直接上測試代碼：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class TestStructGC : MonoBehaviour
{
    public struct StructDef
    {
        public System.Action act;

        public StructDef(System.Action callback)
        {
            act = callback;
        }
    }

    
 
public int memberValue;

    private void Update()
    {
        // 使用匿名函數，不訪問外部函數，0 gc
        {
            UnityEngine.Profiling.Profiler.BeginSample("*** Test1 ***");
            StructDef obj = new StructDef(null);
            UnityEngine.Profiling.Profiler.EndSample();
        }

        
 
// 使用匿名函數，訪問臨時變量，112B gc
        {
            int tmp = 1;
            UnityEngine.Profiling.Profiler.BeginSample("*** Test2 ***");
            StructDef obj = new StructDef(() => { var v = tmp; });
            UnityEngine.Profiling.Profiler.EndSample();
        }

        // 使用匿名函數，訪問外部變量，112B gc
 

        {
            UnityEngine.Profiling.Profiler.BeginSample("*** Test3 ***");
            StructDef obj = new StructDef(() =>
            {
                Debug.LogError(memberValue);
            });
            UnityEngine.Profiling.Profiler.EndSample();
        }

        // 不使用匿名函數，0 gc
        {
            UnityEngine.Profiling.Profiler.BeginSample("*** Test4 ***");
            StructDef obj = new StructDef(actCallback);
            UnityEngine.Profiling.Profiler.EndSample();
        }
    }

    #region optimize for test4
    private System.Action actCallback;

    public TestStructGC()
    {
        actCallback = LocalCallback;
    }

    private void LocalCallback()
    {
        Debug.LogError(memberValue);
    }
    #endregion
}

參考：https://blog.uwa4d.com/archives/Anonymous_heapmemory.html

匿名函數訪問外部變量有gc