We Keep Coding

What Game Engine/Platform to use in order to Write Once and Deploy Everywhere?

Basically, I want to design a strategy game, where most of the game will be spent in menu areas and making "decisions" (like a Tycoon style game), however, there will still be some graphics, and I would prefer 3D, but if not possible, can resort to 2D. The graphics aren't user controlled, but the moving objects will be the result of the "decisions" the players make.
I really just want to write it once, and have it run on the following Platforms:
Windows Desktop
iOS (iPhone, iPod, iPad)
Android Phones (not strictly required)
(any other platforms would just be a bonus, but at the absolute minimum, all i really want is Windows Desktop and iOS [iPhone, iPod, iPad]).
Now, I don't want to learn objective-c or any other languages which i may have no use for in the future, I really just want to be able to program in Visual Studio (VB.Net or C#, since I can convert from vb.net to c# I will just be writing it in vb.net).
Is it possible to write in vb.net/c# and deploy to all these platforms? I heard of MonoTouch, so i can convert from vb to c# but is this seemless or difficult to do? Do I have to learn and write in another language in order to make tweaks to make it work on the iPhone or Android? So, with MonoTouch, can I still use XNA or is there a different engine that I need to learn/use with Visual Studio?
Alternatively, if I cannot do this all in VB.NET or Visual Studio, what programming language and/or graphics engine should I learn in order to be able to write once and deploy everywhere that is relevant? (So, not fussed about Linux, Mac's or other less used OS's).
Right now, I haven't learnt XNA but am wondering if I should or need to, or if i should learn another engine & language. Basically, I don't really know what I need to learn/know in order to be able to write once and deploy on Windows Desktop, iOS (iDevice's) and maybe Android. So, my question really is, what do I need to learn in terms of both the programming language and game/graphics engine. Right now I know VB.NET and would prefer to use what I know, but if not possible, then am prepared to learn another language & engine combination to be able to achieve what I want, if this is what I need to do, I would like to know what language & engine I need to learn.

A good option for this is Unity3D. It allows you to develop a game, and from a single source, publish to PC, Mac, iOS, Android, and other platforms.
It does support C# development (which was one option you listed), though it typically is handled via their own editor instead of within Visual Studio. That being said, they do support Visual Studio as the code editor for projects.

I never tried it out (but I tried MonoTouch - and it worked like a charm), but MonoGame could be the solution for you.
It's C# with XNA for deploying everywhere. For MonoTouch there was a rather hacky way to work with VisualStudio - but for my part it was a good way to work with MonoDevelop (a really good OpenSource-IDE btw.).

C# was invented by Microsoft for use on their products and XNA and Visual Studios are again microsoft development tools, if you're looking to place this across several platforms it might be less headache to consider Java or C++. also as said above, go look for this info it is more then readily available after some 5 minutes of search

Is Mono for OSX viable for learning C#?

I need to learn C#. All of my computers are Macs running the latest version of OS X. I do not want to use Windows, but I will if I must.
That being said, as a new programmer, can I learn C# efficiently with this Mono platform on OS X?
Edit I'm looking to learn C# to gain programming knowledge and start indie game development. After a reasonable understanding is achieved I will then look at some gaming frameworks/platforms and or a more OS specific language.

In a word: Yes.
First of all, thanks Jonathan Pobst for updating What Is Mono page. With that said, Mono supports all versions of C# at present, that is 1.0 to 4.0.
Since your plan is to learn the language first before looking into other things, such as frameworks and tools, you should be fine with Mono. Once you start getting into platform-specific development (i.e., Windows GUI), then you might want to consider switching over to a Windows machine.
http://mono-project.com/What_is_Mono
http://en.wikipedia.org/wiki/C_Sharp_(programming_language)#Versions

If you are going to be developing for Windows, then using a Windows machine is your best bet. If you are simply trying to learn the language, Mono should be fine.

You should be able to, but just keep in mind that while most of the example code will work, some may not on the Mono framework as the implementation is not 100% compatible with .NET Framework. Also, of course some things like WPF aren't be supported by Mono.
I have used Monodevelop on Linux and it mostly works but because I also develop on Windows with VS2010, I am put off by using the inferior Monodevelop but as a newbie it shouldn't matter to you much.

