public static void ToStringItems()
{
var extStringList = new List <string> {
".A", ".B", "CCC"
};
var extList = extStringList.Select(x => (Extension)x).ToList();
var instance = new ExtensionList(extList);
var allExtString = instance.ToStringItems();
var answer = string.Join(",", extStringList);
Assert.IsTrue(allExtString.Equals(answer));
}