SSH.Net异步文件下载 [英] SSH.Net Async file download

问题描述

我正在尝试使用SSH.NET从SFTP服务器异步下载文件.如果我以同步方式进行操作，则可以正常工作，但是如果以异步方式进行操作，则会得到空文件.这是我的代码:

I am trying to download files asynchronously from an SFTP-server using SSH.NET. If I do it synchronously, it works fine but when I do it async, I get empty files. This is my code:

var port = 22;
string host = "localhost";
string username = "user";
string password = "password";
string localPath = @"C:\temp";

using (var client = new SftpClient(host, port, username, password))
{
    client.Connect();
    var files = client.ListDirectory("");

    var tasks = new List<Task>();

    foreach (var file in files)
    {                        
        using (var saveFile = File.OpenWrite(localPath + "\\" + file.Name))
        {
            //sftp.DownloadFile(file.FullName,saveFile); <-- This works fine
            tasks.Add(Task.Factory.FromAsync(client.BeginDownloadFile(file.FullName, saveFile), client.EndDownloadFile));
        }                        
    }

    await Task.WhenAll(tasks);
    client.Disconnect();

}

推荐答案

由于saveFile在using块中声明，因此在您开始执行任务后便立即将其关闭，因此无法完成下载.实际上，令您惊讶的是，我很惊讶.

Because saveFile is declared in a using block, it is closed right after you start the task, so the download can't complete. Actually, I'm surprised you're not getting an exception.

您可以提取代码以下载到这样的单独方法中:

You could extract the code to download to a separate method like this:

var port = 22;
string host = "localhost";
string username = "user";
string password = "password";
string localPath = @"C:\temp";

using (var client = new SftpClient(host, port, username, password))
{
    client.Connect();
    var files = client.ListDirectory("");

    var tasks = new List<Task>();

    foreach (var file in files)
    {                        
        tasks.Add(DownloadFileAsync(file.FullName, localPath + "\\" + file.Name));
    }

    await Task.WhenAll(tasks);
    client.Disconnect();

}

...

async Task DownloadFileAsync(string source, string destination)
{
    using (var saveFile = File.OpenWrite(destination))
    {
        var task = Task.Factory.FromAsync(client.BeginDownloadFile(source, saveFile), client.EndDownloadFile);
        await task;
    }
}

这样，在完成下载文件之前，不会关闭文件.

This way, the file isn't closed before you finish downloading the file.

查看SSH.NET源代码，看起来DownloadFile的异步版本未使用真正的"异步IO(使用IO完成端口)，而是仅在新线程中执行下载.因此，使用BeginDownloadFile/EndDownloadFile并没有真正的优势.您最好在自己创建的线程中使用DownloadFile:

Looking at the SSH.NET source code, it looks like the async version of DownloadFile isn't using "real" async IO (using IO completion port), but instead just executes the download in a new thread. So there's no real advantage in using BeginDownloadFile/EndDownloadFile; you might as well use DownloadFile in a thread that you create yourself:

Task DownloadFileAsync(string source, string destination)
{
    return Task.Run(() =>
    {
        using (var saveFile = File.OpenWrite(destination))
        {
            client.DownloadFile(source, saveFile);
        }
    }
}

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