XamarinWebAPI - Sample project for using ASP.NET Web API in Xamarin iOS apps
This lab goes through the process of creating an ASP.NET Web API project in Visual Studio, and then writing an iOS app using Xamarin to consume the API. With these tools though, you can just as easily write an application that works for iOS, Andriod and Windows Phone. This application can also work for Windows Phone or Android by creating a different UI for those platforms. Xamarin has a number of tutorials on how to do this.