Allow reading Excel files that are currently open

[goswinr]

This allows to use the type provider while the Excel file is open in Excel 365.

[goswinr]

I could not get this repro to build, So I made my own without Paket or Fake to test this change
https://github.com/goswinr/ExcelProvider2

[quintusm]

HI @goswinr

I am aware of the building issues, and working on it. It is unfortunately quite a big change to bring it up to date.

Thanks for the pull request. I will have a look at it and merge it. Looking at it quickly I cannot see that it is a problem at all.

[quintusm]

@goswinr

I have merged your pull request. Once again thanks for contributing!

[goswinr]

Thanks, so you currently cannot publish a new nuget ?

[goswinr]

I am not sure if there is now a small risk that concurrent writing and reading the file might yield a corrupt stream.
But probaly the Exception handler would catch that, no ?

[quintusm]

HI

I have tried to do some reading on this topic. It seems that for reading .Net tries to keep track of changes. If it detects changes it will discard the buffer and read the stream from the start again., as per the documentation. If you try to write, which we are not doing, it would throw an exception.

So I think the chances of something getting corrupted is small enough that the risk is worth taking...

[quintusm]

Thanks, so you currently cannot publish a new nuget ?

I will try to do a new nuget soon. Sorry for the delay.

I don't need to do all the changes I am busy with before being able to push a nuget,