Develop Reference

Library compatibility between C# .NET vs C# Mono

I wanted to tryout C# for general purpose programming (not web development). I program in Windows environment, but I would like to avoid coding specifically for Windows (.NET), because I want to keep the option open for a future migration to Linux.
Are there any specific libraries in C# .NET that wouldn't work in C# Mono for general purpose programming work (not interested in Windows Forms, Silverlight and stuff like that) ?
Is there any internet link of things/features that provides a list that works on C# .NET wouldn't work on C# Mono or vice versa? I didn't readily find anything in google per se.
Note: I would be interested in specific answers, not opinions of which is better or worse (thanks!)

It is possible for a CLR assembly (even in the form of a DLL, as mentioned in the comments) to be read by Mono, as long as it does not have dependencies that do not exist in Mono, because...
...not every piece of code that compiles for .NET will compile for Mono, since there are lots of Windows-specific things in .NET (not strictly part of C#) that aren't implemented (WPF, ASP.NET async stack) or don't make sense at all in Linux (COM is one such example, I think).
Fortunately, there is a list of what .NET features are implemented in Mono. Even more fortunately, it seems they have an app that tells you a priori whether your code makes use of anything not implemented in Mono (but I have never tried it).

if you install xamarin (you need Pro or bigger so you have VS integration) you can create Portable Class Library that targets xamarin (which is based on mono) and visual studio will allow you to only use classes that are compatible with mono.
http://docs.xamarin.com/guides/cross-platform/application_fundamentals/pcl/introduction_to_portable_class_libraries/
apparently you can also use xamarin studio to create a PCL and there is a free version of that.

Possible to create an application c# with no framework

for strict constrains in my scenarios, I have very few room to install my application, and .net framework is not installable (any version).
If the application is really simple (more or less), it is possible to create an application in visual studio (in c#) with no dependendency from the .net framework?
Thanks!

Short answer: NO.
There is no way to create a .NET application without any framework.

It is possible to compile a C# app such that it has no dependencies on any of the built-in .NET types & libraries, by using the /nostdlib switch (see http://msdn.microsoft.com/en-us/library/fa13yay7.aspx). You then need to supply your own System namespace.
However, this doesn't remove the need for the .NET framework on the target machine if you use the standard C# compiler. As well as containing the built-in types, the framework also includes the JIT IL compiler, the CLR extra, which all .NET executables and dll's are reliant on.
There are ways of compiling C# code such that it doesn't need the framework though. The Xamarin product for example (http://xamarin.com/), supports compiling C# code to native iOS apps, which are wholely independent of the .NET framework. I'm not aware of any equivalent for "desktop" OS's though.

Writing, compiling and running a C# program without .Net means running a special C# compiler that produces native code instead of managed code. I think such a compiler exists from WinRT for mobile phones, which uses COM instead of .Net (And C++/CX instead of C++/CLI). Code it produces does not depend on the .Net Framework, but does depend on the WinRT runtime.

You may create mono GTK# application and then use mkbundle to generate independent executable. You can use Visual Studio to build your logic and use Xamarin studio to build GTK# GUI. For more information about mkbundle see this and this.

To reply to your query. It really is not possible to create a .Net application without the .Net frame work. And moreover if you have installed Visual Studio by default it would have asked you to install .Net framework or would have installed it by default. In that scenario there is already .Net framework installed in your PC.
Thanks

dlls on windows and dylibs on macintosh

I have joined a project that has been written in xcode using objective-C that is using some frameworks. This project is aimed at adding Wii-mote functionality to both the windows and Macintosh versions of Unity3d.
My question is how should I go about getting a windows dll that mirrors the functionality of the Macintosh .bundle/.dylib?
Do I need to start translating the code from Objective-C to C#?
or,
Do I need to find/code my own windows .dll from scratch?
or,
Is there a library type that is compatible with both windows and Macintosh?
Thanks in advance for your help!

There is not a library type that is compatible with both systems. It might be possible to use C# libraries on both systems if you're using Mono on the Mac to run C# code, but you already stated that it's an Objective-C project, so I'm guessing that option is out.
So yes, you'll need to write your own .dll on the Windows side in your language of choice.

Why mono touch can write iOS apps in C# and can submit to AppStore?

Is this the mono touch using a C# interrupter or compiler to transfer the C# code to Objective C? But based on the Apple guideline, a interrupter or compiler application is not allow to submit on the AppStore. How is this magic going on? Thanks.

