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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.

Using IqualProp in C#

I want to develop a tool that will assess the quality of a video. I did some R&D on this and found an interface called "IQualProp", that is part of DirectShow API. I am not trying to get a sample code in C# on how to use this interface, but i am not finding anywhere. There are samples in c++ to use this "IQualProp", but not in C#. Any help on this will be great.

IQualProp iqp = your_obj as IQualProp;
After doing this you can access all the properties of the interface.

Use of Java code in other platforms using Mono

I am working on a project which is written in Java (6.0) and I want to use it in almost all the platforms (iOS/Android/Windows/Windows Phone/Mac/Linux) so I thought of Mono and converting the code to C# but it's 40,000 line of code so it would take so much time to write that. Then I realized that I can use JavaBinding to convert a lot of it and use it but I am not sure if I can use it in Windows or more important iOS.
So basically my question is that "Does java binding converts Java code to C# or it's just a call to the java code? If so, I can't use it for my MonoTouch project"

Two approaches are possible,
Use Sharpen to convert Java code to C# code. A perfect example, is to port Android to C# http://blog.xamarin.com/android-in-c-sharp/
Use IKVM to host Java code on Mono. http://www.ikvm.net/
JavaBinding is only useful for Mono for Android, and I don't think it applies to Windows/iOS.

Embedding a Jar into a C# Form

Ok this situation is a bit difficult. But I am trying to embed a Java Jar into a C# form. Not as a new window or new process.
The Jar will be a game that uses the LWJGL library.
The C# Form will be a "wrapper" for it with tools and more.
Anyone have any ideas? Is this even possible?

According to the IKVM Home Page
IKVM.NET is an implementation of Java
for Mono and the Microsoft .NET
Framework. It includes the following
components:
A Java Virtual Machine implemented in
.NET A .NET implementation of the Java
class libraries Tools that enable Java
and .NET interoperability

One would approach it from using a DLL not a jar. Also this sounds like a bit of a security issue.

They're completely different runtimes; you'll need a process for both the CLR and the JVM; no way around that.
You can try the Java to C# converter provided by MS, and then rebuild: http://www.microsoft.com/downloads/details.aspx?FamilyID=46bea47e-d47f-4349-9b4f-904b0a973174&displaylang=en

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.