[API Proposal]: Span.StartsWith/EndsWith(T)

[MihaZupan]

Background and motivation

string.StartsWith(char) and string.EndsWith(char) are useful shortcuts for this.Length > 0 && this[0] == 'a'.
They also sometimes save you from having to declare temporary locals SomeCall().StartsWith('a').
For strings, they are also more efficient now but that's not the main motivation and likely wouldn't apply to Span extensions.

The fact that Span is missing such a helper feels like an oversight on an otherwise helpful API surface of utility extensions and I often end up writing a helper class that polyfills these. A quick GH search shows ~10 other such places.

It also means that some analyzers light up on string, but not on ReadOnlySpan<char>, e.g. the recently added CA1858 - "Use StartsWith instead of IndexOf".

I didn't find an existing issue for these so figured they're at least worth discussing.
At least unlike most? other extensions (e.g. IndexOf, StartsWith(ReadOnlySpan, ReadOnlySpan) ...), these methods are trivial to polyfill without losing out on potential perf.

Edit: more discussion around these APIs also at #85374

API Proposal

namespace System;

public static class MemoryExtensions
{
    // Existing
    public static bool StartsWith(this System.ReadOnlySpan<char> span, System.ReadOnlySpan<char> value, System.StringComparison comparisonType);
    public static bool StartsWith<T>(this System.ReadOnlySpan<T> span, System.ReadOnlySpan<T> value) where T : System.IEquatable<T>?;
    public static bool StartsWith<T>(this System.Span<T> span, System.ReadOnlySpan<T> value) where T : System.IEquatable<T>?;
    // ... same pattern for EndsWith

    // Proposed
    public static bool StartsWith<T>(this System.ReadOnlySpan<T> span, T value) where T : System.IEquatable<T>?;
    public static bool StartsWith<T>(this System.Span<T> span, T value) where T : System.IEquatable<T>?;
    public static bool EndsWith<T>(this System.ReadOnlySpan<T> span, T value) where T : System.IEquatable<T>?;
    public static bool EndsWith<T>(this System.Span<T> span, T value) where T : System.IEquatable<T>?;
}

API Usage

-static bool IsNameHidden(ReadOnlySpan<char> fileName) => fileName.Length > 0 && fileName[0] == '.';
+static bool IsNameHidden(ReadOnlySpan<char> fileName) => fileName.StartsWith('.');

Alternative Designs

While most helpers on MemoryExtensions are available for both Span and ReadOnlySpan of any T, the vast majority of use for this one will likely end up being ReadOnlySpan<char>.

public static bool StartsWith(this System.ReadOnlySpan<char> span, char value);
public static bool EndsWith(this System.ReadOnlySpan<char> span, char value);

Risks

No response

[ghost]

Tagging subscribers to this area: @dotnet/area-system-memory
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

string.StartsWith(char) and string.EndsWith(char) are useful shortcuts for this.Length > 0 && this[0] == 'a'.
They also sometimes save you from having to declare temporary locals SomeCall().StartsWith('a').
For strings, they are also more efficient now but that's not the main motivation and likely wouldn't apply to Span extensions.

The fact that Span is missing such a helper feels like an oversight on an otherwise helpful API surface of utility extensions and I often end up writing a helper class that polyfills these. A quick GH search shows ~10 other such places.

It also means that some analyzers light up on string, but not on ReadOnlySpan<char>, e.g. the recently added CA1858 - "Use StartsWith instead of IndexOf".

I didn't find an existing issue for these so figured they're at least worth discussing.
At least unlike most? other extensions (e.g. IndexOf, StartsWith(ReadOnlySpan, ReadOnlySpan) ...), these methods are trivial to polyfill without losing out on potential perf.

