光脚丫学LINQ(002)：筛选数据

时间：2010-10-28 来源：光脚丫思考

也许最常用的查询操作是应用布尔表达式形式的筛选器。此筛选器使查询只返回那些表达式结果为 true 的元素。使用 where 子句生成结果。实际上，筛选器指定从源序列中排除哪些元素。在下面的示例中，只返回那些地址位于伦敦的 customers。

NorthwindDataContext db = new NorthwindDataContext();   
  
var LondonCustomers = from Customer in db.Customers   
                      where Customer.City == "London"  
                      select Customer;   
  
foreach (var Customer in LondonCustomers)   
{   
    Console.WriteLine("---------------------");   
    Console.WriteLine("Customer ID : {0}", Customer.CustomerID);   
    Console.WriteLine("Customer Name : {0}", Customer.ContactName);   
    Console.WriteLine("City : {0}", Customer.City);   
}  
NorthwindDataContext db = new NorthwindDataContext();

var LondonCustomers = from Customer in db.Customers
                      where Customer.City == "London"
                      select Customer;

foreach (var Customer in LondonCustomers)
{
    Console.WriteLine("---------------------");
    Console.WriteLine("Customer ID : {0}", Customer.CustomerID);
    Console.WriteLine("Customer Name : {0}", Customer.ContactName);
    Console.WriteLine("City : {0}", Customer.City);
}

您可以使用熟悉的 C# 逻辑 AND 和 OR 运算符来根据需要在 where 子句中应用任意数量的筛选表达式。例如，若要只返回位于“伦敦”AND 姓名为“Thomas Hardy”的客户，您应编写下面的代码：

var LondonCustomers = from Customer in db.Customers   
                      where Customer.City == "London" && Customer.ContactName == "Thomas Hardy"  
                      select Customer;  
var LondonCustomers = from Customer in db.Customers
                      where Customer.City == "London" && Customer.ContactName == "Thomas Hardy"
                      select Customer;

若要返回位于伦敦或巴黎的客户，您应编写下面的代码：

var LondonCustomers = from Customer in db.Customers   
                      where Customer.City == "London" || Customer.City == "Paris"  
                      select Customer;