private T AddFileSystemItem <T>(T item) where T : FileSystemItem
{
if (item == null)
{
throw new ArgumentNullException(nameof(item));
}
if (FileSystemItems.ContainsKey(item.Key))
{
throw new ArgumentException($"Item {item.Key} is already added");
}
FileSystemItems.Add(item.Key, item);
return(item);
}
