The most performant way to count substrings in .NET

Software

I benchmarked 14 common approaches to counting substrings in .NET. The approaches differed up to 60-70x in execution time with memory allocations ranging from zero to over 160KB.

Setup

Tests searched for substring “the” in two strings on an Apple M3 using .NET 8 with BenchmarkDotNet’s ShortRun configuration. The big string was the first chapter of The Hobbit, and the small string was the first 100 chars of the big string.

Results

Approach Small (ns) Large (ns) Allocated
Span 17.71 8,227 0 B
IndexOf (Ordinal) 18.93 8,662 0 B
IndexOf (OrdinalIgnoreCase) 20.47 10,463 0 B
String.Replace 37.33 24,645 216 B / 87,963 B
Cached, compiled Regex 127.17 40,968 560 B / 162,880 B
Instantiating a Regex inline 416.44 49,698 2,528 B / 164,848 B
Static Regex (Regex.Match) 154.42 50,996 560 B / 162,880 B
String.Split 145.47 70,195 304 B / 111,058 B
IndexOf (InvariantCulture) 1,216.64 523,154 0 B / 1 B
IndexOf (InvariantCultureIgnoreCase) 1,314.57 534,426 0 B / 1 B
IndexOf (CurrentCultureIgnoreCase) 1,329.19 536,436 0 B / 1 B
IndexOf (CurrentCulture – default) 1,224.49 553,913 0 B / 1 B

Allocated column shows small/large text allocations.

Key Findings

  • Ordinal string operations are 60x faster than culture-aware operations.
  • Span and IndexOf with StringComparison.Ordinal both achieve zero allocations and optimal performance.
  • Regex approaches allocate 160KB+ for large texts despite reasonable performance.
  • Split creates an array of all segments, explaining its 111KB allocation.
    • With larger strings, this creates an object on the Large Object Heap, which has different garbage collection characteristics, and should be avoided.

Recommendation

If you’re a backend or line of business developer modeling your domain, you probably want IndexOf with the Ordinal or OrdinalIgnoreCase comparer, depending on domain semantics.

See also

Dictionary vs FrozenDictionary: when does the extra overhead break-even?

Software

FrozenDictionary offers faster reads but slower creation. Here’s when the trade-off makes sense.

The tl;dr-

Based on benchmark data, here are the break-even points where FrozenDictionary becomes worthwhile:

Collection Size Cache Hits Cache Misses
10 elements 276 reads 125 reads
100 elements 1,831 reads 804 reads
1,000 elements 22,157 reads 9,634 reads
10,000 elements 217,271 reads 104,890 reads

(Based on string keys and OrdinalIgnoreCase comparison.)

  • Cache misses break-even faster – FrozenDictionary is much faster for failed lookups (0.33ns vs 6-7ns), so collections with cache misses justify the switch sooner.
  • Creation cost scales ~linearly with size – Creation cost grows in a fairly linear way, so larger collections will need more reads to justify use.
  • Read performance is consistent – Once created, FrozenDictionary maintains good performance regardless of collection size.

Switch to FrozenDictionary when you expect:

  • Small collections (10-100 elements): Hundreds to low thousands of reads
  • Large collections (1K+ elements): 10K+ reads with cache misses, or 20K+ reads with all hits
  • A high percentage of misses: you can cut the number of reads in ~half if you expect a high percentage of key misses.

Instantiation costs

FrozenDictionary‘s creation penalty is substantial but decreases as collection size increases:

Elements Dictionary FrozenDictionary Overhead
10 90.30 ns 867.24 ns 9.6x
100 900.73 ns 6,285.94 ns 7.0x
1,000 10,597.66 ns 65,989.60 ns 6.2x
10,000 138,642.89 ns 781,551.17 ns 5.6x

Hit + miss costs

Elements dict Hit dict Miss Frozen  Hit frozen Miss
10 5.48 ns 6.54 ns 2.66 ns 0.33 ns
100 5.77 ns 7.04 ns 2.83 ns 0.34 ns
1,000 5.45 ns 6.08 ns 2.95 ns 0.33 ns
10,000 5.64 ns 6.49 ns 2.68 ns 0.36 ns

