Linq 左连接

左连接：

var LeftJoin = from emp in ListOfEmployees 
　　　　　　     join dept in ListOfDepartment on emp.DeptID equals dept.ID
               into JoinedEmpDept 
               from dept in JoinedEmpDept.DefaultIfEmpty() 
               select new  
               { 
                     EmployeeName = emp.Name, 
                     DepartmentName = dept != null ? dept.Name : null  
               };