Yes that should be fine seeing as MonoMac has come out. http://www.mono-project.com/MonoMac
If your wanting to use it for games with OpenGL, download this repo and look at the openGL stuff to get started. Then go to places like NeHe to learn more about GL ect..
You will need to download & install git to pull the repo:: http://git-scm.com/download
MonoMac Samples:: https://github.com/mono/monomac/tree/master/samples/
NeHe:: http://nehe.gamedev.net/
Code Sampler:: http://www.codesampler.com/

Depends.
To just learn the C# language and CLR core libraries which are not graphics related, C# / MonoDevelop is okay.
To learn graphics programming, game programming for OS X, I suggest you first learn Objective-C, Cocoa and then MonoDevelop/MonoMac. The reason is that the Cocoa API's are designed around the Objective-C language which has fundamental differences to C#. Using Cocoa via C# is easy to begin with, but quickly makes little sense unless you understand Cocoa patterns & Objective-C.

Can C++ and C# be used for applications for iPhone

Can I write applications for iPhone in C++ or C#?
Where can I find simulators for iPhone for testing my apps.
How to write them?

You need at least a small Objective C stub to hook into the system and deal with provided services (including getting input), but your program can be primarily in C++ if you would like. Apple seems to disallow C#; tools such as MonoTouch appear to be banned by the current developer agreement.
With a Macintosh, you go to Apple's developer website and download the tools and SDKs for free. They only run on the Mac.

You will want to start here for iPhone development:
http://gemma.apple.com/iphone/index.action
This is where you will get the iPhone SDK, simulator, and other essential tools.

You can use C, Obj-C, and C# (via MonoTouch).
C++ is a partial via Objective-C++. If you're serious about iPhone development though, I would suggest learning Objective-C.
Apple seems to reserve the right to kill the ability to run any application that wasn't writting in either C or Objective-C.

As far as I know, you can use C# for the iPhone using MonoTouch.
Another tool is unity, which is geared towards games and is based on the same codebase.
Both of these are proprietary and cost $$$.

How toI run a game made with XNA on the iPhone/iTouch?

How could I run a game made with XNA on the iPhone/iTouch? Which steps/tools (existing ones or imaginary...) should be used?
Note: The goal is to avoid modifying existing C# code
UPDATE :
If I understand correctly, I must be able to:
Run my XNA code on Mono (monoxna or SilverSprite, promising?)
Run Mono on iPhone (MonoTouch)

Not only is it possible but here is a video of someone doing XnaTouch on MonoTouch: First game to IPhone build with XnaTouch (XNA for IPhone)
Here is the mono article about doing it http://www.mono-project.com/MonoTouch

I don't believe there is a good answer to your question. XNA doesn't target the iPhone, so the chances of being able to effectively port an XNA game without modifying the C# source code isn't likely to happen.
Instead, I'd recommend that you take a look at the various frameworks that exist to help you craft cross-platform games. Unity often comes up in these discussions, but it isn't free.
If cross-platform isn't your goal, but free iPhone development is, then I'd recommend looking at Cocos.
Edit: The MonoTouch project may be able to assist you in the future, but doesn't help you out right now. Still, it's something to keep an eye on.
Edit: The landscape has changed a lot in the ~5 years since this question was posted. If you have an XNA project that you want to get running on iOS, then Xamarin.iOS (formerly MonoTouch) plus MonoGame is a near-perfect fit. MonoGame is missing a huge chunk of the XNA content pipeline, which means you'll either have to abandon it or have a VS2010 instance somewhere compiling your assets.

The MonoTouch project may eventually help here, since it allows you to write C# targeting the iPhone (it is statically compiled to native code).

MonoGame is a free OpenGL implementation of the XNA 4.0 Framework. It is built upon the excellent range of Mono compilers and is compatible with MonoTouch (iOS), Mono for Android (Android), MonoMac (Mac OS X), Mono for Windows and now Linux!
A list of 12 games currently using MonoGame that are on the Apple iOS App Store can be found #
http://monogame.codeplex.com/
https://github.com/mono/MonoGame/wiki/Released-Games

I believe XNA depends on Direct3D 9 (see http://msdn.microsoft.com/en-us/xna/aa937793.aspx), which may hint at the hurdles one might encounter in both porting to mono as well as having sufficient graphics horsepower on iPhone.

