Take an IEnumerable<T> and cast to IList<T> (if possible), otherwise materialize the list with .ToList()

CashOrMaterialize.cs

namespace ConsoleApp1
{
    public static class IEnumerableExtensions
    {
        public static IList<T> CastOrMaterializeList<T>(this IEnumerable<T> source)
        {
            if (source is IList<T>)
                return (IList<T>)source;

            return source
                .ToList();
        }
    }
}