About passing FrameSet from asynchronous thread to main thread (C #)

I want to pass the FrameSet acquired in the asynchronous thread to the main thread.
I tried the following code, but when the source FrameSet is released, the copy destination is also released.
How can this be achieved?

<code (C#)>

FrameSet fs;

Task.Factory.StartNew(() =>
{
    while (!tokenSource.Token.IsCancellationRequested)
    {
        using (var frames = pipeline.WaitForFrames())
        {
            fs = frames;
        }
    }
}, tokenSource.Token);