先上代码，疑问代码在最后一个方法：

private CancellationTokenSource m_cts; 

public void btnCancel_Click(object sender, EventArgs e)
{
    if (m_cts != null) m_cts.Cancel();
}

public async  void btnRun_Click(object sender, EventArgs e)
{
    m_cts = new CancellationTokenSource();
    btnRun.Enabled = false;
    try
    {
        Task<Bitmap> imageDownload = GetBitmapAsync(txtUrl.Text); 
        await UntilCompletionOrCancellation(imageDownload, m_cts.Token);
        if (imageDownload.IsCompleted)
        {
            Bitmap image = await imageDownload;
            panel.AddImage(image);
        }
        else imageDownload.ContinueWith(t => Log(t));
    }
    finally { btnRun.Enabled = true; }
}

private static async  Task UntilCompletionOrCancellation(
    Task asyncOp, CancellationToken ct)
{
    var tcs = new TaskCompletionSource<bool>();
    using(ct.Register(() => tcs.TrySetResult(true)))//这段代码中的using
        await Task.WhenAny(asyncOp, tcs.Task);
    return asyncOp;
}

问题：
using(ct.Register(()=>tcs.TrySetResult(true)))这行代码为什么用using?我知道using会自动释放资源，可我用Task从来没有手动释放资源，
使用Task需要手动释放资源吗？