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

问题:

Given a simple inheritance hierarchy: Person -> Student, Teacher, Staff

Say I have a list of Persons, L. In that list are some Students, Teachers, and Staff.

Using LINQ and C#, is there a way I could write a method that could retrieve only a particular type of person?

I know I can do something like:

var peopleIWant = L.OfType< Teacher >();

But I want to be able to do something more dynamic. I would like to write a method that will retrieve results for any type of Person I could think of, without having to write a method for every possible type.

回答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.

回答2:

This should do the trick.

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

回答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.

回答4:

For general list, using delegate:

public List<T> FilterByType(List<T> items, Type filterType)
{
    return items.FindAll(delegate(T t)
    {
        return t.GetType() == filterType;
    });
}