FormatString.cs

internal static class FormatString
{
    private static readonly IReadOnlyList<string> Empty = new string[0];
    private static readonly ThreadLocal<SortedSet<int>> Indexes = new ThreadLocal<SortedSet<int>>(() => new SortedSet<int>());

    /// <summary>
    /// Call with "first: {0}, second {1} returns new []{"0", "1"};
    /// </summary>
    /// <param name="format">The format string</param>
    /// <returns>An unordered list of format items found in <paramref name="format"/></returns>
    internal static IReadOnlyCollection<string> GetFormatItems(string format)
    {
        if (string.IsNullOrEmpty(format))
        {
            return Empty;
        }

        var matches = Regex.Matches(format, @"{(?<index>\d+)}");
        var items = matches.Cast<Match>()
                           .Select(x => x.Groups["index"].Value)
                           .ToList();
        return items;
    }

    internal static int CountUnique(IReadOnlyCollection<string> items)
    {
        if (items.Count == 0)
        {
            return 0;
        }

        var indexes = Indexes.Value;
        indexes.Clear();

        foreach (var item in items)
        {
            int index;
            if (!int.TryParse(item, out index))
            {
                throw new InvalidOperationException($"Format item is not an int: {item}");
            }

            indexes.Add(index);
        }

        return indexes.Count;
    }

    /// <summary>Checks that <paramref name="items"/> are 0-n with no gaps.</summary>
    /// <param name="items">The format items</param>
    /// <returns>True if <paramref name="items"/> are 0-n with no gaps</returns>
    internal static bool AreItemsValid(IReadOnlyCollection<string> items)
    {
        if (items.Count == 0)
        {
            return true;
        }

        var indexes = Indexes.Value;
        indexes.Clear();

        foreach (var item in items)
        {
            int index;
            if (!int.TryParse(item, out index))
            {
                return false;
            }

            if (index < 0 || index >= items.Count)
            {
                return false;
            }

            indexes.Add(index);
        }

        return indexes.Min == 0 && indexes.Max == indexes.Count - 1;
    }
}

FormatStringTests.cs

public class FormatStringTests
{
    [TestCase("first: {0}, second {1}", "0, 1")]
    [TestCase("second: {1}, first {0}", "1, 0")]
    [TestCase("second: {1}, first {0}, first: {0}", "1, 0, 0")]
    public void GetFormatItems(string format, string expecteds)
    {
        var expected = expecteds.Split(',')
                                .Select(x => x.Trim())
                                .ToArray();
        var actual = FormatString.GetFormatItems(format);
        CollectionAssert.AreEqual(expected, actual);
    }

    [TestCase("0", true)]
    [TestCase("0, 0, 0", true)]
    [TestCase("1, 0, 0", true)]
    [TestCase("0, 0, 1", true)]
    [TestCase("1", false)]
    [TestCase("1, 1", false)]
    [TestCase("0, 2", false)]
    public void AreItemsValid(string itemsString, bool expected)
    {
        var items = itemsString.Split(',')
                            .Select(x => x.Trim())
                            .ToArray();
        var actual = FormatString.AreItemsValid(items);
        Assert.AreEqual(expected, actual);

        actual = FormatString.AreItemsValid(items);
        Assert.AreEqual(expected, actual);
    }

    [Test]
    public void AreItemsValidWhenEmpty()
    {
        Assert.IsTrue(FormatString.AreItemsValid(new string[0]));
    }

    [TestCase("0", 1)]
    [TestCase("0, 0, 0", 1)]
    [TestCase("1, 0, 0", 2)]
    [TestCase("0, 0, 1", 2)]
    public void Count(string itemsString, int expected)
    {
        var items = itemsString.Split(',')
                            .Select(x => x.Trim())
                            .ToArray();
        var actual = FormatString.CountUnique(items);
        Assert.AreEqual(expected, actual);

        actual = FormatString.CountUnique(items);
        Assert.AreEqual(expected, actual);
    }
}