strings.md

Strings

Slicing strings

Before Span, we used String.Substring to get a part of a string, which allocates a new string and copies the characters to it.

Now we can just slice the Span using the range operator, an $O(1)$ operation. We can then pass this Span to a method that accepts a ReadOnlySpan<char> parameter, such as Int32.Parse. See Basics for a partial list of Span-aware APIs.

Example: Parsing a substring

int.Parse("12345".AsSpan()[1..3]) // results in 23

Example: Concatenating strings

const string text = "abcdefghijklmnopq";

string ConcatSubstring() => text[10..] + "---" + text[..5];
string ConcatAsSpan() => string.Concat(text.AsSpan()[10..], "---", text.AsSpan()[..5]);

BenchmarkDotnet results - note the reduction in allocated memory:

Method	Mean	Error	StdDev	Gen 0	Allocated
Substring	30.28 ns	0.633 ns	0.621 ns	0.0306	128 B
AsSpan	17.30 ns	0.377 ns	0.476 ns	0.0134	56 B

Creating strings of a known size using String.Create

When creating a new string whose size is known in advance, we can use the String.Create method to avoid additional allocations. This method works by allocating a string and providing a writable Span within a delegate. It's safe (meaning the string can't be mutated after creation) since the Span can't escape from the delegate.

Important

Avoid capturing variables in the delegate as they incur an allocation - pass all data using the state parameter. Use the static keyword to enforce this.

Example: Reversing a string

static string Reverse(this string s) =>
    string.Create(s.Length, s, static (span, str) =>
    {
        str.AsSpan().CopyTo(span);
        span.Reverse();
    });

Example: Creating a random string

Tip

Can be written using RandomNumberGenerator.GetString(size, "abcdefghijklmnopqrstuvwxyz"), which is also cryptographically secure.

static string GetRandomString(int size, char min = 'a', char max = 'z') =>
    string.Create(size, (min, max), static (span, state) =>
    {
        for (int i = 0; i < span.Length; i++)
        {
            span[i] = (char)Random.Shared.Next((int)state.min, (int)state.max + 1);
        }
    });

Example: Passing a Span<T> to the Create method (Advanced)

ref structs such as Span can't be used in generic type parameters (since there's currently no way to prevent boxing), so if we want to base one string on another, we'll have to use unsafe code - a pointer - in order to pass it into the generic SpanAction<T, TArg> delegate.

Important

Be careful when using unsafe code - it could easily lead to memory corruption. Extensively cover it with tests.

#pragma warning disable CS8500
unsafe static string ReplaceNonAlphanumeric(this ReadOnlySpan<char> s, char replacement)
{
    return string.Create(s.Length, (replacement, spanPtr: (IntPtr)(&s)), static (span, state) =>
    {
        var sourceSpan = *(ReadOnlySpan<char>*)state.spanPtr;
        for (int i = 0; i < span.Length; i++)
        {
            var c = sourceSpan[i];
            span[i] = char.IsLetterOrDigit(c) ? c : state.replacement;
        }
    });
}
#pragma warning restore CS8500

Using ReadOnlySpan<char> as the parameter rather than a string, allows us a lot of flexibility - we can stack-allocate the string, and we can transform it without allocations, as we'll see in the next section.

Mutating strings

We can use Span<char> to mutate strings while minimizing allocations. The MemoryExtensions class contains extension methods that are similar to System.String methods.

For example, we can use MemoryExtensions.Trim, and use the range operator to get a substring.

" abcde ".AsSpan().Trim()[..3] // results in "abc"

To use a method that transforms the string, such as ToLowerInvariant, we can use String.Create, chaining multiple mutations within the delegate.

static string SubstringToLower(this string s, int length) =>
    string.Create(length, (s, length), static (span, state) => state.s.AsSpan()[..state.length].ToLowerInvariant(span));

We can also create an alternative design for the ReplaceNonAlphanumeric method that does not use unsafe code by adding a parameter for the destination span.

static void ReplaceNonAlphanumeric(this ReadOnlySpan<char> source, Span<char> destination, char replacement)
{
    for (int i = 0; i < destination.Length; i++)
    {
        var c = source[i];
        destination[i] = char.IsLetterOrDigit(c) ? c : replacement;
    }
}

To use this method, we'll need to allocate the destination span. If our final destination is a String we can use String.Create, similarly to how we called ToLowerInvariant.

string.Create(length: 3, " a^b ", static (span, str) =>
    str.AsSpan().Trim()[..span.Length].ReplaceNonAlphanumeric('_', span)); // results in "a_b"

Interpolated string handlers

Interpolated string handlers are a C# feature that optimizes string creation by breaking them into multiple "append" calls, rather than allocating a string using String.Format, and potentially creating multiple interim strings (for example, formatting a number). This allows us to "hack" the compiler into all sorts of interesting optimizations.

Handlers available in .NET:

• DefaultInterpolatedStringHandler is used in a String.Create overload, and emitted by the compiler for regular interpolated strings.
• AppendInterpolatedStringHandler is used in StringBuilder.Append. No longer is it needed to break an interpolated string into multiple Append calls for efficiency.
• AssertInterpolatedStringHandler and WriteIfInterpolatedStringHandler are used in Debug.Assert and Debug.WriteIf respectively and completely skip writing according to the condition argument.
• TryWriteInterpolatedStringHandler is used by MemoryExtensions.TryWrite and allows us to efficiently write strings into a Span<char>. There's a similar one in Utf8.TryWrite to write into a Span<byte>.

dotNext also includes:

• BufferWriterInterpolatedStringHandler that writes into an IBufferWriter<char>.
• PoolingInterpolatedStringHandler which allows using a Memory<T> pool rather than allocating new buffers.

Tip

We can reuse any of the above handlers to write custom ones.

