我目前在练习15.2的第15章中，我们的任务是创建一个Razor页面，该页面生成按国家/地区分组的客户列表。当您单击客户名称时，它将带您到一个新页面，该页面显示该客户的完整联系方式及其订单列表。

Taking baby steps, I have built a page that has a card that will have the header as the country and then list each customer name on the card. However, my foreach loop is spitting out each data column under Customer.Country. So if there are 11 countries with Germany, it makes 11 cards with Germany as the title (see image).

It is also populating all of the customer's names as well under each country.

I found that I can use GroupBy() but as I explain below, that causes an invalid cast exception.

customers.cshtml

@page

@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

@using NorthwindEntitiesLib

@model NorthwindWeb.Pages.CustomersModel

customers.cshtml.cs

using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

using Microsoft.AspNetCore.Mvc;

using Microsoft.AspNetCore.Mvc.RazorPages;

using NorthwindContextLib;

using NorthwindEntitiesLib;

namespace NorthwindWeb.Pages

{

public class CustomersModel : PageModel

{

private Northwind db;

public CustomersModel(Northwind injectedContext)

{

db = injectedContext;

}

public IEnumerable Customers { get; set; }

public void OnGet()

{

Customers = db.Customers

.ToArray();

}

}

}

NorthwindEntitesLib

namespace NorthwindEntitiesLib

{

public class Customer

{

public string CustomerID { get; set; }

public string CompanyName { get; set; }

public string ContactName { get; set; }

public string ContactTitle { get; set; }

public string Address { get; set; }

public string City { get; set; }

public string Region { get; set; }

public string PostalCode { get; set; }

public string Country { get; set; }

public string Phone { get; set; }

public string Fax { get; set; }

public ICollection Orders { get; set; }

}

}

I have tried using GroupBy to group the Countries and then populate the list but it gives an error for an invalid casting of type string.

@foreach (Customer customer in Model.Customers.GroupBy(c =>c.Country))

{

}

InvalidCastException：无法将类型为“ System.Linq.Grouping`2 [System.String，NorthwindEntitiesLib.Customer]”的对象强制转换为“ NorthwindEntitiesLib.Customer”。