ExtensionMethods.cs

public static TValue GetOrSet<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, Func<TKey, TValue> create)
{
    TValue value;
    return dictionary.TryGetValue(key, out value) ? value : (dictionary[key] = create(key));
}

public static TValue GetOrSet<TKey, TValue>(this IDictionary dictionary, TKey key, Func<TKey, TValue> create)
{
    var value = (TValue) dictionary[key];
    if (!Equals(value, default(TValue))) return value;
    var created = create(key);
    dictionary[key] = created;
    return created;
}