Parallel.Foreach访问冲突问题

Question

我正在尝试从数据库（oracle）读取blob并将文件保存到系统中。由于blob大小可能很大，我附加字节。这里的扫描字符串是ConcurrentDictionary类型。

ConcurrentQueue<Exception> exceptions = new ConcurrentQueue<Exception>();
Parallel.ForEach(scannedLettersDictionary.Where(y => y.Value.ScannedLetterContent != null).Select(x => x), file =>
{
    try
    {
        using (FileStream stream = new FileStream(ScannedLettersDirectory + file.Value.LetterId + ".tiff", FileMode.Create, FileAccess.Write))
        {
            byte[] bytes = new byte[65536];
            int numBytesToRead = (int)file.Value.ScannedLetterContent.Length;
            int bytesRead = 1;

            using (BinaryWriter writer = new BinaryWriter(stream))
            {
                while (numBytesToRead > 0 && bytesRead > 0)
                {
                    bytesRead = file.Value.ScannedLetterContent.Read(bytes, 0, Math.Min(numBytesToRead, bytes.Length));
                    writer.Write(bytes);
                    numBytesToRead -= bytesRead;
                }

                writer.Flush();
                writer.Dispose();
            }
        }
    }
    catch (AggregateException ag) { exceptions.Enqueue(ag); }
    catch (Exception e) { exceptions.Enqueue(e); }
});

if (exceptions.Count > 0)
{
    foreach (var exp in exceptions)
    {
        LogError(exp, 66, exp.Message, _letterId);
    }
}

问题出在这条线上。它给了我访问冲突例外。

 bytesRead = file.Value.ScannedLetterContent.Read(bytes, 0, Math.Min(numBytesToRead, bytes.Length)); 

如何在Parallel.ForEach中使byte []线程安全。我不想把锁对象放在它之外，因为它不会为我提供任何用途，并且相当于for循环（性能明智）。

感谢任何帮助。