error CS0246: The type or namespace name ‘XXX‘ could not be found

error CS0246: The type or namespace name ‘XXX‘ could not be found

报错内容：error CS0246: The type or namespace name 'XXX' could not be found (are you missing a using directive or an assembly reference?)

(1)报错场景：
测试Unity打包AssetBundle的API，部分代码如下：
```
    [MenuItem("Game/StartBuild")]
    static void StartBuild()
    {
        StartBuildMenu();
    }   

    public static void StartBuildMenu()
    {
        var buildList = new List<AssetBundleBuild>();

        var assetPath = AssetsDir;
        if (Directory.Exists(assetPath))
        {
            // 1.取得所有需要打包的资源
            DirectoryInfo di = new DirectoryInfo(assetPath);
            var filters = fileFilter.Split('|');
            var files = filters.SelectMany(filterChar =>
            {
                var ret = di.GetFiles(filterChar, SearchOption.AllDirectories);
                return ret;
            }).ToList();
            // var assetPaths = fileFilter.Split('|').SelectMany(filterChar =>
            //     di.GetFiles(filterChar, SearchOption.AllDirectories)).ToArray();

            // 2.创建打包对象
            foreach (var file in files)
            {
                AssetBundleBuild build = new AssetBundleBuild();
                build.assetBundleName = GetFileAbName(file.FullName);
                var start = file.FullName.IndexOf("Assets");
                var assetName = file.FullName.Substring(start);
                build.assetNames = new string[] {assetName};
                buildList.Add(build);
            }

            if (!Directory.Exists(ABSaveDir))
                Directory.CreateDirectory(ABSaveDir);
            BuildPipeline.BuildAssetBundles(ABSaveDir, buildList.ToArray(), BuildAssetBundleOptions.None, BuildTarget.Android);
        }
    }
```

执行StartBuildMenu后报错：
error CS0246: The type or namespace name 'MenuItemAttribute' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'MenuItem' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'AssetBundleBuild' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'BuildAssetBundleOptions' could not be found (are you missing a using directive or an assembly reference?)

(2)解析错误日志：
报错都是提示缺少 using 指令或程序集引用，并且发现缺少引用的都是UnityEditor下的类型

(3)解决办法：
在unity中所有使用UnityEditor命名空间的脚本，必须放到Assets目录下的Editor文件夹中，这个Editor文件夹可以直接放到Assets目录下的任何地方，只要名字是Editor就可以。