BFS.cs

public class Tree<K, V>
    where K : class, IComparable<K>
    where V : class
{
    private Node<K, V> root;

    public V BreadthFirstSearch(K key) 
    {
        Queue<Node<K, V>> queue = new Queue<Node<K, V>>();
        root.Visited = true;
        queue.Enqueue(root);
        
        while(queue.Any())
        {
            var node = queue.Dequeue();

            if (node.Key == key)
            {
                return node.Value;
            }
            foreach (var child in node.GetAdjacentUnvisitedNodes())
            {
                child.Visited = true;
                queue.Enqueue(child);
            }
        }
        return default(V);
    }

    private class Node<K, V>
        where K : class, IComparable<K>
        where V : class
    {
        public K Key { get; set; }
        public V Value { get; set; }
        public bool Visited { get; set; }
        public IList<Node<K, V>> Nodes { get; set; }
        
        public IList<Node<K, V>> GetAdjacentUnvisitedNodes() 
        {
            return this.Nodes.Where(n => !n.Visited);
        }
    }
}