using System;

using System.Collections.Generic;

using System.Linq;

​

namespace StackOverflowTest

{

    // https://stackoverflow.com/a/56157672/69809

    public class Program

    {

        public static void Main(string[] args)

        {

            var registry = new Dictionary<string, Test>();

​

            AddNext(registry);  // 1

            AddNext(registry);  // 2

            AddNext(registry);  // 3

​

            Delete(registry, 2);

            AddNext(registry);  // 2

​

            Delete(registry, 1);

            Delete(registry, 2);

            AddNext(registry);  // 1

            AddNext(registry);  // 2

            AddNext(registry);  // 3

        }

        class Test

        {

            public int SequenceNum { get; set; }

        }

​

        static void Delete(Dictionary<string, Test> registry, int val)

        {

            Console.WriteLine("Deleting: " + val);

            registry.Remove(val.ToString());

        }

​

        static void AddNext(Dictionary<string, Test> registry)

        {

            var next = GetNextSequenceNo(registry);

            Console.WriteLine("Next seq num. is " + next);

            registry[next.ToString()] = new Test { SequenceNum = next };

        }

​

        static int GetNextSequenceNo(Dictionary<string, Test> registry)

        {

            // order the values ascending

            var vals = registry.Values.OrderBy(s => s.SequenceNum).ToList();

​

            // find the first value where .SequenceNum is different from (idx + 1)

            var firstGap = vals.TakeWhile((s, idx) => s.SequenceNum == idx + 1).Count();

​

            // take the sequenceNum from the previous item and increment

            if (firstGap > 0)

                return vals[firstGap - 1].SequenceNum + 1;

            else

                return 1;

        }

    }

}

​