流未打开以供阅读

Question

我正在从云端下载一个 PowerPoint 文件。为了限制下载文件的多次调用，我使用MemoryCache 对其进行缓存。下面的代码第一次运行正常，但第一次运行后抛出异常：

“未打开流进行读取。”

下面是我正在使用的代码：

var cache = MemoryCache.Default;
var memoryStream = new MemoryStream();
if(cache.contains(key)) {
    memoryStream = (MemoryStream)cache.Get(key);
} else {
    var file = getFile();
    cache.Add(key, file);
    memoryStream = (MemoryStream)cache.Get(key);
}

using(memoryStream) {
    Open file and do manupulation then save the file in cloud.
}

更新代码：

var cache = MemoryCache.Default;
byte[] data = (byte[])cache.Get(key);
if(data == null)
{
    data_stream = getData(); // get memorystream
    cache.Add(key, data_stream.ToArray(), new CacheItemPolicy());
    data = (byte[])cache.Get(key);
}

using(Stream outputStream = new MemoryStream(decktemplate)) {
    Open file and do manupulation then save the file in cloud.
}

仍然收到以下错误：“算术运算导致溢出。”

下面是堆栈树：

   at MS.Internal.IO.Zip.ZipIOExtraField.ParseRecord(BinaryReader reader, ZipIOZip64ExtraFieldUsage zip64extraFieldUsage, UInt16 expectedExtraFieldSize)
   at MS.Internal.IO.Zip.ZipIOCentralDirectoryFileHeader.ParseRecord(BinaryReader reader, Encoding encoding)
   at MS.Internal.IO.Zip.ZipIOCentralDirectoryBlock.ParseRecord(BinaryReader reader, Int64 centralDirectoryOffset, Int32 centralDirectoryCount, Int64 expectedCentralDirectorySize)
   at MS.Internal.IO.Zip.ZipIOCentralDirectoryBlock.SeekableLoad(ZipIOBlockManager blockManager)
   at MS.Internal.IO.Zip.ZipIOBlockManager.get_CentralDirectoryBlock()
   at MS.Internal.IO.Zip.ZipArchive.GetFiles()
   at System.IO.Packaging.ZipPackage.ContentTypeHelper..ctor(ZipArchive zipArchive, IgnoredItemHelper ignoredItemHelper)
   at System.IO.Packaging.ZipPackage..ctor(Stream s, FileMode mode, FileAccess access, Boolean streaming)
   at System.IO.Packaging.Package.Open(Stream stream, FileMode packageMode, FileAccess packageAccess, Boolean streaming)
   at DocumentFormat.OpenXml.Packaging.OpenXmlPackage.OpenCore(Stream stream, Boolean readWriteMode)
   at DocumentFormat.OpenXml.Packaging.PresentationDocument.Open(Stream stream, Boolean isEditable, OpenSettings openSettings)
   at DocumentFormat.OpenXml.Packaging.PresentationDocument.Open(Stream stream, Boolean isEditable)

Answer 1

将MemoryStream 作为对象 存储在缓存中是一个非常糟糕的主意，因为多个线程可能同时访问它。最好将 payload 存储为byte[] 或ArraySegment<byte>，并使用它为每个调用者创建一个不同 MemoryStream。否则，你会经常被自己绊倒：

byte[] payload = (byte[])cache.Get(key);
if (payload is null)
{
    payload = GetFilePayload();
    cache.Add(key, payload);
}
using(var memoryStream = new MemoryStream(payload, writable: false))
{
    // ...
}