public void TestEnumerator()
{
ImmutableSortedTreeList <int> .Builder list = ImmutableSortedTreeList.CreateBuilder <int>();
ImmutableSortedTreeList <int> .Enumerator enumerator = list.GetEnumerator();
Assert.Equal(0, enumerator.Current);
Assert.False(enumerator.MoveNext());
Assert.Equal(0, enumerator.Current);
Assert.False(enumerator.MoveNext());
Assert.Equal(0, enumerator.Current);
// Adding an item to the list invalidates it, but Current is still unchecked
CollectionAssert.EnumeratorInvalidated(list, () => list.Add(1));
Assert.Throws <InvalidOperationException>(() => enumerator.MoveNext());
Assert.Equal(0, enumerator.Current);
enumerator = list.GetEnumerator();
Assert.Equal(0, enumerator.Current);
Assert.True(enumerator.MoveNext());
Assert.Equal(1, enumerator.Current);
// Reset has no effect due to boxing the value type
((IEnumerator <int>)enumerator).Reset();
Assert.Equal(1, enumerator.Current);
Assert.False(enumerator.MoveNext());
Assert.Equal(1, enumerator.Current);
}