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Determine who has a file open using C# - is there a way now?

I need to be able to check who has a file open using C#. I found a few people asked this question before but they were all a long time ago with the latest being in 2012. I was wondering if, in the last 5 years, has Microsoft added this ability to .net or maybe someone came out with a nuget package that is capable.
Here is the latest answer I found

You can use windows internal function NTQuerySystemInformation with undocumented parameter SystemHandleInformation for it. I don't know about c# implementation, but I know delphy code for it, and vb6 code. Check this links, it will help you to make your c# implementation.
Delphi - get what files are opened by an application
https://forum.sysinternals.com/topic14546.html

As far as I know, the standard library still does not have suitable functions for this. There is a popular question about this: https://stackoverflow.com/a/937558/5665527.
Also, if you want to know who opened the file remotely - net file function may be useful (which is accessible via cmd). I believe that there are similar WinAPI functions on MSDN that could be implemented in a similar way.

How to write Firefox plugin with C# and mono?

this question was asked many times:
2004
http://lists.ximian.com/pipermail/mono-devel-list/2004-October/008213.html
2005
http://web.archiveorange.com/archive/v/0SAXwxPyVBCC31hdB3Mm
http://codeverge.com/netscape.mozilla.jseng/xpcom-and-mono-or-just-c/1577232
2007
http://forum.mozilla-russia.org/viewtopic.php?id=17385
http://rsdn.ru/forum/cpp.applied/2514021.flat
2009
http://ru-csharp.livejournal.com/153546.html
2010
https://forums.mozilla.org/viewtopic.php?f=13&t=1445
https://www.linux.org.ru/forum/development/5069566
and always without answers...
Language bindings page doesn't contain C# or mono (but does so for Java and Python):
https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Language_Bindings
There is some code - http://off.net/~shaver/mono/
But i don't see C++ part of it (and I think it is necessary as a "XPCOM Component Loader")
There is also something what is called MonoConnect
https://bugzilla.mozilla.org/show_bug.cgi?id=296672
But i didn't found it's docs in google.
I know that it is possible to implement plugin as CLR host as written here:
http://www.mono-project.com/docs/advanced/embedding/
but i don't want to reinvent the wheel,
that is why I am asking where to find an existing implementation of "Component Loader".

Here is your answer: http://lists.ximian.com/pipermail/mono-devel-list/2004-October/008215.html, first answer in your first link to a mailing list.
Here is a github link to the issue in MonoConnect. I just took some strings in the diff which is in the tracker(the link you provided) and searched in google for it, got a github repo.
As well as this: http://people.mozilla.org/~chofmann/l10n/tree/mozilla/extensions/mono/
As far as written code, this is the most you will probably get.

Where do I find the latest Google Docs API with the source code of the samples?

I've been trying to use Google Spreadsheet API in C# by following these instructions.
I downloaded the API from the official Downloads page. However, there are two things wrong with it:
The API seems outdated, because I could not find the class OAuth2Parameters mentioned in the tutorial, only OAuthParameters.
The sample directory only contains executable files and no source code. I could really use the source code of the Spreadsheet.exe sample, as it fits my needs pretty well.

You can grab the source code from the project page - I've just done that and built it with no problems. OAuth2Parameters is in the oauthparameters.cs file. I don't know if there's a binary release of the latest version of the libraries, I'm afraid.
(Note that I work for Google, but this post isn't written on behalf of Google - I haven't had much to do with the GData API, although I know someone who has, so if this answer doesn't help I may be able to get more information on Monday...)

Importing mpp file into SQL Server through ASP.NET?

I have an assignment which is to import mpp file into database via ASP.NET (C#).
Can anyone please advice me on this matter.
Any help is very much appreciated.
Thanks.

Depending on the situation, using Interop isn't necessarily a good choice (have to have Project installed on the machine).
You can use MPXJ to open the mpp file and read the information in it. It's free and open-source and works fairly well, though since it's a port from Java, you have to do some things in a Java way in your .NET code.
A paid option is ASPOSE.Tasks, which is a little nicer to use, but pretty expensive.

Project programmability is available in C# but docs are only available for VBA.
This should get you going - all the stuff that is available in VBA can be accessed from C# using the Project Primary Interop Assembly from the list here.
Check out Application.FileSaveAs.

How to write FireFox extension with Visual Studio using C# programming language?

I was wondering if it's possible to write Firefox extension using .Net Framework?
Had anybody such experience in writing Firefox extensions using C# programming language?
Any good sites or guidelines would be very helpful.
FFPlugin http://img81.imageshack.us/img81/3049/ffplugin.jpg
Here is an example of what I need. Extension to enter phone number and to send SMS Message.
I draw it in Paint, so I'm sorry for having no artistic touch.

Assuming you're asking about writing an extension (and not a plugin), the best place to get started is with the info at the Mozilla Dev Center.
Konamiman is correct: extensions are mostly XUL (an XML grammar) and Javascript, packaged into a ZIP file with the extension .xpi. The various links from the page I linked should answer basic questions, like the one you posted above:
https://developer.mozilla.org/en/Building_an_Extension
https://developer.mozilla.org/en/Extension_Frequently_Asked_Questions

Firefox extensions are written primarily in Javascript. See here: http://www.rietta.com/firefox/Tutorial/backend.html. So unless you can find a way to convert C# code/.NET Framework objects to Javascript, you are out of luck.

As has been observed by other respondents, Firefox extensions are primarily written in Javascript: but you're in luck, because Script# is a free tool that enables developers to author C# source code and subsequently 'compile' it into Javascript.

I'm not sure how you'd do for C#, but you can use XPCOM to call native code libraries: that's how the Glasser extension, for instance, is able to use the Windows-specific Aero effects.
IBM hosts a very good XPCOM tutorial at DeveloperWorks.