How should I use properties when dealing with read-only List<T> members

You can expose it AsReadOnly. That is, return a read-only IList<T> wrapper. For example ...

public ReadOnlyCollection<int> List
{
    get { return _lst.AsReadOnly(); }
}

Just returning an IEnumerable<T> is not sufficient. For example ...

void Main()
{
    var el = new ExposeList();
    var lst = el.ListEnumerator;
    var oops = (IList<int>)lst;
    oops.Add( 4 );  // mutates list

    var rol = el.ReadOnly;
    var oops2 = (IList<int>)rol;

    oops2.Add( 5 );  // raises exception
}

class ExposeList
{
  private List<int> _lst = new List<int>() { 1, 2, 3 };

  public IEnumerable<int> ListEnumerator
  {
     get { return _lst; }
  }

  public ReadOnlyCollection<int> ReadOnly
  {
     get { return _lst.AsReadOnly(); }
  }
}

Steve's answer also has a clever way to avoid the cast.

There is limited value in attempting to hide information to such an extent. The type of the property should tell users what they're allowed to do with it. If a user decides they want to abuse your API, they will find a way. Blocking them from casting doesn't stop them:

public static class Circumventions
{
    public static IList<T> AsWritable<T>(this IEnumerable<T> source)
    {
        return source.GetType()
            .GetFields(BindingFlags.Public |
                       BindingFlags.NonPublic | 
                       BindingFlags.Instance)
            .Select(f => f.GetValue(source))
            .OfType<IList<T>>()
            .First();
    }
}

With that one method, we can circumvent the three answers given on this question so far:

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);
Debug.Assert(a.Count == 7); // modified original again

Also:

public static class AlexeyR
{
    public static IEnumerable<T> AsReallyReadOnly<T>(this IEnumerable<T> source)
    {
        foreach (T t in source) yield return t;
    }
}

IEnumerable<int> f = a.AsReallyReadOnly(); // really?

IList<int> g = f.AsWritable(); // apparently not!
g.Add(8);
Debug.Assert(a.Count == 8); // modified original again

To reiterate... this kind of "arms race" can go on for as long as you like!

The only way to stop this is to completely break the link with the source list, which means you have to make a complete copy of the original list. This is what the BCL does when it returns arrays. The downside of this is that you are imposing a potentially large cost on 99.9% of your users every time they want readonly access to some data, because you are worried about the hackery of 00.1% of users.

Or you could just refuse to support uses of your API that circumvent the static type system.

If you want a property to return a read-only list with random access, return something that implements:

public interface IReadOnlyList<T> : IEnumerable<T>
{
    int Count { get; }
    T this[int index] { get; }
}

If (as is much more common) it only needs to be enumerable sequentially, just return IEnumerable:

public class MyClassList
{
    private List<int> li = new List<int> { 1, 2, 3 };

    public IEnumerable<int> MyList
    {
        get { return li; }
    }
}

UPDATE Since I wrote this answer, C# 4.0 came out, so the above IReadOnlyList interface can take advantage of covariance:

public interface IReadOnlyList<out T>

And now .NET 4.5 has arrived and it has... guess what...

IReadOnlyList interface

So if you want to create a self-documenting API with a property that holds a read-only list, the answer is in the framework.

JP's answer regarding returning IEnumerable<int> is correct (you can down-cast to a list), but here is a technique that prevents the down-cast.

class ExposeList
{
  private List<int> _lst = new List<int>() { 1, 2, 3 };

  public IEnumerable<int> ListEnumerator
  {
     get { return _lst.Select(x => x); }  // Identity transformation.
  }

  public ReadOnlyCollection<int> ReadOnly
  {
     get { return _lst.AsReadOnly(); }
  }
}

The identity transformation during enumeration effectively creates a compiler-generated iterator - a new type which is not related to _lst in any way.

Eric Lippert has a series of articles on Immutability In C# on his blog.

The first article in the series can be found here.

You might also find useful Jon Skeet's answer to a similar question.

public List<int> li;

Don't declare public fields, it's generally considered bad practice... wrap it in a property instead.

You can expose your collection as a ReadOnlyCollection :

private List<int> li;
public ReadOnlyCollection<int> List
{
    get { return li.AsReadOnly(); }
}