GetIndexer.cs

// 1)
GetIndexer(this List<Form> list)
{
	var ifi = new List<IFormIndexer>();
	foreach (var item in list)
		ifi.Add(item as IFormIndexer);
}

// or 2)
GetIndexer(this List<Form> list)
{
	var ifi = list as List<IFormIndexer>;
}