReferenceException on the first frame after reload.

(Please don't suggest making the Value field static, that is just a simplified example, I do need/want a singleton class)

Answer 1

Consider using a private constructor to force the static field to be initialized with a value when the SingletonTest object is created by the CLR.

Although Unity normally doesn't recommend using constructors with MonoBehvior because they're supposed to be scripts(among other Mono and Unity Quirks). I found this works great for my use cases for singletons(such as static editor Dictionary<T>s that hold loaded metadata etc...

public class SingletonTest : MonoBehaviour
{
    public static SingletonTest Singleton { get; set; } = null;

    public int Value = 42;

    protected SingletonTest()
    {
        Singleton ??= this;
    }
}

Alternatively consider avoiding the assumption that the given field/property is never null.

For example:

void Awake()
{
     // call method example ↓ (null-coalescing operator)
     SingletonTest.Singleton?.Invoke("Something");
     
     // property example
     if(SingletonTest.Singleton != null)
     {
         Debug.Log($"{SingletonTest.Singleton.gameObject.Name}");
     }
}