Read-only list or unmodifiable list in .NET 4.0

From what I can tell, .NET 4.0 still lacks read-only lists. Why does the framework still lack this functionality? Isn't this one of the commonest pieces of functionality for domain-driven design?

One of the few advantages Java has over C# is this in the form of the Collections.unmodifiablelist(list) method, which it seems is long overdue in IList<T> or List<T>.

Using IEnumerable<T> is the easiest solution to the question - ToList can be used and returns a copy.

You're looking for ReadOnlyCollection, which has been around since .NET2.

IList<string> foo = ...;
// ...
ReadOnlyCollection<string> bar = new ReadOnlyCollection<string>(foo);

or

List<string> foo = ...;
// ...
ReadOnlyCollection<string> bar = foo.AsReadOnly();

This creates a read-only view, which reflects changes made to the wrapped collection.

For those who like to use interfaces: .NET 4.5 adds the generic IReadOnlyList interface which is implemented by List<T> for example.

It is similar to IReadOnlyCollection and adds an Item indexer property.

How about the ReadOnlyCollection already within the framework?

If the most common pattern of the list is to iterate through all the elements, IEnumerable<T> or IQueryable<T> can effectively act as a read-only list as well.

In 2.0 you can call AsReadOnly to get a read-only version of the list. Or wrap an existing IList in a ReadOnlyCollection<T> object.

What's wrong with System.Collections.ObjectModel.ReadOnlyCollection?

Create an extension method ToReadOnlyList() on IEnumerable, then

IEnumerable<int> ints = new int[] { 1, 2, 3 };
var intsReadOnly = ints.ToReadOnlyList();
//intsReadOnly [2]= 9; //compile error, readonly

here is the extension method

public static class Utility
{
	public static IReadOnlyList<T> ToReadOnlyList<T>(this IEnumerable<T> items)
	{
		IReadOnlyList<T> rol = items.ToList();
		return rol;
	}
}

see Martin's answer too