ChatClient client = new(model: "gpt-4o", "OPENAI_API_KEY");

ChatCompletion completion = client.CompleteChat("Say 'this is a test.'");

Console.WriteLine($"[ASSISTANT]: {completion}");

public static class Enumerable
{
    // Extension block
    extension<TSource>(IEnumerable<TSource> source) // extension members for IEnumerable<TSource>
    {
        // Extension property:
        public bool IsEmpty => !source.Any();
        // Extension indexer:
        public TSource this[int index] => source.Skip(index).First();

        // Extension method:
        public IEnumerable<TSource> Where(Func<TSource, bool> predicate) { ... }
    }

    // extension block, with a receiver type only
    extension<TSource>(IEnumerable<TSource>) // static extension members for IEnumerable<Source>
    {
        // static extension method:
        public static IEnumerable<TSource> Combine(IEnumerable<TSource> first, IEnumerable<TSource> second) { ... }

        // static extension property:
        public static IEnumerable<TSource> Identity => yield return default;
    }
}

if (DateTime.Now > token.Expiry)
{
    return Unauthorized();
}