The code

2 minute fix: Ventura + Logitech Brio + dock

Software

My Logitech Brio stopped working after I upgraded from Monterey to Ventura. It’s always been connected to an OWC dock, along with a bunch of other peripherals. Maybe I can save you 15-20 minutes by sharing what I did:

  1. Download the Logitech firmware update for the Brio.
  2. Connect the webcam directly to the computer. In my case, I had to attach a USB-A to USB-C adapter because my MacBook Pro only has USB-C ports. (Contrary to some forum posts, I didn’t need a whole new cable, an adapter worked fine.)
  3. Run the firmware update tool. My firmware was at v1.1, and the latest was 2.9. The update took less than a minute to complete.
  4. Remove the USB-A to USB-C adapter, and reconnect the camera to the dock in the same place it was before.

And that was it. You can use Photo Booth to test at steps 3 and 4 to make sure it’s working along the way. I also rebooted, and everything stayed fixed after the restart.

Circuit breakers in stock market trading

Software

Because I can never remember them.

Level 1 halt (7% drop)

  • Trading will halt for 15 minutes if drop occurs before 3:25 p.m.
  • At or after 3:25 p.m.—trading shall continue, unless there is a Level 3 halt.

Level 2 halt (13% drop)

  • Trading will halt for 15 minutes if drop occurs before 3:25 p.m.
  • At or after 3:25 p.m.—trading shall continue, unless there is a Level 3 halt.

Level 3 halt (20% drop)

  • At any time during the trading day—trading shall halt for the remainder of the trading day.

Backdoor, megabackdoor, and regular Roth IRA conversions

Finance

This is an honest-to-goodness note to my future self.

Backdoor Roth IRA

Definition: Making non-deductible contributions to a traditional IRA, then converting to Roth IRA.

Mechanics: Contribute up to $7,000 ($8,000 if 50+) to a traditional IRA without taking the tax deduction. Then convert this money to a Roth IRA. No additional taxes owed since the contribution was already made with after-tax dollars.

When it can be done: Any time, but primarily useful when your income exceeds direct Roth IRA contribution limits ($153k-$161k for 2025).

Notes: Works best with empty traditional IRA accounts to avoid pro-rata rule complications.

Mega-backdoor Roth IRA

Definition: Converting after-tax 401k contributions to Roth IRA money.

Mechanics: Make after-tax contributions to your 401k beyond the pre-tax/Roth limits. Then either convert directly to Roth IRA (if plan allows) or roll to a rollover IRA first, then convert to Roth IRA. No additional taxes since money was already taxed.

When it can be done: Whenever your 401k plan allows after-tax contributions and in-service distributions or conversions.

Notes: Total 401k contributions (pre-tax + Roth + after-tax) are limited to $71,000 for 2025 ($78,500 if 50+). Not all 401k plans offer this option.

Introducing Haystack – a grab bag of extension methods for .NET

Programming, Software

Like many developers, I have collected a bunch of useful methods over time. Most of the time, these methods don’t have unit tests, nor do they have performance tests. Many of them have origins at StackOverflow — which uses the MIT license — and many of them don’t.

I started collecting them formally about two years ago. Recently I decided to actually turn them into something I could consume via nuget, because I was getting fed up with copying and pasting code everywhere.

Compatibility

Haystack targets .NET Standard 1.3, which means it works with:

  • .NET 4.6+
  • .NET Core 1.0+
  • Mono 4.6+
  • UWP 10+

Tradeoffs

  • Performance vs maintainability: If I have to choose between maintainability and raw speed, I’ll choose maintainability. To that end, if there were was more than one maintainable approach, I chose the faster of the two, using Benchmark.NET to determine the winner. In some cases, like constant time string comparisons, slower is actually better, so as not to leak information, but only in certain places, so those places where optimizations might leak information are purposely slow, whereas the less security-critical areas use the faster implementation.
  • Correctness: For the most part, each method has unit tests associated with it.

