Primitives: The extensions library for .NET

In this article, you'll learn about the Microsoft.Extensions.Primitives library. The primitives in this article are not to be confused with .NET primitive types from the BCL, or that of the C# language. Instead, the types within the primitive's library serve as building blocks for some of the peripheral .NET NuGet packages, such as:

Change notifications

Propagating notifications when a change occurs is a fundamental concept in programming. The observed state of an object more often than not can change. When change occurs, implementations of the Microsoft.Extensions.Primitives.IChangeToken interface can be used to notify interested parties of said change. The implementations available are as follows:

As a developer, you're also free to implement your own type. The IChangeToken interface defines a few properties:

Instance-based functionality

Consider the following example usage of the CancellationChangeToken:

CancellationTokenSource cancellationTokenSource = new();
CancellationChangeToken cancellationChangeToken = new(cancellationTokenSource.Token);

Console.WriteLine($"HasChanged: {cancellationChangeToken.HasChanged}");

static void callback(object? _) =>
    Console.WriteLine("The callback was invoked.");

using (IDisposable subscription =
    cancellationChangeToken.RegisterChangeCallback(callback, null))

Console.WriteLine($"HasChanged: {cancellationChangeToken.HasChanged}\n");

// Outputs:
//     HasChanged: False
//     The callback was invoked.
//     HasChanged: True

In the preceding example, a CancellationTokenSource is instantiated and its Token is passed to the CancellationChangeToken constructor. The initial state of HasChanged is written to the console. An Action<object?> callback is created that writes when the callback is invoked to the console. The token's RegisterChangeCallback(Action<Object>, Object) method is called, given the callback. Within the using statement, the cancellationTokenSource is cancelled. This triggers the callback, and the state of HasChanged is again written to the console.

When you need to take action from multiple sources of change, use the CompositeChangeToken. This implementation aggregates one or more change tokens and fires each registered callback exactly one time regardless of the number of times a change is triggered. Consider the following example:

CancellationTokenSource firstCancellationTokenSource = new();
CancellationChangeToken firstCancellationChangeToken = new(firstCancellationTokenSource.Token);

CancellationTokenSource secondCancellationTokenSource = new();
CancellationChangeToken secondCancellationChangeToken = new(secondCancellationTokenSource.Token);

CancellationTokenSource thirdCancellationTokenSource = new();
CancellationChangeToken thirdCancellationChangeToken = new(thirdCancellationTokenSource.Token);

var compositeChangeToken =
    new CompositeChangeToken(
        new IChangeToken[]

static void callback(object? state) =>
    Console.WriteLine($"The {state} callback was invoked.");

// 1st, 2nd, 3rd, and 4th.
compositeChangeToken.RegisterChangeCallback(callback, "1st");
compositeChangeToken.RegisterChangeCallback(callback, "2nd");
compositeChangeToken.RegisterChangeCallback(callback, "3rd");
compositeChangeToken.RegisterChangeCallback(callback, "4th");

// It doesn't matter which cancellation source triggers the change.
// If more than one trigger the change, each callback is only fired once.
Random random = new();
int index = random.Next(3);
CancellationTokenSource[] sources = new[]


// Outputs:
//     The 4th callback was invoked.
//     The 3rd callback was invoked.
//     The 2nd callback was invoked.
//     The 1st callback was invoked.

In the preceding C# code, three CancellationTokenSource objects instances are created and paired with corresponding CancellationChangeToken instances. The composite token is instantiated by passing an array of the tokens to the CompositeChangeToken constructor. The Action<object?> callback is created, but this time the state object is used and written to console as a formatted message. The callback is registered four times, each with a slightly different state object argument. The code uses a pseudo-random number generator to pick one of the change token sources (doesn't matter which one) and call its Cancel() method. This triggers the change, invoking each registered callback exactly once.

Alternative static approach

As an alternative to calling RegisterChangeCallback, you could use the Microsoft.Extensions.Primitives.ChangeToken static class. Consider the following consumption pattern:

CancellationTokenSource cancellationTokenSource = new();
CancellationChangeToken cancellationChangeToken = new(cancellationTokenSource.Token);

IChangeToken producer()
    // The producer factory should always return a new change token.
    // If the token's already fired, get a new token.
    if (cancellationTokenSource.IsCancellationRequested)
        cancellationTokenSource = new();
        cancellationChangeToken = new(cancellationTokenSource.Token);

    return cancellationChangeToken;

void consumer() => Console.WriteLine("The callback was invoked.");

using (ChangeToken.OnChange(producer, consumer))

// Outputs:
//     The callback was invoked.

