Interchanging ReadOnlySpan<T>, string, and char[] in C#

Most think of only using Span<T> and ReadOnlySpan<T> (in C#) in places they need ultimate performance. But there is another use case where they are offly useful. And that is when you start working with string and char[].

Relying on ReadOnlySpan<T>, letting callers decide types

If you make your utility methods deal with ReadOnlySpan<T> (wherever appropriate1) instead of string or char[], you will only have to make slight refactorings to adjust for correct methods used. Most just include indexing and slicing, which can be done with span.Slice(i, len) or span[a..b] instead of string.Substring(i, len). Other common operations such as concatenation does have proper support already throughout the .NET standard library (See: Concatenating ReadOnlySpan<T>#).

With that you can then let the caller of the function choose between sending a range of different types, as string, Span<T>, and char[] all can be implicity casted to ReadOnlySpan<T>.

Let’s say you have the following method:

public class StringHelper
    /// <summary>
    /// Example: <c>"john.doe"</c> as username and <c>['.']</c> as separator, gives <c>"doe"</c>
    /// </summary>
    public ReadOnlySpan<char> GetLastSegment(ReadOnlySpan<char> source, ReadOnlySpan<char> separators)
        // ...

Now in your code production code, let’s say you may have those name name separators specified as an char[] because you also need it in a string.Split operation you’re doing later on. No worries! Just pass in that array as-is:

public class UsernameHelper
    private static readonly char[] _nameSeparators = new [] { '.', '-', '_' };

    public string GetNameSegments(string username)
        return username.Split(_nameSeparators, StringSplitOptions.RemoveEmptyEntries);

    public string GetLastName(string username)
        return StringHelper.GetLastSegment(username, _nameSeparators);

And then later in your tests, you want to declare test cases in NUnit, and you’re fully allowed to do that by representing the name separators as a string instead (which is very helpful as you cannot have array constants in attributes):

public class StringHelperTests
    // You cannot have array definitions in attributes
    [TestCase("john.doe", ".", "doe")]
    [TestCase("steve-smith", "-", "smith")]
    [TestCase("anne-frid.von.bygel", ".-", "bygel")]
    public void CorrectlySplits(string input, string separators, string expected)
        var result = StringHelper.GetLastSegment(input, separators);
        Assert.AreEqual(expected, result);

No extra casting required! Perfectly fine to just feed the separators in as a string instead. Need an empty array of char? Just pass an empty string! They’re stored in memory in the same way, so why not take advantage of that?


The ReadOnlySpan<T> type was introduced to allow for writing hyper-optimized array managing code, but the use cases does not stop there.

The biggest downside of using ReadOnlySpan<char> everywhere instead is partily because it’s a ref struct and has those limitations put on it, but worst (and almost silly, in a way) is that the type name is so long! It clutters the signatures of your methods enourmously, though it does go hand in hand with some clean code principles as it gives you yet another reason to make your functions smaller and take less arguments.

  1. As you cannot have ref structs in classes or records, those are inappropriate. Other examples are in asyncronous code and enumerators.