Abstract getter with concrete setter in C#

Question

I'm trying to write an abstract base class for read-only collections which implement IList. Such a base class should implement the set-indexer to throw a NotSupportedException, but leave the get-indexer as abstract. Does C# allow such a situation? Here's what I have so far:

public abstract class ReadOnlyList : IList {

    public bool IsReadOnly { get { return true; } }

    public object this[int index] {
        get {
            // there is nothing to put here! Ideally it would be left abstract.
            throw new NotImplementedException();
        }
        set {
            throw new NotSupportedException("The collection is read-only.");
        }
    }

    // other members of IList ...
}

Ideally ReadOnlyList would be able to implement the setter but leave the getter abstract. Is there any syntax which allows this?

Answer 1

Delegate the work to protected members which you can then mark as abstract or virtual depending on the desired behaviour. Try something like this:

 // implementor MUST override
 protected abstract object IndexerGet(int index);

 // implementor can override if he wants..
 protected virtual void IndexerSet(int index, object value) 
 {
   throw new NotSupportedException("The collection is read-only.");
 }

 public object this[int index] {
   get {
     return IndexerGet(index);
   }
   set {
     IndexerSet(index, value);
   }
 }

Answer 2

No, you cannot have a property with a member abstract and the other implemented. What I would do is to set the whole property as abstract and redefine it at the inheritors to do whatever you want.