我想在我的项目中使用 datepicker。这是我第一次使用 bootstrap。问题是当我单击日期表单时未显示 datepicker。我已经尝试在标签内部和内部替换 CDN url 位置，但仍然没有工作。我不知道我犯了什么错误，希望你们能帮助我解决这个问题，谢谢。
<!DOCTYPE html>
<html lang="en">
<head>
<!-- Required meta tags -->
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<!-- Bootstrap CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" integrity="sha384-rwoIResjU2yc3z8GV/NPeZWAv56rSmLldC3R/AZzGRnGxQQKnKkoFVhFQhNUwEyJ" crossorigin="anonymous">
<!-- datepicker -->
<link rel="stylesheet" href="https://code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<link rel="stylesheet" href="/resources/demos/style.css">
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$(function() {
$("#datepicker").datepicker();
});
</script>
</head>
<body>
<nav class="navbar navbar-toggleable-md navbar-inverse bg-inverse">
<button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarsExampleDefault" aria-controls="navbarsExampleDefault" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<a class="navbar-brand" href="#">Menu</a>
<div class="collapse navbar-collapse" id="navbarsExampleDefault">
<ul …我是 Xamarin 的新手。我想向Book班级添加信息，然后使用displayalert()方法显示信息并且它有效。但是为什么会出错？希望你们能帮我解决这个问题。先谢谢你。
新书页.xaml
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="Form.NewBookPage">
<StackLayout Margin="10,10,10,0">
<Entry x:Name="nameEntry"
Placeholder="name of the book"/>
<Entry x:Name="authorEntry"
Placeholder="name of the author"/>
<Button Text="save"
Clicked="Button_Clicked" />
</StackLayout>
</ContentPage>
新书页.xaml.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;
namespace Form
{
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class NewBookPage : ContentPage
{
public NewBookPage ()
{
InitializeComponent ();
}
private void Button_Clicked(object sender, EventArgs e)
{
Book book = new …