余小章 @ 大內殿堂

有關where的查詢表達式可參考以下：

http://msdn.microsoft.com/zh-tw/library/bb301979.aspx

 

首先創建我們要查詢的類別

[Serializable]
public class Customer
{
    public int ID { get; set; }

    public string CompanyName { get; set; }

    public string City { get; set; }

    public string Region { get; set; }

    public string Country { get; set; }

    public List<Order> Orders { get; set; }
}

[Serializable]
public class Order
{
    public int ID { get; set; }

    public int CustomerID { get; set; }

    public DateTime? OrderDate { get; set; }
}

 

初始化集合假資料

private List<Customer> _customerList = new List<Customer>()
{
    new Customer() { CompanyName = "台雞店", City = "台北", Country = "台灣", ID = 1,
        Orders =new List<Order>()
        {
            new Order(){ID = 1,CustomerID = 1,OrderDate = DateTime.Now.AddDays(1)},
            new Order(){ID = 2,CustomerID = 1,OrderDate = DateTime.Now.AddDays(2)},
            new Order(){ID = 3,CustomerID = 1,OrderDate = DateTime.Now.AddDays(5)},
        }},
    new Customer() { CompanyName = "頂泰風", City = "天龍", Country = "台灣", ID = 2 },
    new Customer() { CompanyName = "打了沒", City = "台北", Country = "台灣", ID = 3 },
    new Customer() { CompanyName = "台灣水妮", City = "高雄", Country = "台灣", ID = 4 ,
        Orders =new List<Order>()
        {
            new Order(){ID = 1,CustomerID = 4,OrderDate = DateTime.Now.AddDays(1).AddHours(1)},
            new Order(){ID = 2,CustomerID = 4,OrderDate = DateTime.Now.AddDays(3).AddDays(2)},
            new Order(){ID = 3,CustomerID = 4,OrderDate = DateTime.Now.AddDays(5).AddMonths(3)},
        }},
    new Customer() { CompanyName = "老是穿", City = "高雄", Country = "台灣", ID = 5 },
    new Customer() { CompanyName = "酊亡", City = "高雄", Country = "台灣", ID = 6 ,
        Orders =new List<Order>()
        {
            new Order(){ID = 1,CustomerID = 6,OrderDate = DateTime.Now.AddDays(3)},
            new Order(){ID = 2,CustomerID = 6,OrderDate = DateTime.Now.AddDays(7).AddHours(1)},
            new Order(){ID = 3,CustomerID = 6,OrderDate = DateTime.Now.AddDays(10).AddMonths(1)},
        }},
};

 

---

找出含有City==”台北"的物件

法一：

var query = from customer in this._customerList
            where customer.City == "台北"
            select customer;

foreach (var customer in query)
{
    Console.WriteLine("CompanyName : " + customer.CompanyName);
}

 

 

 

result：
CompanyName : 台雞店
CompanyName : 打了沒

法二：

var query = this._customerList.Where(c => c.City == "台北");

foreach (var customer in query)
{
    Console.WriteLine("CompanyName : " + customer.CompanyName);
}

result：
CompanyName : 台雞店
CompanyName : 打了沒

---

找出含有City==”台北" && 索引值<=3

法一：使用List.IndexOf

var query = from customer in this._customerList
            where customer.City == "台北" && this._customerList.IndexOf(customer) <= 3
            select customer;

foreach (var customer in query)
{
    Console.WriteLine("CompanyName : " + customer.CompanyName);
}

result：
CompanyName : 台雞店
CompanyName : 打了沒

法二：

var query = this._customerList.Where((c, index) => c.City == "台北" && index <= 3);
foreach (var customer in query)
{
    Console.WriteLine("CompanyName : " + customer.CompanyName);
}

result：
CompanyName : 台雞店
CompanyName : 打了沒

 

 

---

找出含有City==”台北" && 索引值<=3，在第一筆就離開查詢

法一：

var query = (from customer in this._customerList
             where customer.City == "台北" && this._customerList.IndexOf(customer) <= 3
             select customer).FirstOrDefault();

if (query != null)
{
    Console.WriteLine("CompanyName : " + query.CompanyName);
}

result：
CompanyName : 台雞店

法二：

var query = this._customerList.Where((c, index) => c.City == "台北" && index <= 3).FirstOrDefault();
if (query != null)
{
    Console.WriteLine("CompanyName : " + query.CompanyName);
}

result：
CompanyName : 台雞店

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