From the Introduction to MonoTouch documentation:
MonoTouch allows you to write native iOS applications using C# and the .NET Base Class Library (BCL). Via the magic of Mono’s compiler, applications built on the MonoTouch platform compile down directly to native ARM assembly code.
Since it is compiling down to ARM code, there is no interpretation happening on the device while your app is running, which is why apps are in compliance with the AppStore requirements.

In the documentation, Introduction to MonoTouch (emphasis added):
MonoTouch allows you to write native iOS applications using C# and the .NET Base Class Library (BCL). Via the magic of Mono’s compiler, applications built on the MonoTouch platform compile down directly to native ARM assembly code. This contrasts with compilers for traditional .NET applications that create Intermediate Language (IL) code that compiles at run time. Even things like generics, which used to rely on Just In Time (JIT) compilation are available via Mono’s Ahead Of Time (AOT) compiler.
So there is no interpreter. It compiles to native ARM assembly.

Please see this press release from Apple in 2010 which states that they have removed the restrictions on tools used to create iOS apps. (Hat tip to #ShaChris23's comment on this question.)
That explains why regular compilers are allowed - and as the other answers have explained, MonoTouch is a compiler, not an JIT interpreted/compiled language like Mono/.NET is on other platforms.

Can a C# program be cross-platform?

I'm a newbie to programming, and I'm considering using C# to write a VERY simple program that simply edits a text file.
Will it work on Macs and Linux? What will work?
I want to know what makes a program cross-platform?

C# can be compiled for and ran on multiple platforms (Windows, Linux, macOS) by using .NET Core.
Previously, cross-platform development using C# could only be done by using Mono which is a third-party implementation however it is now recommended to use .NET Core as it is developed by Microsoft.

Now C# is fully cross platform and has variety of frameworks and run times.
Java is a language + framework. But C# is just a modern language. C# language features are very powerful, easy to learn. No confusion between framework and language.
C# can be used in following frameworks
.Net core (Multi platform supports Linux, Windows and Mac), mostly used to develop restful services, web applications and enterprise systems
.Net Framework (Windows only), Same as .net core, but has more windows only features
Unity - One of most used game engine to develop games for PC, Sony, Xbox devices etc..
Xamarin - To develop native Mobile apps (Android, IOS, Mac)
.Net standard - Create a library in .net standard use in all above applications
Java - Code once, Ship same file to all platforms
C++, C, Rust - Code once, Compile for specific platform
C# - It is just a language, pick your framework according to requirement.
Most popular applications like Chrome, Firefox uses C++, Rust and they are compiled for each platform. But still they share the majority of code between these platforms. Some games use C++ to share the code between IOS and Android.
For new developers its good to start with NodeJS and C#. Both are easy and can do any type of applications.

It's hard to answer something like that to somebody, which states that he just starts programming, but I'll try to give you an (somewhat simplified) overview: C# is a language, which is compiled into an executable program. The language itself is portable between plattforms, as long as you have a compiler on each target plattform. On Windows you have the "default" .Net implementation and on Linux/Mac you have Mono. To do something useful - like editing files - you have to use some libraries. There is a standard library which is available on each plattform. As long as you use only that functionality, your program will be cross plattform. Obviously is will not be cross plattform, as soon as you use some library specific to a certain plattform. Editing files is part of the standard library, so you can implement such simple tools in a cross plattform way. When choosing a book, you should look for books which focus on the "raw" language and not on Visual Studio or other specific tools.

A new open source release from Microsoft called CoreCLR is cross-platform and can now be used for this
The CoreCLR repo contains the complete runtime implementation for .NET Core. It includes RyuJIT, the .NET GC, native interop and many other components. It is cross-platform, with multiple OS and CPU ports in progress.
-from the CoreCLR repo

I would not recommend c# as the top choice for multiplatform coding. I would recommend python instead it is very easy to run even as a script and works perfectly on all platforms. If you insist on c# be prepared to deal with the same problem explained by Justin Ethier. Python on the other hand, is the same for all platforms, it is super powerful fast and even has wrappers ready to connect to other languages.

C# is cross-platform thanks to .NET Core, but without a cross-platform User Interface library you can only make very basic console programs with it. In order to make cross-platform apps for Windows, Mac and Linux you need a cross-platform User Interface library like Nevron Open Vision

Yes, C# is fully cross-platform thanks to .NET Core.
.NET Core is a free and open-source, managed computer software framework for Windows, Linux, and macOS operating systems. It is the cross-platform successor to the .NET Framework which only run on Windows. The project is primarily developed and supported by Microsoft (through the .NET Foundation).

C# is cross platform and you can run your program in Windows/Linux and Mac. For Linux, though, you need to run it using mono