Examples

string.ConstantTimeCompare

Constant time string comparison matter in cryptography for various reasons. To that end, fast string comparisons can leak information, so we want to exhaustively check all the bytes in the string, even if we know the strings aren’t equal early on.

const string here = "Here";
const string there = "There";
 
var areSame = here.ConstantTimeEquals(there);   // false

string.TrimStart and string.TrimEnd

It’s useful to be able to remove substrings from the beginning and/or end of a string. With or without a StringComparer overload.

const string trim = "Hello world";
const string hello = "Hello worldThis is a hello worldHello world";
 
var trimFront = hello.TrimStart(trim);   // This is a hello worldHello world
var trimEnd = hello.TrimEnd(trim);       // Hello worldThis is a hello world
var trimBoth = hello.Trim(trim);         // This is a hello world

The library is growing bit-by-bit, and contributions are welcome!

Productivity fetishism

Happiness

I switched to Firefox recently, which has a “Recommended by Pocket” section on the New Tab page. As I expected, many of the recommendations are productivity fetish articles from Lifehacker and similar rubbish. Their job is not to make you more productive–whatever that means–it’s to keep you reading.

Instead:

  1. Discover what’s valuable. Talk to people with high visibility and the insight to match. That could be an executive, or it could be your spouse.
  2. Do only valuable things. Being busy doesn’t mean you’re doing anything worth doing.
  3. Learn your tools. If something feels like it’s harder than it should be, you’re using the wrong tool, or you don’t know your tools well enough.

If you follow these guidelines, you’ll be happier, less stressed, and deliver more value.

9 observations on 6 months of running a moderately-successful open source project

ical.net, Programming

I’ve run ical.net, an RFC-5545 (icalendar) library for .NET since ~May 2016. It’s basically the only game in town if you need to do anything with icalendar-formatted data. (Those .ics files you get as email attachments are icalendar data.)

A lot of these fall into the “pretty obvious” category of observations.

1) Release notes matter

If nothing else, it serves as a historical reference for your own benefit. It also helps your users understand whether it’s worth upgrading. And when your coworkers ask if a version jump is important weeks after you’ve published it, you can point them to the release notes for that version, and they’ll never ask you again.

2) Automation is important

One of the best things I did when I first figured out how to make a nuget package was push as much into my nuspec file as I could. Everything I learned about various do’s and don’ts was pushed into the code in the moment I learned it.

Not everything in ical.net is automated, and I think I’m OK with that for now. For example, a merge doesn’t trigger a new nuget package version. I think that’s probably a feature rather than a bug.

I suspect I’ll reach a second tipping point where

3) Document in public

Scott Hanselman has the right of this:

Keep your emails to 3-4 sentences, Hanselman says. Anything longer should be on a blog or wiki or on your product’s documentation, FAQ or knowledge base. “Anywhere in the world except email because email is where your keystrokes go to die,” he says.

That means I reply to a lot of emails with a variation of “Please ask this on StackOverflow so I can answer it in public.” And many of those answers are tailored to the question, and then I include a link to a wiki page that answers a more general form of the question. Public redundancy is okay.

Accrete your public documentation.

4) Broken unit tests should be fixed or (possibly) deleted

When I took over dday.ical, there were about 70 (out of maybe 250) unit tests that were failing. There was so much noise that it was impossible to know anything about the state of the code. My primary aim was to improve performance for some production issues that we were having, but I couldn’t safely do that without resolving the crazy number of broken unit tests.

The first thing I did was evaluate each and every broken test, and decide what to do. Having a real, safe baseline was imperative, because you never want to introduce a regression that could have been caught.

The corollary to this is that sometimes your unit tests assert the wrong things. So a bugfix in one place may expose an bad assertion in a unit test elsewhere. That happened quite a lot, especially early on.

5) Making code smaller is always the right thing to do

(So long as your unit tests are passing.)

Pinning down what “smaller” means is difficult. Lines of code may be a rough proxy, but I think I mean smaller in the sense of “high semantic density” + “low cognitive load”.

  • Reducing cognitive load can be achieved by simple things like reducing the number of superfluous types; eliminating unnecessary layers of indirection; having descriptive variable and method names; and having a preference for short, pure methods.
  • Semantic density can be increased by moving to a more declarative style of programming. Loops take up a lot of space and aren’t terribly powerful compared to their functional analogs: map, filter, fold, etc. (I personally find that I write more bugs when writing imperative code. YMMV.) You won’t find many loops in ical.net, but you will find a lot of LINQ.

