System.Windows.Data.DataList.VerifyValidItem C# (CSharp) Method

        private void VerifyValidItem(object item)
        {
            if (item == null) {
                throw new ArgumentNullException("item");
            }

            EnsureSnapShot();
            if (_snapShot.Contains(item) == false) {
                throw new ArgumentOutOfRangeException("The specified item is not part of this DataList.");
            }
        }