Dictionary - AddIfNotExists Extension Method

DictionaryExtensionMethods.cs

/// <summary>
/// Very simple extension method for adding an item to a dictionary
/// and not evaluating the function unless the key doesn't exist.
///
/// Example with Entity Framework:
/// dataValueCache.AddIfNotExists(dataID, () => db.DataItems.First(d => d.DataID == dataID).Value);
/// </summary>
/// <param name="key">The dictionary key.</param>
/// <param name="value">The dictionary value.</param>
/// <returns>If the item was added to the dictionary.</returns>
static public bool AddIfNotExists<T, U>(this Dictionary<T, U> source, T key, Func<U> value)
{
    if (!source.ContainsKey(key))
    {
        source.Add(key, value());
        return true;
    }
    else
    {
        return false;
    }
}