定期从 HttpWebRequest 写入文件与下载完成后写入文件？

现在我正在使用此代码来下载文件（带有 Range 标头）。大多数文件都很大，并且在文件下载时当前正在运行 99% 的 CPU。有什么办法可以定期写入文件，这样它就不会一直保留在 RAM 中吗？

private byte[] GetWebPageContent(string url, long start, long finish)
{
    byte[] result = new byte[finish];
    HttpWebRequest request;

    request = WebRequest.Create(url) as HttpWebRequest;

    //request.Headers.Add("Range", "bytes=" + start + "-" + finish);

    request.AddRange((int)start, (int)finish);

    using (WebResponse response = request.GetResponse())
    {
        return ReadFully(response.GetResponseStream());
    }    
}

public static byte[] ReadFully(Stream stream)
{
    byte[] buffer = new byte[32768];
    using (MemoryStream ms = new MemoryStream())
    {
        while (true)
        {
            int read = stream.Read(buffer, 0, buffer.Length);
            if (read <= 0)
                return ms.ToArray();
            ms.Write(buffer, 0, read);
        }
    }
}

而不是将数据写入内存流 http://msdn.microsoft.com/en-us/library/system.io.memorystream.aspx（将数据存储在内存中），将数据写入文件流 http://msdn.microsoft.com/en-us/library/system.io.filestream.aspx（将数据存储在磁盘上的文件中）。

byte[] buffer = new byte[32768];
using (FileStream fileStream = File.Create(path))
{
    while (true)
    {
        int read = stream.Read(buffer, 0, buffer.Length);
        if (read <= 0)
            break;
        fileStream.Write(buffer, 0, read);
    }
}

使用.NET 4.0：

using (FileStream fileStream = File.Create(path))
{
    stream.CopyTo(fileStream);
}