C# SFTP客户端实现文件传输功能

SftpClient client = new SftpClient(ip, port, userName, password);  // 除了port为int外，其他参数都是string，用户名和密码是Linux允许访问OpenSSH账号名和密码,sshd_config AllowUser那里做配置的
client.Connect();
List<TransFileInfo> entries = client.ListDirectory(path)  // path服务端的路径
                                    .Where(t => t.Name != "." && t.Name != ".." && !t.Name.StartsWith('.'))  // 剔除"."当前目录、".."上级目录、"."开头的配置文件
                                    .OrderByDescending(t => t.IsDirectory)
                                    .ThenBy(t => t.Name, StringComparer.OrdinalIgnoreCase)
                                    .Select(t => mew TransFileInfo()
                                    {
                                        Name = t.Name,
                                        Size = t.IsDirectory ? 0 : t.Length,
                                        Time = t.LastWriteTime.ToString("yyyy-MM-dd HH:mm"),
                                        IsDirectory = t.IsDirectory,
                                        FullPath = t.Name
                                    });

SftpClient client = new SftpClient(ip, port, userName, password);
client.Connect();  // SftpClient的创建和连接的成本很低，可以在每次执行时都创建并连接
SftpFileAttributes attrs = client.GetAttributes(deleteFilePath);
if (attrs.IsDirectory)
    // 写个递归删除所有文件夹中的文件就行
else
    client.DeleteFile(deleteFilePath);

client.RenameFile(oldFilePath, newPath);

client.CreateDirectory(dirPath);

long totalLength = client.GetAttributes(remotePath).Size;  // 获取一下远程文件的大小
string tempPath = localPath + ".tmp";  // 先将远程文件保存至临时文件，结束后再删除掉.tmp后缀
long offset = 0;
if (File.Exits(tempPath))
{
    offset = (new FileInfo(tempPath)).Length;  // 因为提供的断点续传功能，所以临时文件的长度作为偏置，续传时就从这里开始
    if (offset >= totalLength)
        offset = totalLength;  // 偏置为totalLength就算是下载完毕了
}

using StfpFileStream remoteStream = client.OpenRead(remotePath);
remoteStream.Seek(offset, SeekOrigin.Begin);  // SeekOrigin.Begin从文件开头开始偏移

using FileStream localStream = new FileStream(tempPath, FileMode.OpenOrCreate, FileAccess.Write, FileShare.None, 256 * 1024);  // 每次只读取256kb
localStream.Seek(offset, SeekOrigin.Begin);

byte[] buffer = new byte[256 * 1024];
while(true)
{
    if(!await Check())
        continue;

    int read = await remoteStream.ReadAsync(buffer.AsMemory(0, buffer.Length));  // Memory类似(指针 + 长度)，由GC回收
    if (read == 0)
        break;

    await localStream.WriteAsync(buffer.AsMemory(0, read));
}

// 回收流资源 + 复制临时文件
localStream.Dispose();
if (File.Exits(localPath))
    File.Delete(localPath);

File.Move(tempPath, localPath);

async void Check()
{
    TaskCompletionSource<bool>? tcs;
    lock(_lock)
    {
        if (isPaused)
            return false;  // 这里省略了，isPaused为true时，while(true)那边不继续

        tcs = _pauseTcs;
    }

    if (tcs != null)
        await tcs.Task.ConfigureAwait(false);  // 在这里等
}

void Resume()
{
    TaskCompletionSource<bool>? tcs;
    lock(_lock)
    {
        if (!isPaused)
            return;

        isPaused = false;
        tcs = _pauseTcs;
        _pauseTcs = null;
    }

    tcs?.TrySetResult(true);    
}

void Cancel()
{
    TaskCompletionSource<bool>? tcs;
    lock(_lock)
    {
        if (isPaused)
            return;

        isPaused = true;
        _pauseTcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationAsynchronously);
    }

    tcs?.TrySetResult(false);    
}