I think a preference for semantic density is a taste that develops over time.

6) Semantic versioning is the bee’s knees

In a nutshell:

Given a version number MAJOR.MINOR.PATCH, increment the:

  1. MAJOR version when you make incompatible API changes,
  2. MINOR version when you add functionality in a backwards-compatible manner, and
  3. PATCH version when you make backwards-compatible bug fixes.

This seems like common sense advice, but by imposing some modest constraints, it frees you from thinking about certain classes of problems:

  • It’s concrete guidance to contributors as to why their pull requests are or are not acceptable, namely: breaking changes are a no-no
  • Maintaining a stable API is a good way to inspire confidence in consumers of your library

And by holding my own feet to the fire, and following my own rules, I’m a better developer.

7) People will want bleeding-edge features, but delivering them might not be the highest-impact thing you can do

.NET Core is an exciting development. I would LOVE for ical.net to have a .NET Core version, and I’ve made some strides in that direction. But the .NET Core tooling is still beta, the progress in VS 2017 RC notwithstanding. I spent some time trying to get a version working–and I did–but I couldn’t see any easy way to automate the compilation of a .NET Core nuget package alongside the normal framework versions without hating my life.

So I abandoned it.

When the tooling is out of beta, I expect maintaining a Core version will be easier and Core adoption will be higher, both of which improve the ROI with respect to development effort.

8) It’s all cumulative

Automation, comprehensive unit test coverage with a mandatory-100% pass rate, lower cognitive load, higher semantic density, etc. All these things help you go faster with a high degree of confidence later on.

9) People are bad at asking questions and opening tickets

And if you’re not okay with that, then being a maintainer might not be a good fit for you.

  • No, I really can’t make sense of your 17,000-line Google Calendar attachment, sorry.
  • No, I won’t debug your application for you, just because it uses ical.net on 2 lines of your 100+ line method, sorry.
  • No, I’m not going to drop everything to help you, no matter how many emails you send me in a 10 minute time interval, sorry.

All of these things are common when you run an open source project that has traction. Ask anyone.

A self-contained, roll-forward schema updater

Programming, Software,

I use Dapper for most of my database interactions. I like it because it’s simple, and does exactly one thing: runs SQL queries, and returns the typed results.

I also like to deploy my schema changes as part of my application itself instead of doing it as a separate data deployment. On application startup, the scripts are loaded and executed in lexical order one by one, where each schema change is idempotent in isolation.

The problem you run into is making destructive changes to schema, which is a reasonable thing to want to do. If script 003 creates a column of UNIQUEIDENTIFIER, and you want to convert that column to NVARCHAR in script 008, you have to go back do some reconciliation between column types. Adding indexes into the mix makes it even hairier. Scripts that are idempotent in isolation are easy to write. Maintaining a series of scripts that can be safely applied in order from beginning to end every time an application starts up is not.

Unless you keep track of which schema alterations have already been applied, and only apply the changes that the application hasn’t seen before. Here’s a short, self-contained implementation:

public class SchemaUpdater
{
  private readonly string _connectionString;
  private readonly ILog _logger;
  private readonly string _environment;
 
  public SchemaUpdater(string connectionString, string environment)
    : this(connectionString, environment, LogManager.GetLogger(typeof(SchemaUpdater))) { }
 
  internal SchemaUpdater(string connectionString, string environment, ILog logger)
  {
    _connectionString = connectionString;
    _environment = environment;
    _logger = logger;
  }
 
  public void UpdateSchema()
  {
    MaybeCreateAuditTable();
    var previousUpdates = GetPreviousSchemaUpdates();
 
    var assemblyPath = Uri.UnescapeDataString(new UriBuilder(typeof(SchemaUpdater).GetTypeInfo().Assembly.CodeBase).Path);
    var schemaDirectory = Path.Combine(Path.GetDirectoryName(assemblyPath), "schema-updates");
 
    var schemaUpdates = Directory.EnumerateFiles(schemaDirectory, "*.sql", SearchOption.TopDirectoryOnly)
      .Select(fn => new { FullPath = fn, Filename = Path.GetFileName(fn) })
      .Where(file => !previousUpdates.Contains(file.Filename))
      .OrderBy(file => file.Filename)
      .Select(file => new { file.Filename, Query = File.ReadAllText(file.FullPath) })
      .ToList();
 
