CloudBlockBlob的OpenReadAsync和DownloadFromStreamAsync功能之间的区别

Question

Azure blob存储中OpenReadAsync的{​​{1}}和DownloadToStreamAsync函数之间有什么区别？在谷歌搜索但找不到答案。

Answer 1

OpenReadAsync和DownloadToStreamAsync都可以启动异步操作以检索blob流。 根据我的测试，您可以通过以下部分更好地了解它们：

基本概念

DownloadToStreamAsync：启动异步操作，将blob的内容下载到流中。

OpenReadAsync：启动异步操作以将blob的内容下载到流中。

用法

a）DownloadToStreamAsync

示例代码：

using (var fs = new FileStream(<yourLocalFilePath>, FileMode.Create))
{
    await blob.DownloadToStreamAsync(fs);
}

b）OpenReadAsync

示例代码：

//Set buffer for reading from a blob stream, the default value is 4MB.
blob.StreamMinimumReadSizeInBytes=10*1024*1024; //10MB
using (var blobStream = await blob.OpenReadAsync())
{
    using (var fs = new FileStream(localFile, FileMode.Create))
    {   
       await blobStream.CopyToAsync(fs);
    }
}

通过Fiddler

捕获网络请求

a）DownloadToStreamAsync

b）OpenReadAsync

根据以上所述，DownloadToStreamAsync只发送一个获取请求以检索blob流，而OpenReadAsync根据您设置的“Blob.StreamMinimumReadSizeInBytes”或默认值发送多个请求来检索blob流。

Answer 2

DownloadToStreamAsync 和 OpenReadAsync 的区别在于 DownloadToStreamAsync 会在返回前将 blob 的内容下载到流中，但 OpenReadAsync 不会触发下载，直到流被消耗。

例如，如果使用它从 ASP.NET 核心服务返回文件流，则应使用 OpenReadAsync 而不是 DownloadToStreamAsync：

使用 DownloadToStreamAsync 的示例（在这种情况下不推荐）：

Stream target = new MemoryStream(); // Could be FileStream
await blob.DownloadToStreamAsync(target); // Returns when streaming (downloading) is finished. This requires the whole blob to be kept in memory before returning!
_logger.Log(LogLevel.Debug, $"DownloadToStreamAsync: Length: {target.Length} Position: {target.Position}"); // Output: DownloadToStreamAsync: Length: 517000 Position: 517000
target.Position = 0; // Rewind before returning Stream:
return File(target, contentType: blob.Properties.ContentType, fileDownloadName: blob.Name, lastModified: blob.Properties.LastModified, entityTag: null);

使用 OpenReadAsync 的示例（在这种情况下推荐）：

// Do NOT put the stream in a using (or close it), as this will close the stream before ASP.NET finish consuming it.
Stream blobStream = await blob.OpenReadAsync(); // Returns when the stream has been opened
_logger.Log(LogLevel.Debug, $"OpenReadAsync: Length: {blobStream.Length} Position: {blobStream.Position}"); // Output: OpenReadAsync: Length: 517000 Position: 0
return File(blobStream, contentType: blob.Properties.ContentType, fileDownloadName: blob.Name, lastModified: blob.Properties.LastModified, entityTag: null);

Answer 3

OpenReadAsync返回Task<Stream>，您可以使用等待。

样本测试方法

CloudBlobContainer container = GetRandomContainerReference();
            try
            {
                await container.CreateAsync();

                CloudBlockBlob blob = container.GetBlockBlobReference("blob1");
                using (MemoryStream wholeBlob = new MemoryStream(buffer))
                {
                    await blob.UploadFromStreamAsync(wholeBlob);
                }

                using (MemoryStream wholeBlob = new MemoryStream(buffer))
                {
                    using (var blobStream = await blob.OpenReadAsync())
                    {
                        await TestHelper.AssertStreamsAreEqualAsync(wholeBlob, blobStream);
                    }
                }
            }

DownloadToStreamAsync是一个虚拟（可以被覆盖）的方法，它返回一个任务并将流对象作为输入。

样本用法。

await blog.DownloadToStreamAsync(memoryStream);