Everyone saying this is possible because there are CLR libraries for Mono, does not even think about the fact that XNA is a WHOLE DIFFERENT set of frameworks:
"The XNA Framework class library is a library of classes, interfaces, and value types that are included in XNA Game Studio."
As another responder noted, this is so far from happening it's not even funny. If you really want to write a game I'd check out Unity. It's commercial, but it looks utterly impressive and you can script your game logic in Javascript or (here's the funny part) C#!
Yes, instead of going through a ton of work to port a whole framework because you want to write a game in C#, why not just use a framework that lets you do that today?

From the horse's mouth:
MonoTouch + SilverSprite = XNA 2D
games on iPhone? :)
What Bill means is that it will eventually be feasible to write a 2D XNA game, then use SilverSprite to run it on Silverlight, then use MonoTouch to run it on the iPhone

This space is definitely heating up. There is now an XNA Touch project on codeplex that aims to bring the XNA API onto the iPhone/iPad platform:
http://xnatouch.codeplex.com/

'Maybe' you could just change XnaTouch's namespace to match those of existing Xna code (i.e. Microsoft.Xna.Framework.*), when developing a MonoTouch project? This could be done on a vendor fork (copy) of the XnaTouch code, easy to manage using Git, Mercurial or Piston etc.
I'm unsure as to whether the XnaTouch team would undertake such a change on its own codebase, so this is probably best done on your own, personal, code branches (interestingly, the Mono.xna project uses the original Microsoft.Xna.Framework.* namespaces, I'm unsure why XnaTouch chose not do so).
JFYI, the current XnaTouch (v1.0) follows Xna 4.0 quite faithfully. I've found a [small] few missing method calls, which I've 'implemented' to throw not implemented exceptions (?). The original-xna4-and-modified-xna-touch code does compile, now I need to implement iphone-specific gui and handling etc.
Hope this helps someone.
Cheers
Rich

For starters you would need a CLR implementation on the iPhone, which doesn't exist at the moment, but it seems someone is trying: MonoTouch.

A friend of mine is developing an architecture port of XNA 4.0 for the iOS platform. It's of course written in Objective-C, but it has the feel of XNA. He has published the source under LGPL at http://code.google.com/p/xni/.

Xna is not officially supported on iPhone; however, you could use ExEn (http://exen.codeplex.com/) or MonoGame (http://monogame.codeplex.com/) to port your Xna games. A bonus for using either of these is that they support Mono for Android.

If you're looking for a free cross-platform sort of thing, you could check out phonegap. I don't have any experience with it but it looks cool.
http://phonegap.com/

Can Adobe Air be the best solution for a Mac and iPhone application?

We are at a crossroad as what development tool to use for our application. We do not know if
we will go for Adobe Air or use the traditional road which is Objective-C and Cocoa. The software is similar to www.riffmasterpro in functionality. Is Air the best solution here?

Well, firstly, Flash isn't supported on iPhone so there is no Air platform for iPhone (at least, none that I know of - correct me if I'm wrong).
Secondly, native Objective-C applications on Mac will always surpass Air applications with regards to how well they tie into the OS. An example of this is TweetDeck. It's an amazing application but it just doesn't feel like a Mac app (no meaningful menus, no growl integration, etc...). Mac users are typically very specific with regards to how their apps behave.
I would advise that if you want to write Mac applications (and iPhone applications) to rather stick with Objective-C.
If your primary concern is cross-platform (with the exception of iPhone) then Air is an option worth considering.

If you're looking to build an app solely for the Mac/iPhone platform, I would suggest using Objective-C instead. Adobe AIR's strength is that it can be installed and used cross-platform with little problem.
One of the downsides of that, though, is that performance will never be as good as something that ties directly into the operation system.

problem of using middle frameworks is that you will never have the potential of the hardware, in other worlds you will not be able to use any feature that Mac/Windows can give to you by the hardware, you wll always be attached to what the framework will give to you and hope that you can do everything that you need...
specially in your music application.
regarding the Adobe Air framework, it is a good idea to cover both Linux, Mac and Windows computers but you will not be able to run it under the iPhone because there is no Adobe Flash supported... Adobe is making a flash player for the iPhone, but I do believe that will ot support Air, at least by the near future.
For the iPhone you can build a Web Application or a SDK Application, the last on, and if you will use SDK 3.0, you will find plenty of good things to use it to, but you need to develop a full application from sctrath.