    foreach (var update in schemaUpdates)
    {
      using (var connection = new SqlConnection(_connectionString))
      {
        try
        {
          var splitOnGo = SplitOnGo(update.Query);
          foreach (var statement in splitOnGo)
          {
            try
            {
              connection.Execute(statement);
            }
            catch (Exception exception)
            {
              Console.WriteLine(exception);
              throw;
            }
          }
 
          connection.Execute("INSERT INTO SchemaRevision (Filename, FileContents) VALUES (@filename, @fileContent)",
            new { filename = update.Filename, fileContent = update.Query });
        }
        catch (Exception e)
        {
          _logger.Fatal(new { Message = "Unable to apply schema change", update.Filename, update.Query, Environment = _environment }, e);
          throw;
        }
      }
    }
  }
 
  public static ICollection<string> SplitOnGo(string sqlScript)
  {
    // Split by "GO" statements
    var statements = Regex.Split(
      sqlScript,
      @"^[\t\r\n]*GO[\t\r\n]*\d*[\t\r\n]*(?:--.*)?$",
      RegexOptions.Multiline |
      RegexOptions.IgnorePatternWhitespace |
      RegexOptions.IgnoreCase);
 
    // Remove empties, trim, and return
    var materialized = statements
      .Where(x => !string.IsNullOrWhiteSpace(x))
      .Select(x => x.Trim(' ', '\r', '\n'))
      .ToList();
 
    return materialized;
  }
 
  internal void MaybeCreateAuditTable()
  {
    const string createAuditTable =
@"IF NOT EXISTS(SELECT 1 FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'SchemaRevision')
BEGIN
CREATE TABLE [dbo].[SchemaRevision]
(
[SchemaRevisionNbr] BIGINT IDENTITY(1,1),
[Filename] VARCHAR(256),
[FileContents] VARCHAR(MAX),
CONSTRAINT PK_SchemaRevision PRIMARY KEY (SchemaRevisionNbr),
)
END";
 
    using (var connection = new SqlConnection(_connectionString))
    {
      connection.Execute(createAuditTable);
    }
  }
 
  internal HashSet<string> GetPreviousSchemaUpdates()
  {
    using (var connection = new SqlConnection(_connectionString))
    {
      var results = connection.Query<string>(@"SELECT Filename FROM SchemaRevision");
      return new HashSet<string>(results, StringComparer.OrdinalIgnoreCase);
    }
  }
}

Update 2017-09-05: I added the SplitOnGo() method to support the GO delimiter, since I’ve had occasion to need it recently. It’s adapted from Matt Johnson’s answer on StackOverflow.

Proposed functionality and API changes for ical.net v3

ical.net, Programming, Software,

Update 2018-04-11: Most all of these changes have been published in ical.net versions 3 and 4. See the release notes for more details.

Downloading remote resources

When I ported ical.net to .NET Core, I removed the ability to download remote payloads from a URI. I did this for many reasons:

  • There are myriad ways of accessing an HTTP resource. There are myriad ways of doing authentication. Consumers of ical.net are in a position to know the details of their environment, including security concerns, so responsibility for these concerns should lie with the developers using the library.
  • Choosing to support HttpClient leaves .NET 4.0 users out in the cold. Choosing to support WebClient brings those people into the fold, but leaves .NET Core and WinRT users out. It also prevents developers working with newer versions of .NET from benefiting from HttpClient.
  • Non-blocking IO leaves developers working with WinForms and framework versions < 4.5 out in the cold. Bringing those developers back into the fold means we can’t make use of async Tasks. Given the popularity of microservices and ical.net’s origins on the server side, this is a non-starter.

We can’t satisfy all use cases if we try to do everything, so instead I’ve decided that we’ll leave over-the-wire tasks to the developers using ical.net.

