HttpWebRequest（跨域下载文件——网络流转换为内存流下载）

1.Stream 转换为 MemoryStream(Stream不支持查找)

MemoryStream StreamToMemoryStream(Stream instream)
        {
            MemoryStream outstream = new MemoryStream();
            const int bufferLen = 4096;
            byte[] buffer = new byte[bufferLen];
            int count = 0;
            while ((count = instream.Read(buffer, 0, bufferLen)) > 0)
            {
                outstream.Write(buffer, 0, count);
            }
            return outstream;
        }

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