public class GenericHashtable<K, V> : Hashtable, IEnumerable<KeyValuePair<K, V>>
{
  public GenericHashtable() : base() {}
  public GenericHashtable(int capacity) : base(capacity) {}
  
 public virtual void Add(K key, V value)
  {
    base.Add(key, value);
  }
  
  [Obsolete("The key must be type of K, value must be type of V!", true)]
  public new void Add(object key, object value)
  {  
    //this method can't be used anymore
  }

  public V this[K key]
  {
    get
    {
      return (V)base[key];
    }
    set
    {
      base[key] = value;
    }
  }

  [Obsolete("The key must be type of K, value must be type of V!", true)]
  public new object this[object key]
  {
    get { return null; } set { }
  }

  public new IEnumerator<KeyValuePair<K, V>> GetEnumerator()
  {
    IDictionaryEnumerator enumerator = base.GetEnumerator();
    while (enumerator.MoveNext())
      yield return new KeyValuePair<K, V>((K)enumerator.Key, (V)enumerator.Value);
  }

  public override string ToString()
  {
    StringBuilder result = new StringBuilder();
    foreach (KeyValuePair<K, V> elem in this)
    {
      result.AppendFormat("[{0}; {1}]\r\n", elem.Key, elem.Value);
    }
    return result.ToString();
  }
}