Trie structure in c#

TrieNode.cs

using System;
using System.Collections.Generic;
using System.Linq;
namespace MyTrie {
    public class TrieNode {
        Dictionary<char, TrieNode> children = new Dictionary<char, TrieNode>();
        public int Size { get; set; }

        public TrieNode() {
            Size = 0;
        }

        private TrieNode GetNode(char c) {
            if (children.ContainsKey(c)) {
                return children[c];
            }
            return null;
        }

        private void SetNode(char c, TrieNode node) {
            if (!children.ContainsKey(c)) {
                children.Add(c, node);
            }
        }

        public bool Search(string word) {
            return Search(word, 0);
        }

        public bool Search(string s, int index) {
            if (index == s.Length) {
                return true;
            }
            TrieNode child = GetNode(s[index]);
            if (child == null) {
                return false;
            }
            return child.Search(s, index + 1);
        }

        public void Add(string s) {
            Add(s, 0);
        }

        private void Add(string s, int index) {
            Size++;
            if (s.Length == index) return;
            char current = s[index];
            TrieNode child = GetNode(current);
            if (child == null) {
                child = new TrieNode();
                SetNode(current, child);
            }
            child.Add(s, index + 1);
        }

        public int FindCount(string s, int index) {
            if (index == s.Length) {
                return Size;
            }
            TrieNode child = GetNode(s[index]);
            if (child == null) {
                return 0;
            }
            return child.FindCount(s, index + 1);
        }

        public List<string> Find(string s, int index) {
            if (index == s.Length) {
                return GetChildWords();
            }
            TrieNode child = GetNode(s[index]);
            if (child == null) {
                return null;
            }
            return child.Find(s, index + 1);
        }

        public List<string> GetChildWords() {
            List<string> l = new List<string>();
            foreach (var child in children) {
                if (l.Count > 0 && l[l.Count - 1] == "") {
                    l[l.Count - 1] = child.Key.ToString();
                } else {
                    l.Add(child.Key.ToString());
                }
                child.Value.GetChildWords(ref l);

            }
            if (l.Count > 0 && l[l.Count - 1] == "") {
                l.RemoveAt(l.Count - 1);
            }
            return l;
        }

        public void GetChildWords(ref List<string> l) {
            foreach (var child in children) {
                l[l.Count - 1] += child.Key;
                if (child.Value.children.Count == 0) {
                    l.Add("");
                    return;
                }
                child.Value.GetChildWords(ref l);
            }

        }
    }
}