Why does my JsonUtility deserialization break in builds but not editor?

IL2CPP Managed Stripping Level removes types and members not statically referenced. Reflection-based deserialization touches members the linker can't see; they're stripped. Editor doesn't strip; build does. Add a link.xml or [Preserve] attribute.

What does Managed Stripping Level Low/Medium/High mean?

Low strips only obvious unused code. Medium strips more aggressively. High strips everything not statically reachable. Default for IL2CPP is Low; if you set Medium/High you must preserve reflection-touched types yourself.

Where does link.xml go?

Anywhere in Assets. Unity's build pipeline picks up all link.xml files and merges them. Common location: Assets/link.xml at the project root.

Fix: Unity IL2CPP Strip Engine Code Breaks Reflection

Quick answer: Add an Assets/link.xml with explicit preserve entries for assemblies and types your code reflects on. Apply [Preserve] to specific reflected members. Lower Managed Stripping Level to Low if you can’t enumerate every reflected type.

JsonUtility.FromJson works fine in Editor. IL2CPP build returns an object with all default values. The fields’ setters were stripped because the linker only sees them touched by reflection.

The Symptom

Reflection-based code (JSON deserialization, Newtonsoft, custom inspector serialization, dependency injection) returns null/default values in IL2CPP builds. Editor and Mono builds work. Errors may say “TypeLoadException” or be silent.

The Fix

Pattern 1: link.xml preserve.

<!-- Assets/link.xml -->
<linker>
  <assembly fullname="YourGame">
    <type fullname="Game.Save.SaveData" preserve="all" />
    <type fullname="Game.Save.*" preserve="all" />
  </assembly>
  <assembly fullname="Newtonsoft.Json" preserve="all" />
</linker>

preserve=“all” keeps types, fields, methods, properties. Use sparingly — preserving everything balloons the build.

Pattern 2: Preserve attribute.

using UnityEngine.Scripting;

[Preserve]
[Serializable]
public class SaveData
{
    [Preserve] public int level;
    [Preserve] public string playerName;
}

Per-class. Useful for one-offs.

Pattern 3: Lower stripping level. Player Settings → Other Settings → Managed Stripping Level → Low. Build is bigger but reflection works without explicit preserves.

JsonUtility Specific

JsonUtility uses [SerializeField] members. If those members are also marked [Preserve] (or the type is in link.xml), the linker keeps them. Without preserve, [SerializeField] alone isn’t enough at High stripping level.

Verifying

Build IL2CPP. Run on device. Reflection code should produce expected values. If something is still missing, the build log lists stripped types in the IL2CPP step (verbose build report).

“link.xml. [Preserve]. Or lower Stripping. Reflection survives.”

Related Issues

For asmdef circular, see asmdef. For Burst BC1006, see Burst.

Preserve what reflection touches. Builds work.