打开App.xaml.cs文件，讲代码改成如下：

using Xamarin.Forms;

namespace one
{
    public partial class App : Application
    {
        public App()
        {
            MainPage = new ContentPage
            {
                Content = new StackLayout
                {
                    VerticalOptions = LayoutOptions.Center,
                    Children = {
                        new Label {
                            HorizontalTextAlignment = TextAlignment.Center,
                            Text = "Welcome to My Xamarin Forms!"
                        }
                    }
                }
            };
        }

        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
        }
    }
}

效果如下：

很明显这是用代码实现了界面的内容。而前面则是用xaml方式布局内容的。