Generic get/set operation for types that are stored in an implementation of <see cref="IDictionary"/>

GetFromDictionary.cs

public static T GetFromDictionary<T>(string key, IDictionary items, Func<T> whenKeyNotFound)
{

    if (items[key] != null)
    {
        // return from dictionary
        return (T)items[key];
    }

    // return new item from caller and add to dictionary
    var item = whenKeyNotFound();
    items.Add(key, item);

    return item;
}