Building apps is still an issue at any level of business, and app developers are still hard to come by and expensive. Five years into the adoption of mobile apps as a full-blown business necessity, not a lot of ground has been gained. Could ExitEvent use an app? Yes. Are we monkeying around with HTML5? You bet. Do we have the funds to go out and build native?

Not yet.

This is where Xamarin comes in. And in less than a year, the Cambridge company has driven enough organic growth out of their technology and their model — think 150,000 developers and 7,500 customers — to have landed $12 million is Series A from Charles River Ventures, Ignition Partners, and Floodgate.

I had a great conversation with CEO Nat Friedman yesterday and let me tell you something, he is psyched about the technology. He should be. The former Chief Technology and Strategy Officer at Novell, he and Miguel de Icaza, founder of the Mono project, spun the Mono technology out of Novell, and built Xamarin around it.

Xamarin makes cross-platform, mobile development software that lets developers build mobile apps with C# and .NET. These are native, no compromise mobile apps for iOS and Android, because it exposes the entire native API. It also compiles in native code, resulting in gains in performance.

This is potentially disruptive in two ways:

1. The native mobile application pool just increased by about 8 million developers

Talking rough numbers, there are about eight million developers on the C#/.NET Platform. I know this, and I used to be one of them (I still code, just not ninja code). Furthermore, Nat says that about 45% of business are still on the .NET platform.

These are medium-to-large companies (Nat says that Xamarin’s target for B2B is usually in that 12-member tech department range) that can start cranking out internal apps without bringing in new knowledge. Costly new knowledge.

But it’s not just B2B. Xamarin’s customers are actually about half App Store/Play Store/Amazon and half internal apps that will never traverse outside of the walls of a company — translate that to about half B2B, half B2C.

Developers are using Xamarin’s software to make public facing consumer apps and games (Infinite Flight is an iPad simulator) all the way to corporate applications (Just Enough is a mobile CRM app that will never be used by a consumer).

This means that not only will the cost of developing corporate apps in house go down, but the cost of, say, building your startup around a native app, will also go down, dramatically.

Not only that, but the experience level will go up. An iOS developer with three years of experience commands an astronomical salary or rate. C# has been around for ten years, and there are a lot of guys in short-sleeve white dress shirts just waiting to jump on the mobile bandwagon.

Nat is also proud of the fact that Xamarin’s technology can upgrade the careers of a whole lot of developers. It’s a quantum leap, he says.

2. The software is up to 90% cross-platform

Let me get technical for just a second. Xamarin’s software breaks an app down into two separate pieces. The first is the presentation layer, like the view in MVC. The second piece contains the backend, the business logic, the data layer services, etc.

All of this is compiled into native code such that, according to Nat, 50-90% of the code can be shared across iOS, Android, and WinMo.

This is another step in the build-once-deploy-everywhere dream we keep having.

Xamarin built up a cache of 7,500 customers, companies like Rdio and ClearChannel, and an army of 150,000 developers in less than a year, without raising a dime. With the cash, Nat says they plan on building on that organic growth, getting the word out, and, you know, converting a few more of those eight-million C# developers in mobile developers.

Xamarin has 30 employees, most of whom came from Mono, and recently opened an office in San Francisco. They’re also in 12 countries around the world.