将BeginRead循环转换为ReadAsync并根据需要取消

我试图构建两个将通过RDP相互通信的应用程序。我有基本的沟通，但我想优化代码并删除所有异常。

在服务器上，我打开FileStream，当我从客户端发送东西时，我使用BeginRead:读取它

public void Connect()
{
    mHandle = Native.WTSVirtualChannelOpen(IntPtr.Zero, -1, CHANNEL_NAME);
    Task.Factory.StartNew(() =>
    {
        IntPtr tempPointer = IntPtr.Zero;
        uint pointerLen = 0;
        if (Native.WTSVirtualChannelQuery(mHandle, WtsVirtualClass.WTSVirtualFileHandle, out tempPointer, ref pointerLen))
        {
            IntPtr realPointer = Marshal.ReadIntPtr(tempPointer);
            Native.WTSFreeMemory(tempPointer);
            var fileHandle = new SafeFileHandle(realPointer, false);
            fileStream = new FileStream(fileHandle, FileAccess.ReadWrite, 0x640, true);
            var os = new ObjectState();
            os.RDPStream = fileStream;
            fileStream.BeginRead(os.Buffer, 0, 8, CallBack, os);
        }
    });
}

和我的回电：

private void CallBack(IAsyncResult iar)
{
    if (iar.IsCompleted || iar.CompletedSynchronously)
    {
        var os = iar.AsyncState as ObjectState;
        int readed = 0;
        try
        {
            readed = os.RDPStream.EndRead(iar);
        }
        catch (IOException ex)
        {
            MessageBox.Show(ex.Message);
        }
        if (readed != 0)
        {
            switch (os.State)
            {
                case FileDownloadStatus.Length:
                    os.Length = BitConverter.ToInt32(os.Buffer, 0);
                    os.State = FileDownloadStatus.Body;
                    os.RDPStream.BeginRead(os.Buffer, 0, os.Length, CallBack, os);
                    break;
                case FileDownloadStatus.Body:
                    var message = Encoding.UTF8.GetString(os.Buffer, 0, readed);
                    Debug.WriteLine(message);
                    var newos = new ObjectState {RDPStream = os.RDPStream};
                    os.RDPStream.BeginRead(newos.Buffer, 0, 8, CallBack, newos);
                    break;
            }
        }
    }
}

和所需物品：

enum FileDownloadStatus
{
    Length,
    Body,
}
class ObjectState
{
    public int Length = 0;
    public FileDownloadStatus State = FileDownloadStatus.Length;
    public byte[] Buffer = new byte[0x640];
    public FileStream RDPStream = null;
}

不幸的是，当我试图关闭我的应用程序时，我得到了异常System.OperationCanceledException。

我想将其转换为ReadAsync，因为这样我就可以使用CancelationToken，但之后我必须升级到.NET 4.5。

我发现了关于CancelationToken的类似问题，但它是用F#写的，我需要C#。

所以我的问题是：
如何停止BeginRead或将此代码转换为.NET 4.5并使用ReadAsync？

    Task<int> task = fileStream.ReadAsync(os.Buffer, 0, 8, cancelToken);
    try
    {
        await task;
        await CallBack(os, task);
    }
    catch (IOException ex)
    {
        // TODO: unpack task exception and display correctly
        MessageBox.Show(ex.Message);
    }

    private async Task CallBack(ObjectState os, Task<int> task)
    {
        if (task.Status != TaskStatus.Canceled)
        {
            int readed = task.Result;