从HashSet中选择所有特定项目

Question

I have a simple class called User :

public class User
{
    public int ID { get; set; }
    public int MI { get; set; }

    public User(int id, int mi)
    {
        ID = ID;
        MI = mi;
    }
}

And later on, I have a HashSet of Users that I want to get the ID's from and assign to a in HashSet as follows :

    HashSet<Users> _users = new HashSet<>();
    //code where several User objects are assigned to _users
    HashSet<int> _usersIDs = new HashSet<int>();
    _usersIDs = _users.Select("ID")

But this doesn't work, how can I successfully assigned all of the int ID's in _users to a new HashSet?

Answer 1

You can do:

HashSet<int> _usersIDs = new HashSet<int>(_users.Select(user=> user.ID));

But you should override GetHashCode for your User class if you are going to use it in a HashSet<T> and possibily Eqauls as well like:

public class User
{
    protected bool Equals(User other)
    {
        return ID == other.ID && MI == other.MI;
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != this.GetType()) return false;
        return Equals((User) obj);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return (ID*397) ^ MI;
        }
    }

    public int ID { get; set; }
    public int MI { get; set; }

    public User(int id, int mi)
    {
        ID = id; //based on @Jonesy comment
        MI = mi;
    }
}