The primacy of strings

To that end… strings will be the primary way to work with ical.net. A developer should be able to instantiate everything from a huge collection of calendars down to a single calendar component (a VEVENT for example) by passing it a string that represents that thing. In modern C#, working directly with strings is more natural than passing Streams around, which is emblematic of old-school Java. It’s also more error prone: I fixed several memory leaks during the .NET Core port due to undisposed Streams)

  • The constructor will be the deserializer. It is reasonable for the constructor to deserialize the textual representation into the typed representation.
  • ToString() will be the serializer. It is reasonable for ToString() to serialize the typed representation into the textual representation.

Constructors as deserializers buys us…

Immutable types and (maybe) a fluid API

One of the challenges I faced when refactoring for performance was reasoning about mutable properties during serialization and deserialization. Today, deserialization makes extensive use of public, mutable properties. In fact, the documentation reflects this mutability:

var now = DateTime.Now;
var later = now.AddHours(1);
 
var rrule = new RecurrencePattern(FrequencyType.Daily, 1)
{
    Count = 5
};
 
var e = new Event
{
    DtStart = new CalDateTime(now),
    DtEnd = new CalDateTime(later),
    Duration = TimeSpan.FromHours(1),
    RecurrenceRules = new List&lt;IRecurrencePattern&gt; {rrule},
};
 
var calendar = new Calendar();
calendar.Events.Add(e);

To be completely honest, this state of affairs makes it quite difficult to make internal changes without breaking stuff. Many properties would naturally be getter-only, because they can be derived from simple internals, like Duration above. Yet they’re explicitly set during deserialization. This is an incredible vector for bugs and breaking changes. (Ask me how I know…)

If we close these doors and windows, it will increase our internal maneuverability.

Fluid API

Look at the code above. Couldn’t it be more elegant? Shouldn’t it be? I don’t yet have a fully-formed idea of what a more fluid API might look like. Suggestions welcome.

Component names

IICalendarTypeNames

The .NET framework guidelines recommend prefixing interface names with “I”. The calendar spec is called “iCalendar”, as in “internet calendar”, which is an unfortunate coincidence. Naming conventions like IICalendarCollection offend my sense of aesthetics, so I renamed some objects when I forked ical.net from dday. I’ve come around to valuing consistency over aesthetics, so I may go back to the double-I where it makes sense to do so.

CalDateTime

The object that represents “a DateTime with a time zone” is called a CalDateTime. I’m not wild about this; we already have the .NET DateTime struct which has its own shortcomings that’ve been exhaustively documented elsewhere. A reasonable replacement for CalDateTime might be a DateTimeOffset with a string representation of an IANA, BCL, or Serialization time zone, with the time zone conversions delegated to NodaTime for computing recurrences. (In fact, NodaTime is already doing the heavy lifting behind the scenes for performance reasons, but the implementation isn’t pretty because of CalDateTime‘s mutability. Were it immutable, it would have been a straightforward engine replacement.)

CalDateTime is the lynchpin for most of the ical.net library. Most of its public properties should be simple expression bodies. Saner serialization and deserialization will have to come first as outlined above.

Divergence from spec completeness and adherence

VTIMEZONE

The iCalendar spec has ways of representing time change rules with VTIMEZONE. In the old days, dday.ical used this information to figure out Standard Time/Summer Time transitions. But as the spec itself notes:

Note: The specification of a global time zone registry is not addressed by this document and is left for future study. However, implementers may find the Olson time zone database [TZ] a useful reference. It is an informal, public-domain collection of time zone information, which is currently being maintained by volunteer Internet participants, and is used in several operating systems. This database contains current and historical time zone information for a wide variety of locations around the globe; it provides a time zone identifier for every unique time zone rule set in actual use since 1970, with historical data going back to the introduction of standard time.

At this point in time, the IANA (née Olson) tz database is the best source of truth. Relying on clients to specify reasonable time zone and time change behavior is unrealistic. I hope the spec authors revisit the VTIMEZONE element, and instead have it specify a standard time zone string, preferably IANA.

To that end… ical.net will continue to preserve VTIMEZONE fields, but it will not use them for recurrence computations or understanding Summer/Winter time changes. It will continue to rely on NodaTime for that.

URL and ATTACH

As mentioned above, ical.net will no longer include functionality to download resources from URIs. It will continue to preserve these fields so clients can do what they wish with the information they contain. This isn’t a divergence from the spec, per se, which doesn’t state that clients should provide facilities to download resources.