enumerate hashset and delete elements from it

Question

I want to go through a HashSet and make a (complicated) check on each element which results in saving the element, removing the element from the HashSet or doing nothing.

Since a foreach loop does not allow me to change the HashSet and an index is not available, I do not know how to perform the task (without doing slow things like copying the HashSet first or applying several LINQ operations which means enumerating the HashSet more than once).

Any suggestions?

Answer 1

You just need to use RemoveWhere() with an appropriate predicate function.

You can have a side-effect of your predicate which copies the element being checked (assuming that's what you mean by "save the element"), if needed. Perhaps that sounds a little hacky, but it would work fine I think.

This works because your predicate function will be presented with each element of the HashSet in an undefined order, and therefore you can decide what to do with each element, as well as returning true to delete it and false to keep it.

[EDIT] Here's a code sample.

using System;
using System.Collections.Generic;

namespace Demo
{
    public class Program
    {
        [STAThread]
        private static void Main(string[] args)
        {
            var hashSet = new HashSet<int> {4, 0, 6, -1, 23, -8, 14, 12, -9, 5, 2};
            var itemProcessor = new ItemProcessor();

            hashSet.RemoveWhere(itemProcessor.Process);

            Console.WriteLine("Max = {0}, Min = {1}", itemProcessor.Max, itemProcessor.Min);
            Console.WriteLine("\nHashSet contents:");

            foreach (int number in hashSet)
            {
                Console.WriteLine(number);
            }
        }
    }

    public sealed class ItemProcessor
    {
        private int max = int.MinValue;
        private int min = int.MaxValue;

        // Removes all negative numbers and calculates max and min values.

        public bool Process(int item)
        {
            max = Math.Max(item, max);
            min = Math.Min(item, min);

            return (item < 0);
        }

        public int Max { get { return max; } }
        public int Min { get { return min; } }
    }
}