准备三个 ContentPage 对象，暂时无需xaml：
MainPage.cs、ModelessPage.cs、ModalPage.cs。

MainPage.cs：

using System;

using Xamarin.Forms;

namespace one
{
    public class MainPage : ContentPage
    {
        public MainPage()
        {
            Title = "Main Page";

            Button gotoModelessButton = new Button { 
                Text = "Go to modeless Page",
                HorizontalOptions = LayoutOptions.Center,
                VerticalOptions = LayoutOptions.CenterAndExpand
            };

            gotoModelessButton.Clicked += async (sender, e) => {
                await Navigation.PushAsync(new ModelessPage());
            };

            Button gotoModalButton = new Button { 
                Text = "Go to Modal Page",
                HorizontalOptions = LayoutOptions.Center,
                VerticalOptions = LayoutOptions.CenterAndExpand
            };

            gotoModalButton.Clicked += async (sender, e) => {
                await Navigation.PushModalAsync(new ModalPage());
            };


            Content = new StackLayout
            {
                Children = {
                    gotoModelessButton,
                    gotoModalButton
                }
            };
        }
    }
}

ModelessPage.cs：

using System;

using Xamarin.Forms;

namespace one
{
    public class ModelessPage : ContentPage
    {
        public ModelessPage()
        {
            Title = "Modeless Page";

            Button goBackButton = new Button {
                Text = "Back to Main",
                HorizontalOptions = LayoutOptions.Center,
                VerticalOptions = LayoutOptions.Center
            };

            goBackButton.Clicked += async (sender, e) =>  {
                await Navigation.PopAsync();
            };

            Content = goBackButton;
        }
    }
}

ModalPage.cs：

using System;

using Xamarin.Forms;

namespace one
{
    public class ModalPage : ContentPage
    {
        public ModalPage()
        {
            Title = "Modal Page";

            Button goBackButton = new Button
            {
                Text = "Back to Main",
                HorizontalOptions = LayoutOptions.Center,
                VerticalOptions = LayoutOptions.Center
            };

            goBackButton.Clicked += async (sender, e) =>  {
                await Navigation.PopModalAsync();
            };


            Content = goBackButton;
        }
    }
}

加载内容：

using Xamarin.Forms;

namespace one
{
    public partial class App : Application
    {
        public App()
        {
            MainPage = new NavigationPage(new MainPage());
        }

        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
        }
    }
}

运行~~：

点击"Go to modeless Page"：

点击 "Back to Main" 即可返回到首页，与 顶部左边的 Back 按钮同等功能。

点击"Go to Modal Page"：

点击 "Back to Main" 即可返回到首页。

总结一下：

Navigation.PushAsync() 类似 iOS 中 UINavigationController 的 pushViewController;

Navigation.PushModalAsync() 类似 iOS 中 presentViewController；