Support reading OLE stream as Stream

[phuclv90]

I need to save a stream (or parts of it) to a file, but in CFStream the only methods that can be used to get data are Read(byte[] buffer, long position, int count) and byte[] GetData(), thus I have to get a byte[] buffer every time and write it to file. As the buffer is larger than 85000 bytes, it's put on the large object heap and the GC won't collect it right away even if it's not used anywhere else. As a result for big streams my app becomes a memory hog when saving big streams and I have to call GC.Collect() manually

I've written a custom class to wrap CFStream that extends System.IO.Stream and calls CFStream.Read() inside its Read overload, but the result is that performance is almost 10 times slower. I debugged and found out that there are a lot of small reads from the stream and a new StreamView is created even when reading just a single byte. After reading GC.Collect() is called, thus there are a lot of GC wake ups in 1 second

I ended up working around the issue by wrapping another layer of System.IO.BufferedStream. But it looks like the issue can be solved much easier and more efficient by exporting StreamView which is currently an internal class. We just need to make it public, or probably some other small changes to make it work

[poizan42]

Is OpenMcdf.Extensions.CFStreamExtension.AsIOStream not good enough?

[phuclv90]

@poizan42 I didn't know about that. There's a single OpenMcdf.dll in the current project and I've looked around it to no avail. Searching repo doesn't help either because there are so many CFStream in the result

[poizan42]

It's in OpenMcdf.Extensions: https://www.nuget.org/packages/OpenMcdf.Extensions

[phuclv90]

I've checked the source code and it uses cfStream.Read which still results in terrible performance for small/random reads/writes

[IS4Code]

StreamView directly doesn't have to be public, but CFStream should have a method Open/GetStream which constructs it. The only thing missing is to disable writing when the file is opened with CFSUpdateMode.ReadOnly.

[jeremy-visionaid]

This should be now be addressed as part of #194 Small reads should be fast and efficient in the proof of concept.

[jeremy-visionaid]

I'm hopeful that 3.0.0-preview1 should get released to NuGet in the next few days. @ironfede has kindly granted me write access to the repo, so I'll go ahead and close this one as fixed by #194.