在包含对象属性的列表中查找

Question

I'm trying to find which items in my list fill a certain criteria.

I have a List<Employee> , and each Employee has a List<Role> attribute. Each Role has an ID as an attribute. I'm trying to find all Employee s that have a certain Role ID in the list. Here is my non-working sample:

var query = EmployeeList.Where(employee=> employee.Roles.Contains(role => role.ID == roleID)).ToList();

Answer 1

Use Enumerable.Any

var query = EmployeeList.Where(employee => employee.Roles
                                                   .Any(role => role.Id == roleID))
                                                   .ToList();

Answer 2

You can just change your Contains to Any (as you're just checking whether any of the roles of the employee matches your condition):

var query = EmployeeList.Where(employee => employee.Roles
                                                 .Any(role => role.ID == roleID))
                        .ToList();

Note that this won't be a terribly efficient approach - every employee has to be checked, and every role of every employee. If you need to do this often with the same set of employees but different role IDs, you could build a lookup from role ID to employees:

var lookup = EmployeeList.SelectMany(e => e.Roles,
                                     (e, r) => new { Employee = e, Role = e })
                         .ToLookup(pair => pair.Role.ID,
                                   pair => pair.Employee);

You can then just use:

foreach (var employee in lookup[roleID])
{
    ....
}