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Do you know any nice interface for C# Interactive standalone?

You can open the C# Interactive window in Visual Studio to use C# as a scripting an shell language. Unfortunately, this requires you to have Visual Studio open all the time.
I know I can run csi.exe itself, but this does not give me any syntax highlighting or auto completion features.
How can I run the C# Interactive Windows standalone?

I would suggest LINQPad (I don't use it myself but I know this can do that kind of things). I think it's pretty powerful
https://www.linqpad.net/

VSCode has built in support for .NET Interactive Notebooks - if you don't see it, you can install the '.NET Interactive Notebooks' extension

SonarQube plugin development-c#

I want to develop sonarqube plugin for c#. I developed some source code analysis tool in c# and I want to integrate into sonarqube. I just want to know which IDE I must use for this work and how to setup project in that IDE . can I code this plugin in visual studio?
ThankYou

All plugins must be written in Java. You can execute your tool by Java code.
You should read Developing Plugins guide on the SonarQube Wiki.

Since the plugin must be written in Java you can't use Visual Studio. I would recommend IntelliJ and the plugin-project that I'm working on right now is Maven-type.
This is also a good guide on how to develop the plugin https://deors.wordpress.com/2014/03/20/sonarqube-plugins-1/

Can I program in C# on a Mac?

I want to learn C# for Unity and my personal computer is a MacBook Air. Can I program in C# on a Mac?

Your first option is Microsoft Visual Studio for Mac which was released in 2017. If you're used to VS ide then I suggested you download this.
If not then you can have a look into MonoDevelop. You can download from here
MRE = Mono Runtime Environment
MDK = Mono Development Kit.
MDK = MRE + Extra tools, libraries, .NET PCL (Portable Class Library) profiles, etc.
If you have an application that you want to run that needs Mono you can install just the MRE.
If you are doing development, writing C# applications, whilst you can sometimes get away with just having the MRE installed, you may hit some missing features such as not having the .NET PCL profiles. So I would install the MDK if you are doing development. Reference

Yes, it is. Microsoft has released Visual Studio Code 2017. It works on Windows, Mac OS, and Linux; It is quite nice development text-editor. It works similar to the full-version, available on Windows - with a few draw-backs, and it will allow to write programs, as well as run and test them right on your Mac.
Microsoft Docs offer a quick getting started tutorial and a Hello World program out-of-the-box with the Visual Studio Code.
You will need to install a few dependencies, however. Which may sound kinda of obvious:
Visual Studio Code
.Net Core SDK
C# extension from the marketplace.
I had good luck getting started and Visual Studio got all of the dependencies for me; so I simply went into the program, opened the integrated terminal (View > Integrated Terminal) and ran the command dotnet new console, and boom, a few seconds later I had a Hello World program, written in C#, running on my MacBook Pro.

There is now a Visual Studio for Mac available as a free community version as well as professional and enterprise versions by subscription.
JetBrains also has an IDE for C#, called Rider, that runs on the Mac. If you use IntelliJ or any of their other products you might like it - it feels better to me than MonoDevelop or Visual Studio for Mac. It is a commercial product but JetBrains offers very generous license terms (as in free) for students and open source projects. They also have a substantial discount for startups.

It is possible, but you won't be able to use Microsoft's tools, you'll need to use a third-party program like Xamarin Studio (MonoDevelop).
Edit
at the time of this answer there wasn't a Microsoft tool/IDE that could be used for developing .net / C# programs on a Mac. Now there are two:
Visual Studio Code
Visual Studio for Mac

Note that while everyone mentions Mono and Xamarin, which you should absolutely look into for C# development, Unity works on Mac directly, specifically Mac OSX 10.6+
while both use the C# language, some of the paradigms espoused by Unity are a little different from standard C# development. I personally recommend learning both.
ADDENDUM: Note that MS has recently announced that they'll be moving the standard .NET implementation cross-platform, and are doing so with the help of the Mono team, so while there's currently a confusing dual set of tools, it'll be shifting away from this in the future

It is entirely possible, if you install a version of Windows using Bootcamp you can switch between the two at start up and have the full functionality of both by installing Visual Studio on the Windows partition. Alternatively, you could use VMware such as Virtual Box and use Windows within your Mac OS.

Is there an Eclipse C# Highlight, Code Completion plugin

I need an Eclipse 3 plugin for C# syntax highlight and hopefully code completion.
I am running both Mac and Windows versions of Eclipse.
I have evaluated "Improve C#" but it seems to be not working.

Have a look at Emonic. It is an Eclipse plug-in which allows you to build C# programs with Mono or Microsoft .NET.

Learn C# on mac?

Is mono the only route , any specific visual studio like editors that you recommend?

Without meaning to state the obvious and miss the point, if you mean a Mac computer rather than a Mac OS, you could install bootcamp or use parallels to run windows on the Mac and then use Visual Studio (there are also free versions of Visual Studio)

Yeah, mono is really your only option, unless some undergrad somewhere has developed some very experimental thing I don't know about. As for an IDE, well I believe the only thing half way stable that will work right now on Mac OS X is Monodevelop:
http://tirania.org/blog/archive/2008/Feb-07-2.html
I mean, you could run Parallels and develop your code on a Windows VM with Visual Studio and as long as you follow certain guidelines about portability, you could use VS to develop your mono apps. Although, you probably might as well get familiar with Monodevelop. It would be interesting to see if SharpDevelop ever gets ported.

Well, you could also try Silverlight...
Pro:
It is an official Microsoft implementation, so it is more likely to work
Its support for recent stuff like C# 3.0 is much better
Con:
It is browser-only, Silverlight apps do not run standalone
You won't get the whole .NET Class Library, only a subset, so it is somewhat limited
You won't get Visual Studio on a Mac
For IDE, I suggest Eclipse Tools for Microsoft Silverlight (apparently it is Windows-only at the moment) you should use MonoDevelop or SharpDevelop or something like that.

Use Xamarin Studio IDE.
https://xamarin.com/mac
Uses Mono and C#. It has any features you might expect from a modern IDE and combines with XCode for GUI.

Mono is pretty much your only route right now, though there are incessant rumours (I wouldn't give much thought to them, though) that Microsoft is planning to port C# to Mac in the future. I'd be very surprised if that happened, though.
As for IDEs, I can't help there... If I want C#, I stick to Visual Studio (run it through Parallels or BootCamp, if you really want to use VS).
Edit: As Graham points out, there is Cocoa#, but I'd caution that simply because the project has stalled, and there's unlikely to be much future for it. :(

I would go the virtualization route, either Parallels or VMware Fusion. Both will run Windows XP and Visual Studio very well on a modern Mac. Windows has the best tools for .NET and C# development and it only makes to use them, especially when you're just starting out.

Virtual Box is my new favorite and open source (means free) VM software. Don't pay for Parallels. Also, with BootCamp, you have to reboot the machine to switch between Windows and OS X, so Virtual Box is the way to go.