using System.Collections.Generic;

using System.Linq;

public class Matcher

{

    // The know position action block would populate this list

    private Dictionary<int,KnownPosition> _knownPositions = new Dictionary<int, KnownPosition>();

    private int _walkIndex = 0;

    public void DoMatching()

    {

        _walkIndex++;

        var matches = _knownPositions.Where(x=> x.Key <= _walkIndex);

        if (matches.Any())

        {

            var ordered = matches.OrderByDescending(x=> x.Key);

            ResetParticleFilterToKnownPosition(ordered.First().Value);                  

        }

        //Continue with what we normally do in the matcher

    }

    private void ResetParticleFilterToKnownPosition(object knownPosition){}

}

​

public class KnownPosition{

}