/// <summary>
/// Add an item to storage using the specified unique key.
/// </summary>
/// <param name="key">
/// Unique identifier of the item.
/// </param>
/// <param name="value">
/// The item to add.
/// </param>
public void Add <T>(string key, T value)
{
if (stateStorage.ContainsKey(key))
{
throw new ArgumentException(RM.Get_Error_KeyAlreadyExists(key));
}
stateStorage.Add(key, value);
}