SerializableReadonlyStruct

Warning

This package is currently very experimental.

An IL Post-processor for Unity to make readonly structs serializable.

readonly fields cannot be serialized by default in Unity, but this package allows you to serialize readonly fields in structs.

using System;
using UnityEngine;
using SerializableReadonlyStruct;

public class Example : MonoBehaviour
{
    [SerializeField] private S s;
}

[Serializable, SerializableReadonly]
public readonly struct S
{
    [SerializeField] private readonly int a;
}

Installation

Add git URL to Package Manager:

https://github.com/ruccho/SerializableReadonlyStruct.git?path=/Packages/com.ruccho.serializable-readonly-struct

Compatibility

Actually, [SerializableReadonly] just removes the readonly keyword from the struct definition from the compiled DLLs. readonly is an annotation only for the compiler and the runtime does not care about it (unless the metadata is used by reflection). The effect of readonly is still valid in the compiled code.

If [SerializableReadonly] is used in precompiled assemblies (e.g. libraries), other csprojs will reference the compiled assembly which has no readonly keyword. This means the fields seem to be mutable on IDE but actually immutable in the compilation.

#region Assembly-CSharp.dll

public class Example : MonoBehaviour
{
    [SerializeField] private S s;
    
    private void Start()
    {
        s.a = 100; // This is legal on the IDE, but cause a compilation error on Unity
    }
}

#endregion

// If SomeLibrary.dll doesn't have Unity-generated csproj, it will precompiled and referenced by Assembly-CSharp.dll
#region SomeLibrary.dll

[Serializable, SerializableReadonly]
public readonly struct S
{
    public readonly int a;
}

#endregion