如何使用 Linq 检查对象列表属性是否允许来自另一个列表的值

Question

I have a class Person, in this class i have some properties. One of these properties is string Role. the user can input the role in an xml file that i should parse. I have a List of allowedRoles where the allowed roles are hardcoded. I want to check if the Role property and compare it to the allowedRoles list. I want to use linq to filter out the unallowed Roles.

public class Person
{
   Public string Name {get; set;}

   Public string Role {get; set;}
}

I have a list with allowed roles

var allowedRoles = new List<string>{"Developer","Admin","Recruiter"};

At a later stage i when i parsed the xml. I get a list of Person where i need to validate the role property against my AllowedRoles list. I dont want to use a loop. I would prefer using a linq expression to filter out the unvalid roles where Im left with a list of Persons that have the allowed roles. so I have a list of Person:

var persons = new List<Person>();

How do i use my AllowedRoles list to filter out the unallowed Roles from Persons?

Answer 1

One way of achieving this would be to use Where combined with Contains , for example:

var filteredPeople = persons.Where(p => allowedRoles.Contains(p.Role));

Answer 2

You can simply do:

var allowedPersons = persons.Where(p => allowedRoles.Any(r => r == p.Role));

Answer 3

You can use Linq Where extension method with the Contains method of List<string> like that:

static void Test()
{
  var persons = new List<Person>
  {
    new Person { Name = "1", Role = "Admin" },
    new Person { Name = "2", Role = "Customer" },
    new Person { Name = "3", Role = "Recruiter" },
    new Person { Name = "4", Role = "User" },
  };

  var allowedRoles = new List<string> { "Developer", "Admin", "Recruiter" };

  var personsAllowed = persons.Where(person => allowedRoles.Contains(person.Role));

  foreach ( var person in personsAllowed )
    Console.WriteLine(person.Name);
}

To get unallowed simply use the not operator:

var personsUnallowed = persons.Where(person => !allowedRoles.Contains(person.Role));

The contains method assumes that strings identically matches and are same case.

Output allowed

1
3

Output unallowed

2
4

Answer 4

With an extension method, you can write it in a more expressive way:

var filteredPeople = persons.Where(p => p.Role.IsIn(allowedRoles));

And your extension method:

public static class ExtensionMethods
 {
 public static bool IsIn<T>(this T item, List<T> list)
  {
  return list.Contains(item);
  }
 }

This reads nearly like real english: "where role is in allowed roles".