Convert from System.Collections.Generic.IList<object> to System.Collections.IList

I have implemented the following class:

class LazyList<T> : IList<T>
{
    public LazyList(IEnumerable<T> enumerable) { }
}

I need to use this class with non-generic legacy System.Collections. I have solved half of the problem by using IEnumerable.Cast<object>() as described here https://stackoverflow.com/a/776407/2109230/.

However, I need to cast LazyList<T> to non-generic IList to complete the refactor process. How can I achieve this?

UPDATE 1:

LazyList basically helps me refactor a method that returns IList after iterating an IDataReader. I use data returned from this method to send some updates over WebSocket. The point is to send data constantly, not after processing the entire IDataReader. I have used yield return to return a lazy IEnumerable, but other methods require the refactored method to return IList. So, I came up with the idea of LazyList.

There are actually two methods, one returning IList<T>, which I have refactored successfully, and another returning IList. I wanted to see if there is a simpler way to refactor the version returning IList, without implementing non-generic LazyList. However, I did that and it wasn't that bad.

Solution

You should follow the pattern the List<T> does - implement both interfaces and use explicit interface implementations to remove some duplication (see the source code) and limit the exposed contract. Something to get you started:

class LazyList<T> : IList<T>, IList
{
    public LazyList(IEnumerable<T> enumerable) { }
  
    public void Add(T item) => throw new NotImplementedException();
    public int Count { get; }

    public int Add(object? item)
    {
        Add((T)item!); // call to the generic implementation
        return Count - 1;
    }

    int IList.IndexOf(object? item)
    {
        if (IsCompatibleObject(item))
        {
            return IndexOf((T)item!); // call to generic 
        }
        return -1;
    }

    public int IndexOf(T item) => throw new NotImplementedException();
    
    private static bool IsCompatibleObject(object? value) => (value is T) || (value == null && default(T) == null);

   // ...
}