private Dictionary<int, Definition> m_typeDefinitions = new Dictionary<int, Definition>()

public Definition GetDefinition( EntityType type )
{
    Definition def = null;
    int typeIndex = (int) type;

    m_typeDefinitions.TryGetValue( typeIndex, out def );

    // return "null" can be considered as a valid value,
    // and you handle the lack of stuff found in later code
    return def;
}