在C＃中使用内存中文件的最佳方法是什么？

Question

I am building an ASP.NET web application that creates PowerPoint presentations on the fly. I have the basics working but it creates actual physical files on the hard disk. That doesn't seem like a good idea for a large multi-user web application. It seems like it would be better if the application created the presentations in memory and then streamed them back to the user. Instead of manipulating files should I be working with the MemoryStream class? I am not exactly sure I understand the difference between working with Files and working with Streams. Are they sort of interchangeable? Can anyone point me to a good resource for doing file type operations in memory instead of on disk? I hope I have described this well enough.

Corey

Answer 1

You are trying to make decision that you think impacts performance of your application based on "doesn't seem like a good idea" measurement, which is barely scientific. It would be better to implement both and compare, but first you should list your concerns about either implementations.

Here are some ideas to start:

• there are really not much difference between temporary files and in-memory streams. Both would have content in physical memory if they are small enough, both will hit the disk if there is memory pressure. Consider using temporary Delete on close files for your files if cleaning files up is the main concern.
• OS already doing very good job for managing large files with caching, one would need to make sure pure in-memory solution at least matches it.
• MemoryStream is not the best implementation for reasonably sized streams due its "all data is in single byte array" contract (see my answer at https://stackoverflow.com/a/10424137/477420 ).
• Managing multiple large in-memory streams (ie for multiple users) is fun for x86 platform, less of a concern for x64 ones.
• Some API simply don't provide a way working with Stream-based classes and require physical file.

Answer 2

Files and streams are similar, yes. Both essentially stream a byte array...one from memory, one from the hard drive. If the API you are using allows you to generate a stream, then you can easily do that and serve it out to the user using the Response object.

The following code will take a PowerPoint memory object (you'll need to modify it for your own API, but you can get the general idea), save it to a MemoryStream , then set the proper headers and write the stream to the Response (which will then let the user save the file to their local computer):

SaveFormat format = SaveFormat.PowerPoint2007;
Slideshow show = PowerPointWriter.Generate(report, format);
MemoryStream ms = new MemoryStream();
show.Save(ms, format);

Response.Clear();
Response.Buffer = true;
Response.ContentType = "application/vnd.ms-powerpoint";
Response.AddHeader("Content-Disposition", "attachment; filename=\"Slideshow.ppt\"");
Response.BinaryWrite(ms.ToArray());
Response.End();

Answer 3

Yes, I would recommend the MemoryStream. Typically any time you access a file, you are doing so with a stream. There are many kinds of streams (eg network streams, file streams, and memory streams) and they all implement the same basic interface. If you are already creating the file in a file stream, instead of something like a string or byte array, then it should require very little coding changes to switch to a MemoryStream.

Basically, a steam is simply a way of working with large amounts of data where you don't have to, or can't, load all the data at into memory at once. So, rather than reading or writing the entire set of data into a giant array or something, you open a stream which gives you the equivalent of a cursor. You can move your current position to any spot in the stream and read or write to it from that point.