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public static bool TryAdd<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, TValue addValue)
{
  bool canAdd = !dict.ContainsKey(key);
  
  if (canAdd)
  {
    dict.Add(key, addValue);
  }
  
  return canAdd;
}

public static bool TryAdd<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, Func<TKey, TValue> addValueFactory)
{
  bool canAdd = !dict.ContainsKey(key);
  
  if (canAdd)
  {
    dict.Add(key, addValueFactory(key));
  }
  
  return canAdd;
}