如何取消异步等待

public static async Task<T> WithCancellation<T>( 
    this Task<T> task, CancellationToken cancellationToken) 
{ 
    var tcs = new TaskCompletionSource<bool>(); 
    using(cancellationToken.Register( 
                s => ((TaskCompletionSource<bool>)s).TrySetResult(true), tcs)) 
        if (task != await Task.WhenAny(task, tcs.Task)) 
            throw new OperationCanceledException(cancellationToken); 
    return await task; 
}

FileStream fs =...;
byte [] b = …; 
CancellationToken token = …; 
Task op = fs.ReadAsync(b, 0, b.Length); 
try 
{ 
    await op.WithCancellation(token); 
} 
catch(OperationCanceledException) 
{ 
    op.ContinueWith(t => /* handle eventual completion */); 
    … // whatever you want to do in the case of cancellation 
}

FileStream fs = …; 
byte [] b = …; 
CancellationToken token = …; 
Task op = fs.ReadAsync(b, 0, b.Length, token); 
try 
{ 
    await op.WithCancellation(token); 
} 
catch(OperationCanceledException) 
{ 
    if (!op.IsCompleted) 
        op.ContinueWith(t => /* handle eventual completion */); 
    … // whatever you want to do in the case of cancellation 
}