当下载大文件时，如果网络不稳定或者程序异常退出，会导致下载失败，甚至重试多次仍无法完成下载。为此OSS提供了断点续传下载功能。

以下代码用于断点续传下载：

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using Aliyun.OSS;
using Aliyun.OSS.Common;
 
var endpoint = "<yourEndpoint>";
var accessKeyId = "<yourAccessKeyId>";
var accessKeySecret = "<yourAccessKeySecret>";
var bucketName = "<yourBucketName>";
var objectName = "<yourObjectName>";
var localFilename = "<yourLocalFilename>";
var downloadFilename = "<yourDownloadFilename>";
var checkpointDir = "<yourCheckpointDir>";
// 创建OssClient实例。
var client = new OssClient(endpoint, accessKeyId, accessKeySecret);
try
{
    // 通过DownloadObjectRequest设置多个参数。
    DownloadObjectRequest request = new DownloadObjectRequest(bucketName, objectName, downloadFilename)
    {
        // 指定下载的分片大小。
        PartSize = 8 * 1024 * 1024,
        // 指定并发线程数。
        ParallelThreadCount = 3,
        // checkpointDir用于保存断点续传进度信息。如果某一分片下载失败，再次下载时会根据文件中记录的点继续下载。如果checkpointDir为null，断点续传功能不会生效，每次失败后都会重新下载。
        CheckpointDir = checkpointDir,
    };
    // 断点续传下载。
    client.ResumableDownloadObject(request);
    Console.WriteLine("Resumable download object:{0} succeeded", objectName);
}
catch (OssException ex)
{
    Console.WriteLine("Failed with error code: {0}; Error info: {1}. \nRequestID:{2}\tHostID:{3}",
        ex.ErrorCode, ex.Message, ex.RequestId, ex.HostId);
}
catch (Exception ex)
{
    Console.WriteLine("Failed with error info: {0}", ex.Message);
}

断点续传下载详情请参见断点续传下载。