xamarin.forms-dependencyservice.cs

/// XF Project
namespace UsingDependencyService
{
    public interface IToDo {
        void ToDo (); //note that interface members are public by default
    }
}

namespace UsingDependencyService.Service
{
    public void DoSomething() 
    {
        ...
        DependencyService.Get<IToDo>().ToDo(); // DependencyService Call
        ...
    }
}

/// Android Project
[assembly: Dependency (typeof (ToDo_Android))]
namespace UsingDependencyService.Android
{
    public class ToDo_Android : IToDo
    {
        public void ToDo ()
        {
            ... // here your things to do
        }
    }
}

/// iOS Project
[assembly: Dependency (typeof (ToDo_iOS))]
namespace UsingDependencyService.iOS
{
    public class ToDo_iOS : IToDo
    {
        public void ToDo ()
        {
            ... // here your things to do
        }
    }
}