/// <summary>
/// Return the object value associated with the provided string key.
/// If no value exists for that key, return null.
/// </summary>
/// <param name="key">the key of the key/value property</param>
/// <returns>the object value related to the string key</returns>
public override object GetProperty(string key)
{
PropertyType pt = m_vertexType.FindProperty(key);
return(m_vertexType.GetPropertyValue(m_id, pt));
}