/// <summary>
/// Adds a key/value pair to the dictionary.
/// </summary>
/// <typeparam name="TK">key type</typeparam>
/// <typeparam name="TV">value type</typeparam>
/// <param name="dictionary"></param>
/// <param name="key"></param>
/// <param name="addFunc">add function</param>
/// <param name="updateFunc">update function</param>
/// <returns>The new value for the key.</returns>
/// <remarks>
/// Add or updates the key/value pair to the dictionary.
/// </remarks>
private static TV AddOrUpdateInternal <TK, TV>(IDictionary <TK, TV> dictionary, TK key, NonNullable <Func <TK, TV> > addFunc, NonNullable <Func <TK, TV, TV> > updateFunc)
{
dictionary[key] = !dictionary.ContainsKey(key)
? addFunc.Value.Invoke(key)
: updateFunc.Value.Invoke(key, dictionary[key]);
return(dictionary[key]);
}