加入多个序列

考虑实体 Customer，Purchase 和 PurchaseItem 如下：

public class Customer
{
   public string Id { get; set } // A unique Id that identifies customer    
   public string Name  {get; set; }
}

public class Purchase
{
   public string Id { get; set }
   public string CustomerId {get; set; }
   public string Description { get; set; }
}

public class PurchaseItem
{
   public string Id { get; set }
   public string PurchaseId {get; set; }
   public string Detail { get; set; }
}

考虑以下实体的以下示例数据：

var customers = new List<Customer>()             
 {
    new Customer() {
        Id = Guid.NewGuid().ToString(),
        Name = "Customer1"            
    },
            
    new Customer() {
        Id = Guid.NewGuid().ToString(),
        Name = "Customer2"            
    }
 };        
    
 var purchases = new List<Purchase>() 
 {
     new Purchase() {                
         Id = Guid.NewGuid().ToString(),
         CustomerId = customers[0].Id,
         Description = "Customer1-Purchase1"            
     },

     new Purchase() {
         Id = Guid.NewGuid().ToString(),
         CustomerId = customers[0].Id,
         Description = "Customer1-Purchase2"            
     },
     
     new Purchase() {
         Id = Guid.NewGuid().ToString(),
         CustomerId = customers[1].Id,
         Description = "Customer2-Purchase1"            
     },

     new Purchase() {
         Id = Guid.NewGuid().ToString(),
         CustomerId = customers[1].Id,
         Description = "Customer2-Purchase2"            
     }
  };
    
 var purchaseItems = new List<PurchaseItem>() 
 {
     new PurchaseItem() {                
         Id = Guid.NewGuid().ToString(),
         PurchaseId= purchases[0].Id,
         Detail = "Purchase1-PurchaseItem1"            
     },

     new PurchaseItem() {                
         Id = Guid.NewGuid().ToString(),
         PurchaseId= purchases[1].Id,
         Detail = "Purchase2-PurchaseItem1"            
     },
     
     new PurchaseItem() {                
         Id = Guid.NewGuid().ToString(),
         PurchaseId= purchases[1].Id,
         Detail = "Purchase2-PurchaseItem2"            
     },

     new PurchaseItem() {                
         Id = Guid.NewGuid().ToString(),
         PurchaseId= purchases[3].Id,
         Detail = "Purchase3-PurchaseItem1"
     }
 };

现在，考虑下面的 linq 查询：

var result = from c in customers
            join p in purchases on c.Id equals p.CustomerId           // first join
            join pi in purchaseItems on p.Id equals pi.PurchaseId     // second join
            select new
            {
               c.Name, p.Description, pi.Detail
            };

要输出上述查询的结果：

foreach(var resultItem in result)
{
    Console.WriteLine($"{resultItem.Name}, {resultItem.Description}, {resultItem.Detail}");
}

查询的输出将是：

Customer1，Customer1-Purchase1，Purchase1-PurchaseItem1

Customer1，Customer1-Purchase2，Purchase2-PurchaseItem1

Customer1，Customer1-Purchase2，Purchase2-PurchaseItem2

Customer2，Customer2-Purchase2，Purchase3-PurchaseItem1

.NET 小提琴现场演示