如何编写LINQ在lambda表达式

Question

加入命令我写了下面的JOIN语句在LINQ：

var result = (from od in orders 
join em in employees on od.EmployeeID equals em.EmployeeID 
join ct in customers on od.CustomerID equals ct.CustomerID 

select new MyJoin 
{ 
    OrderID = od.OrderID, 
    //OrderDate = od.OrderDate, 
    ShipCountry = od.ShipCountry, 
    CompanyName = ct.CompanyName, 
    ContactName = ct.ContactName, 
    EmployeeName = (em.FirstName + ' '+em.LastName), 
}) 

我怎么会写这样的lambda表达式？

Answer 1

我提供ReSharper的，作为一种服务：

var result = orders.Join(employees, 
         od => od.EmployeeID, 
         em => em.EmployeeID, 
         (od, em) => new { od, em }) 
        .Join(customers, 
         od => od.CustomerID, 
         ct => ct.CustomerID, 
         (obj, ct) => new MyJoin 
             { 
              OrderID = obj.od.OrderID, 
              OrderDate = obj.od.OrderDate, 
              ShipCountry = obj.od.ShipCountry, 
              CompanyName = ct.CompanyName, 
              ContactName = ct.ContactName, 
              EmployeeName = (obj.em.FirstName + ' ' + obj.em.LastName), 
             }); 

您正在使用Join扩展方法：

IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(
      this IEnumerable<TOuter> outer, 
      IEnumerable<TInner> inner, 
      Func<TOuter, TKey> outerKeySelector, 
      Func<TInner, TKey> innerKeySelector, 
      Func<TOuter, TInner, TResult> resultSelector); 

对于第一个电话，这是它与你的LINQ语法以上：

outer: orders 
inner: employees 
outerKeySelector: od.EmployeeID 
innerKeySelector: em.EmployeeID 
resultSelector: <this is implied in your LINQ statements> 

该outerKeySelector和innerKeySelector与来自LINQ的equals的效果进行了比较。