Example: TryWrite

The following method checks if the host & port part of a Uri matches a list of patterns. Rather than using Uri.GetLeftPart which allocates a string, we'll use TryWrite. Because it uses an interpolated string handler, there are no additional allocations (for example, to convert the port number into a string). Regex also works directly with Span<char>.

[SkipLocalsInit]
bool MatchesUriPattern(IEnumerable<Regex> patterns, Uri uri)
{
    var maxLength = uri.Scheme.Length + "://".Length + uri.Host.Length + ":".Length + 5;
    using SpanOwner<char> hostAndPort = maxLength <= 256 ? new(stackalloc char[256], maxLength) : new(maxLength);
    hostAndPort.Span.TryWrite($"{uri.Scheme}://{uri.Host}:{uri.Port}", out var written);
    var hostAndPortSpan = hostAndPort.Span[..written];

    foreach (var pattern in patterns)
    {
        if (pattern.IsMatch(hostAndPortSpan))
        {
            return true;
        }
    }

    return false;
}

Example: Using substrings in interpolated strings

Interpolated string handlers have overloads for ReadOnlySpan<char>, which allows us to get substrings without allocating new strings.

Tip

Consider String.Create and String.Concat before using an interpolated string, as they would be more efficient.

string Format(string str, int num)
{
    var index = str.IndexOf(':');
    return $"{str.AsSpan()[..index]} {num} {str.AsSpan()[(index + 1)..]}";
}

Example: Using DefaultInterpolatedStringHandler instead of StringBuilder

StringBuilder is a reference type and often in order to avoid allocating new ones, we pool them, or store them in reusable thread-static fields. As an alternative, we can use DefaultInterpolatedStringHandler as an append-only value type string builder.

Tip

StringBuilder works more like a linked list of arrays while DefaultInterpolatedStringHandler works more like a dynamic array. This means that StringBuilder might perform better when we can't approximate the final size of the string.

Tip

When possible, it's preferable to use stackalloc to provide the initial buffer as it performs better. If we don't provide an initial buffer, it uses a rented array with a size calculated from the parameters literalLength and formattedCount.

Important

The handler uses ArrayPool<char> internally when it grows out of the initial buffer, so we must call ToStringAndClear to return the rented array rather than ToString.

string BuildString(int count)
{
    var initialBuffer = (stackalloc char[256]);
    var builder = new DefaultInterpolatedStringHandler(0, 0, CultureInfo.InvariantCulture, initialBuffer);
    builder.AppendLiteral("hello");
    for (int i = 0; i < count; i++)
    {
        builder.AppendLiteral(", ");
        builder.AppendFormatted(i);
    }
    
    return builder.ToStringAndClear();
}

Hashing a string

To get a hexadecimal string representing the hash of a string, we'll use the SHA256 algorithm (it could be replaced with others available in .NET).

Important

The methods below will produce different hashes.

Using MemoryMarshal

We can use MemoryMarshal to reinterpret the string's char array as bytes (an $O(1)$ operation) rather than encode it.

However it can (depending on the input) double the time to hash the string as we'll get $length * 2$ bytes.

static string GetSha256(this string s)
{
    var hash = (stackalloc byte[32]);
    SHA256.HashData(MemoryMarshal.AsBytes(s.AsSpan()), hash);
    return Convert.ToHexString(hash);
}

UTF-8 encoding with ArrayPool

UTF-8 encoding can produce a lower byte count for many strings, which would make the hash function faster. We'll use SpanOwner again to stackalloc or rent an array.

const int StackallocThreshold = 256;

[SkipLocalsInit]
static string GetSha256(this string s)
{
    int inputByteCount = Encoding.UTF8.GetByteCount(s);
    using SpanOwner<byte> encodedBytes = inputByteCount <= StackallocThreshold ? new(stackalloc byte[StackallocThreshold], inputByteCount) : new(inputByteCount);
    int encodedByteCount = Encoding.UTF8.GetBytes(s, encodedBytes.Span);
    var hash = (stackalloc byte[32]);
    SHA256.HashData(encodedBytes.Span[..encodedByteCount], hash);
    return Convert.ToHexString(hash);
}

Splitting strings

MemoryExtensions methods Split and SplitAny allow us to split strings with no allocations. Unlike String.Split, it writes the results to a Span<Range>, which means we have to pre-allocate the ranges. If there are more matches than the ranges provide, the last range will contain the remainder of the string.

Tip

We're using collection expressions to create a ReadOnlySpan<string> of separators (of course, it can also be allocated statically once).

var stringToSplit = ";;11==22";
var spanToSplit = stringToSplit.AsSpan();
var ranges = (stackalloc Range[2]);
ReadOnlySpan<string> separators = [ "==", ";;" ];
var count = spanToSplit.SplitAny(ranges, separators, StringSplitOptions.RemoveEmptyEntries);
Debug.Assert(count == 2);
var result = (int.Parse(spanToSplit[ranges[0]]), int.Parse(spanToSplit[ranges[1]])); // results in (11, 22)

Optimized value searches using StringValues

Methods like ContainsAny and IndexOfAny can benefit from various optimizations, depending on the characters searched. For example, if the value contain only ASCII characters, or whether it's up to 5 characters, or represents a contiguous range (e.g. a-z). Determining the optimization is done once when StringValues is created.

Many .NET classes, such as JSON parsing, regular expressions and Uri have been enhanced with it.

private static readonly SearchValues<char> s_lineEndings = SearchValues.Create("\n\r\f\u0085\u2028\u2029");

int CountLineEndings(ReadOnlySpan<char> s)
{
    int count = 0;

    int pos;
    while ((pos = s.IndexOfAny(s_lineEndings)) >= 0)
    {
        count++;
        s = s.Slice(pos + 1);
    }

    return count;
}