xamarin 的四个文件：App.xaml，App.xaml.cs，onePage.xaml，onePage.xaml.cs。

App.xaml：

<Application xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="one.App">
    <Application.Resources>
        <!-- Application resource dictionary -->
    </Application.Resources>
</Application>

App.xaml.cs：

using Xamarin.Forms;

namespace one
{
    public partial class App : Application
    {
        public App()
        {
            InitializeComponent();

            MainPage = new onePage();
        }

        protected override void OnStart()
        {
            // Handle when your app starts
        }

        protected override void OnSleep()
        {
            // Handle when your app sleeps
        }

        protected override void OnResume()
        {
            // Handle when your app resumes
        }
    }
}

onePage.xaml：

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:local="clr-namespace:one" x:Class="one.onePage">
    <Label Text="Welcome to Xamarin Forms!" VerticalOptions="Center" HorizontalOptions="Center" />
</ContentPage>

onePage.xaml.cs：

namespace one
{
    public partial class onePage : ContentPage
    {
        public onePage()
        {
            InitializeComponent();
        }
    }
}

效果如下：