How do you convert an List to read only in C

For example, AsReadOnly is the main method

List<int> a = new List<int> {1, 2, 3, 4, 5};  

IList<int> b = a.AsReadOnly(); // block modification...  

IList<int> c = b.AsWritable(); // ... but unblock it again  

c.Add(6);  
Debug.Assert(a.Count == 6); // we've modified the original  

IEnumerable<int> d = a.Select(x => x); // okay, try this...  

IList<int> e = d.AsWritable(); // no, can still get round it  

e.Add(7);