一个LINQ查询，用于在使用实体框架核心时从多个实体获取计数

using System;
using System.Linq;

// 假设有两个实体类：Person 和 Order
public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Order
{
    public int Id { get; set; }
    public int PersonId { get; set; }
    public decimal Amount { get; set; }
}

public class Program
{
    public static void Main()
    {
        // 假设有一个包含多个 Person 和 Order 实例的数据源
        var people = new[]
        {
            new Person { Id = 1, Name = "Alice" },
            new Person { Id = 2, Name = "Bob" },
            new Person { Id = 3, Name = "Charlie" }
        };

        var orders = new[]
        {
            new Order { Id = 1, PersonId = 1, Amount = 100 },
            new Order { Id = 2, PersonId = 1, Amount = 200 },
            new Order { Id = 3, PersonId = 2, Amount = 150 },
            new Order { Id = 4, PersonId = 3, Amount = 300 }
        };

        // 使用LINQ查询从多个实体获取计数
        var count = (from person in people
                     join order in orders on person.Id equals order.PersonId
                     select person).Count();

        Console.WriteLine("Count: " + count);
    }
}