错误：名称＆＃39;任务＆＃39;在当前上下文中不存在，同时遵循unity firebase存储文档

Question

关键是，我尝试按照firebase文档使用统一存储。专门用于上传图像文件。 https://firebase.google.com/docs/storage/unity/upload-files

这是我的代码

// Get a reference to the storage service, using the default Firebase App
Firebase.Storage.FirebaseStorage storage = Firebase.Storage.FirebaseStorage.DefaultInstance;

// Create a storage reference from our storage service
Firebase.Storage.StorageReference storage_ref = storage.GetReferenceFromUrl("gs://AR-Festival-2018.appspot.com");


// Data in memory
byte[] custom_bytes = SaveLoad.imageTexture[activeIndex].EncodeToPNG();

// Create a reference to the file you want to upload
Firebase.Storage.StorageReference rivers_ref = storage_ref.Child("images/rivers.jpg");

// Upload the file to the path "images/rivers.jpg"
rivers_ref.PutBytesAsync(custom_bytes)
  .ContinueWith ((Task<StorageMetadata> task) => {
    if (task.IsFaulted || task.IsCanceled) {
        Debug.Log(task.Exception.ToString());
        // Uh-oh, an error occurred!
    } else {
        // Metadata contains file metadata such as size, content-type, and download URL.
        Firebase.Storage.StorageMetadata metadata = task.Result;
        string download_url = metadata.DownloadUrl.ToString();
        Debug.Log("Finished uploading...");
        Debug.Log("download url = " + download_url);
    }
});

简单地复制粘贴它会导致术语“任务”的结果。和＆＃39; StorageMetadata＆＃39;消息不存在： 名称＆＃39;任务＆＃39;在当前上下文中不存在

我错过了什么吗？我需要包含任何统一包以使任务工作吗？ 我在c＃

中使用了团结2017.3.1

更新：

在Unity Log中，我收到此错误，

Assets / AR Part / Script / Gallery.cs（218,20）：错误CS0246：类型或命名空间名称＆＃39;任务＆＃39;无法找到。你错过了`System.Threading.Tasks＆＃39;使用指令？