PreserveAttribute

Description

PreserveAttribute prevents byte code stripping from removing a class, method, field, or property.

When you create a build, Unity will try to strip unused code from your project. This is great to get small builds. However, sometimes you want some code to not be stripped, even if it looks like it is not used. This can happen for instance if you use reflection to call a method, or instantiate an object of a certain class. You can apply the [Preserve] attribute to classes, methods, fields and properties. In addition to using PreserveAttribute, you can also use the traditional method of a link.xml file to tell the linker to not remove things. PreserveAttribute only works on IL2CPP platforms. The link.xml file works for Mono and IL2CPP platforms.

#pragma strict

import UnityEngine;
import UnityEngine.Scripting;
import System.Reflection;

public class NewBehaviourScript extends MonoBehaviour {
    function Start() {
        ReflectionExample.InvokeBoinkByReflection();
    }
}
public class ReflectionExample {
    public static function InvokeBoinkByReflection() {
        typeof(ReflectionExample).GetMethod("Boink", BindingFlags.NonPublic | BindingFlags.Static).Invoke(null, null);
    }
    // No other code directly references the Boink method, so when when stripping is enabled,
    // it will be removed unless the [Preserve] attribute is applied.
    @Preserve
    static function Boink() {
        Debug.Log("Boink");
    }
}

using UnityEngine;
using System.Collections;
using System.Reflection;
using UnityEngine.Scripting;

public class NewBehaviourScript : MonoBehaviour
{
    void Start()
    {
        ReflectionExample.InvokeBoinkByReflection();
    }
}

public class ReflectionExample
{
    static public void InvokeBoinkByReflection()
    {
        typeof(ReflectionExample).GetMethod("Boink", BindingFlags.NonPublic | BindingFlags.Static).Invoke(null, null);
    }

    // No other code directly references the Boink method, so when when stripping is enabled,
    // it will be removed unless the [Preserve] attribute is applied.
    [Preserve]
    static void Boink()
    {
        Debug.Log("Boink");
    }
}

For 3rd party libraries that do not want to take on a dependency on UnityEngine.dll, it is also possible to define their own PreserveAttribute. The code stripper will respect that too, and it will consider any attribute with the exact name "PreserveAtribute" as a reason not to strip the thing it is applied on, regardless of the namespace or assembly of the attribute.