在 for 循环中使用 await 作为异步函数

Question

我正在通过 MS Graph 运行一堆转换为 pdf 的 api 调用，然后将 base64 编码的字符串返回到前端以在 pdf.js 中进行处理。 但是，我想通过添加 async 和 await 来加快速度，以便在最短的时间内执行尽可能多的 api 调用（不在 MS Graph 中使用批处理）。 但是，尚不清楚此代码是通过 for 循环循环到下一个调用还是单独等待我不想要的每个 await 结果。 我想为每个请求继续循环，然后在所有调用完成后返回最终结果。

        public async Task<ActionResult> PDFVersionsToBase64([FromForm] string ID)
    {
        //Using FormData on frontend
        //checking ID exists on searching in dashboard
        if (String.IsNullOrEmpty(ID))
        {
            return Ok(new { Result = "Failed" });
        }
        else
        {

            String query = "select * from dbo.function(@ID)";
            using (var connection = new SqlConnection(connectionString))
            {
                var documents = connection.Query<QuestionDocuments>(query, new { ID = ID});

                foreach (var file in documents)
                {
                    var fullPath = Path.Combine(fileDirectory, file.path);
                    using (var memoryStream = await oneDrive.DriveItemConvertToPdfAsync(SharedDriveID, fullPath, "path"))
                    {
                        byte[] buffer = new byte[memoryStream.Length];
                        memoryStream.Read(buffer, 0, (int)memoryStream.Length);
                         file.Base64 = Convert.ToBase64String(buffer);
                    }
                }

                return Ok(documents);

            }
        }
    }

        public async Task<Stream> DriveItemConvertToPdfAsync(string DriveID, string PathOrDriveItemID, string SearchType)
    {
        Stream response = null;
        try
        {
            var queryOptions = new List<QueryOption>()
            {
                new QueryOption("format", "pdf")
            };

            if (SearchType == "path")
            {
                response= await _graphServiceClient.Me.Drives[DriveID].Root
                .ItemWithPath(PathOrDriveItemID)
                .Content
                .Request(queryOptions)
                .GetAsync();
            }
            else if (SearchType == "id")
            {
                response = await _graphServiceClient.Me.Drives[DriveID].Items[PathOrDriveItemID]
                .Content
                .Request(queryOptions)
                .GetAsync();
            }
        }
        catch (ServiceException ex)
        {
            Console.WriteLine($"Error deleting file: {ex.ToString()}");
        }
        return response;
    }

Answer 1

您需要启动所有任务，然后同时等待它们。

var files = documents.ToArray();

var tasks = files.Select(file =>
    oneDrive.DriveItemConvertToPdfAsync(
        SharedDriveID, 
        Path.Combine(fileDirectory, file.Path), 
        "path"));

var streams = await Task.WhenAll(tasks);

for(var i = 0; i < files.Length; i++)
{
    var file = files[i];
    var stream = streams[i];
    using (var memoryStream = stream)
    {
        byte[] buffer = new byte[memoryStream.Length];
        memoryStream.Read(buffer, 0, (int)memoryStream.Length);
         file.Base64 = Convert.ToBase64String(buffer);
    }
}

或者，如果您想改用流的ReadAsync方法：

var tasks = documents.Select(async file => {
    var stream = await oneDrive.DriveItemConvertToPdfAsync(
        SharedDriveID, 
        Path.Combine(fileDirectory, file.Path), 
        "path");
        
    using (var memoryStream = stream)
    {
        byte[] buffer = new byte[memoryStream.Length];
        await memoryStream.ReadAsync(buffer, 0, (int)memoryStream.Length);
        file.Base64 = Convert.ToBase64String(buffer);
    }
});

await Task.WhenAll(tasks);