使用 LINQ 和 EF Core 查询

Question

我正在执行一项任务，该任务要求我搜索数据库以返回用户输入的城市中的公司名称。

我创建的方法可以正常工作并完成预期的工作，但是有两个问题：

1. 它在第一个foreach语句中打印了 6 次而不是仅一次。
2. 它没有在第一个foreach语句中返回正确数量的客户。

这是完整的方法：

static void ReturnCities()
{
    WriteLine("Enter the name of a city: ");
    string? city = ReadLine();

    using (var db = new Northwind())
    {
        // query to find all customers in the
        // city that was entered
        var query = db.Customers
                      .Where(customer => customer.City == city)
                      .Select(customer => new
                              {
                                  customer.City
                              });

        // prints a statement with how many customers
        // are in the city entered
        foreach (var item in query)
        {
            WriteLine("There are {0} customers in {1}: ",
                      arg0: item.City.Count(),
                      arg1: item.City);
        }

        // query to return a list of all customers 
        // in the city entered       
        var query2 = db.Customers
                       .Where(customer => customer.City == city)
                       .Select(customer => new
                               {
                                   customer.CompanyName
                               });

        foreach (var item2 in query2)
        {
            WriteLine($"    {item2.CompanyName} ");
        }
    }
}

output 目前是：

Enter the name of a city: 
London
There are 6 customers in London: 
There are 6 customers in London: 
There are 6 customers in London:
There are 6 customers in London:
There are 6 customers in London:
There are 6 customers in London:
    Around the Horn
    B's Beverages
    Consolidated Holdings
    Eastern Connection
    North/South
    Seven Seas Imports

我不确定是什么导致了除foreach语句之外的第一个问题，可能是因为它为查询中的每个项目返回了一个打印语句，但我无法修复它。

我目前仍在尝试解决第二个问题，但是对于这两个问题的任何和所有提示都表示赞赏！

Answer 1

您的查询将只返回Your query will just return an IQueryable<string> { "london", "london", "london" }因为Select() 。

您可以简单地对您的代码进行 1 次查询...

var query = db.Customers
    .Where(customer => new() {
        customer.City,
        customer.CompanyName
    });

var count = query.Count();
// output: 6

foreach(var customer in query) 
{
    Console.Writeline(customer.CompanyName);
}

// output:
//    Around the Horn
//    B's Beverages
//    Consolidated Holdings
//    Eastern Connection
//    North/South
//    Seven Seas Imports