LINQ: From a list of type T, retrieve only objects of a certain subclass S

Solution 1:

you can do this:

IList<Person> persons = new List<Person>();

public IList<T> GetPersons<T>() where T : Person
{
    return persons.OfType<T>().ToList();
}

IList<Student> students = GetPersons<Student>();
IList<Teacher> teacher = GetPersons<Teacher>();

EDIT: added the where constraint.

Solution 2:

This should do the trick.

var students = persons.Where(p => p.GetType() == typeof(Student));

Solution 3:

You could do this:

IEnumerable<Person> GetPeopleOfType<T>(IEnumerable<Person> list)
    where T : Person
{
    return list.Where(p => p.GetType() == typeof(T));
}

But all you've really done is rewrite LINQ's OfType() method with a safer version that uses static type checking to ensure you pass in a Person. You still can't use this method with a type that's determined at runtime (unless you use reflection).

For that, rather than using generics, you'll have to make the type variable a parameter:

IEnumerable<Person> GetPeopleOfType(IEnumerable<Person> list, Type type)
{
    if (!typeof(Person).IsAssignableFrom(type))
        throw new ArgumentException("Parameter 'type' is not a Person");

    return list.Where(p => p.GetType() == type);
}

Now you can construct some type dynamically and use it to call this method.