Much like previous examples, you'll need an implementation of IChangeToken that is produced by the changeTokenProducer. The producer is defined as a Func<IChangeToken> and it's expected that this will return a new token every invocation. The consumer is either an Action when not using state, or an Action<TState> where the generic type TState flows through the change notification.

String tokenizers, segments, and values

Interacting with strings is commonplace in application development. Various representations of strings are parsed, split, or iterated over. The primitives library offers a few choice types that help to make interacting with strings more optimized and efficient. Consider the following types:

  • StringSegment: An optimized representation of a substring.
  • StringTokenizer: Tokenizes a string into StringSegment instances.
  • StringValues: Represents null, zero, one, or many strings in an efficient way.

The StringSegment type

In this section, you'll learn about an optimized representation of a substring known as the StringSegment struct type. Consider the following C# code example showing some of the StringSegment properties and the AsSpan method:

var segment =
    new StringSegment(
        "This a string, within a single segment representation.",
        14, 25);

Console.WriteLine($"Buffer: \"{segment.Buffer}\"");
Console.WriteLine($"Offset: {segment.Offset}");
Console.WriteLine($"Length: {segment.Length}");
Console.WriteLine($"Value: \"{segment.Value}\"");

Console.Write("Span: \"");
foreach (char @char in segment.AsSpan())

// Outputs:
//     Buffer: "This a string, within a single segment representation."
//     Offset: 14
//     Length: 25
//     Value: " within a single segment "
//     " within a single segment "

The preceding code instantiates the StringSegment given a string value, an offset, and a length. The StringSegment.Buffer is the original string argument, and the StringSegment.Value is the substring based on the StringSegment.Offset and StringSegment.Length values.

The StringSegment struct provides many methods for interacting with the segment.

The StringTokenizer type

The StringTokenizer object is a struct type that tokenizes a string into StringSegment instances. The tokenization of large strings usually involves splitting the string apart and iterating over it. With that said, String.Split probably comes to mind. These APIs are similar, but in general, StringTokenizer provides better performance. First, consider the following example:

var tokenizer =
    new StringTokenizer(
        new[] { ' ' });

foreach (StringSegment segment in tokenizer)
    // Interact with segment

In the preceding code, an instance of the StringTokenizer type is created given 900 auto-generated paragraphs of Lorem Ipsum text and an array with a single value of a white-space character ' '. Each value within the tokenizer is represented as a StringSegment. The code iterates the segments, allowing the consumer to interact with each segment.

Benchmark comparing StringTokenizer to string.Split

With the various ways of slicing and dicing strings, it feels appropriate to compare two methods with a benchmark. Using the BenchmarkDotNet NuGet package, consider the following two benchmark methods:

  1. Using StringTokenizer:

    StringBuilder buffer = new();
    var tokenizer =
        new StringTokenizer(
            new[] { ' ', '.' });
    foreach (StringSegment segment in tokenizer)
  2. Using String.Split:

    StringBuilder buffer = new();
    string[] tokenizer =
            new[] { ' ', '.' });
    foreach (string segment in tokenizer)

Both methods look similar on the API surface area, and they're both capable of splitting a large string into chunks. The benchmark results below show that the StringTokenizer approach is nearly three times faster, but results may vary. As with all performance considerations, you should evaluate your specific use case.

Method Mean Error StdDev Ratio
Tokenizer 3.315 ms 0.0659 ms 0.0705 ms 0.32
Split 10.257 ms 0.2018 ms 0.2552 ms 1.00


  • Mean: Arithmetic mean of all measurements
  • Error: Half of 99.9% confidence interval
  • Standard deviation: Standard deviation of all measurements
  • Median: Value separating the higher half of all measurements (50th percentile)
  • Ratio: Mean of the ratio distribution (Current/Baseline)
  • Ratio standard deviation: Standard deviation of the ratio distribution (Current/Baseline)
  • 1 ms: 1 Millisecond (0.001 sec)

For more information on benchmarking with .NET, see BenchmarkDotNet.

The StringValues type

The StringValues object is a struct type that represents null, zero, one, or many strings in an efficient way. The StringValues type can be constructed with either of the following syntaxes: string? or string?[]?. Using the text from the previous example, consider the following C# code:

StringValues values =
        new[] { '\n' }));

Console.WriteLine($"Count = {values.Count:#,#}");

foreach (string? value in values)
    // Interact with the value
// Outputs:
//     Count = 1,799

The preceding code instantiates a StringValues object given an array of string values. The StringValues.Count is written to the console.

The StringValues type is an implementation of the following collection types:

  • IList<string>
  • ICollection<string>
  • IEnumerable<string>
  • IEnumerable
  • IReadOnlyList<string>
  • IReadOnlyCollection<string>

As such, it can be iterated over and each value can be interacted with as needed.

See also