API Proposal

namespace System;

public static class MemoryExtensions
{
    // Existing
    public static bool StartsWith(this System.ReadOnlySpan<char> span, System.ReadOnlySpan<char> value, System.StringComparison comparisonType);
    public static bool StartsWith<T>(this System.ReadOnlySpan<T> span, System.ReadOnlySpan<T> value) where T : System.IEquatable<T>?;
    public static bool StartsWith<T>(this System.Span<T> span, System.ReadOnlySpan<T> value) where T : System.IEquatable<T>?;
    // ... same pattern for EndsWith

    // Proposed
    public static bool StartsWith<T>(this System.ReadOnlySpan<T> span, T value) where T : System.IEquatable<T>?;
    public static bool StartsWith<T>(this System.Span<T> span, T value) where T : System.IEquatable<T>?;
    public static bool EndsWith<T>(this System.ReadOnlySpan<T> span, T value) where T : System.IEquatable<T>?;
    public static bool EndsWith<T>(this System.Span<T> span, T value) where T : System.IEquatable<T>?;
}

API Usage

-static bool IsNameHidden(ReadOnlySpan<char> fileName) => fileName.Length > 0 && fileName[0] == '.';
+static bool IsNameHidden(ReadOnlySpan<char> fileName) => fileName.StartsWith('.');

Alternative Designs

While most helpers on MemoryExtensions are available for both Span and ReadOnlySpan of any T, the vast majority of use for this one will likely end up being ReadOnlySpan<char>.

public static bool StartsWith(this System.ReadOnlySpan<char> span, char value);
public static bool EndsWith(this System.ReadOnlySpan<char> span, char value);

Risks

No response

Author:	MihaZupan
Assignees:	-
Labels:	api-suggestion, area-System.Memory
Milestone:	-

[Clockwork-Muse]

Likely these don't exist because you can do:

// For non-reference types
MemoryExtensions.StartsWith(somespan, stackalloc[] {somevalue});

[Sergio0694]

@MihaZupan should we also consider StartsWith/EndsWith overloads taking a SearchValues<T> parameter?

[MihaZupan]

Do you mean something like a StartsWithAny?

static bool StartsWithAny<T>(ReadOnlySpan<T> span, SearchValues<T> values) =>
    span.Length > 0 && values.Contains(span[0]);

Things like span[0] is 'a' or 'b' or 'c' do come up sometimes, but I don't know how commonly compared to a single value. Wanting to do that with more than a few values such that you'd want a SearchValues feels niche.

Something like a bool StartsWithAny(ReadOnlySpan<char> span, SearchValues<string> values) could have an optimized implementation, but I'm not sure of the use cases.

Or did I misunderstand your question?

[Sergio0694]

Yup, no that was my question indeed — I can see how it might probably be niche, plus the fact you can just roll your own version still leveraging SearchValues<T> by just doing values.Contains(span[0]) seems sufficient if anyone needs it I guess. As in, people could just add an extension like that using the code you showed, and it'd just as fast as a built-in API 🙂

Thanks!

[SimonCropp]

any chance of this getting into v9?

[Sergio0694]

Wondering if it could make more sense to just add params to those span parameters once C# 13 supports it? 🤔

[MihaZupan]

@dotnet/area-system-memory should this one be marked as ready for review?

[tannergooding]

We do provide similar overloads on string and it is quite convenient overall, so I'd lean towards marking this ready-for-review.

We could make some overloads params ReadOnlySpan<T> value, but not everywhere due to trailing parameters like StringComparison.

CC. @stephentoub, @GrabYourPitchforks for additional input.

[stephentoub]

so I'd lean towards marking this ready-for-review

Seems fine.

Assuming the first-class span support lands, we wouldn't need the Span-based overloads, just the ReadOnlySpan-based overloads.

We could make some overloads params ReadOnlySpan value

I'm not following. All of the proposed overloads just take a T value... what would be params? The proposal would be to instead have them take a params ReadOnlySpan such that it becomes e.g. StartsWithAny?

[tannergooding]

The proposal would be to instead have them take a params ReadOnlySpan such that it becomes e.g. StartsWithAny?

Right, sorry, that's more what I meant. Was mostly just thinking now that we have params ROSpan<T> there are more ways to handle cases like this in a friendly manner.

Having an explicit overload that just takes 1 value and does the least amount of checks/overhead is still goodness I think, though.

[GrabYourPitchforks]

Having an explicit overload that just takes 1 value and does the least amount of checks/overhead is still goodness I think, though.

Agreed. This seems like the most common use case.