Interpolated String Formatter

example.cs

var url = new InterpolatedStringFormatter("/api/{product}/order", productName).ToString();

InterpolatedStringFormatter.cs

    public readonly struct InterpolatedStringFormatter : IReadOnlyList<KeyValuePair<string, object>>
    {
        internal const int MaxCachedFormatters = 1024;
        private const string NullFormat = "[null]";
        private static int _count;
        private static ConcurrentDictionary<string, StringValuesFormatter> _formatters = new ConcurrentDictionary<string, StringValuesFormatter>();
        private readonly StringValuesFormatter _formatter;
        private readonly object[] _values;
        private readonly string _originalMessage;

        // for testing purposes
        internal StringValuesFormatter Formatter => _formatter;

        public InterpolatedStringFormatter(string format, params object[] values)
        {
            if (values != null && values.Length != 0 && format != null)
            {
                if (_count >= MaxCachedFormatters)
                {
                    if (!_formatters.TryGetValue(format, out _formatter))
                        _formatter = new StringValuesFormatter(format);
                }
                else
                {
                    _formatter = _formatters.GetOrAdd(format, f =>
                    {
                        Interlocked.Increment(ref _count);
                        return new StringValuesFormatter(f);
                    });
                }
            }
            else
            {
                _formatter = null;
            }

            _originalMessage = format ?? NullFormat;
            _values = values;
        }

        public KeyValuePair<string, object> this[int index] {
            get {
                if (index < 0 || index >= Count)
                    throw new IndexOutOfRangeException(nameof(index));

                if (index == Count - 1)
                    return new KeyValuePair<string, object>("{OriginalFormat}", _originalMessage);

                return _formatter.GetValue(_values, index);
            }
        }

        public int Count {
            get {
                if (_formatter == null)
                    return 1;

                return _formatter.ValueNames.Count + 1;
            }
        }

        public IEnumerator<KeyValuePair<string, object>> GetEnumerator()
        {
            for (var i = 0; i < Count; ++i)
            {
                yield return this[i];
            }
        }

        public override string ToString()
        {
            if (_formatter == null)
                return _originalMessage;

            return _formatter.Format(_values);
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }