If you're using an implementation of System.Collections.IEnumerable you can do like following to convert it to a List. The following uses Enumerable.Cast method to convert IEnumberable to a Generic List.
//ArrayList Implements IEnumerable interface
ArrayList _provinces = new System.Collections.ArrayList();
_provinces.Add("Western");
_provinces.Add("Eastern");
List<string> provinces = _provinces.Cast<string>().ToList();
If you're using Generic version IEnumerable<T>, The conversion is straight forward. Since both are generics, you can do like below,
IEnumerable<int> values = Enumerable.Range(1, 10);
List<int> valueList = values.ToList();
But if the IEnumerable is null, when you try to convert it to a List, you'll get
ArgumentNullException saying Value cannot be null.
IEnumerable<int> values2 = null;
List<int> valueList2 = values2.ToList();
Therefore as mentioned in the other answer, remember to do a null check before converting it to a List.