[API Proposal]: MemoryExtensions.AsSpan(this string? text, Range range)

[stephentoub]

Background and motivation

Some APIs return Range instances to specify subsets. You can use such a Range with a string like str[range] to get the substring, but that allocates a string. MemoryExtensions exposes AsSpan methods that take Ranges, but only for T[] and ArraySegment<T>, not string. It does have a Range-based string extension, but only AsMemory.

API Proposal

namespace System;

public static class MemoryExtensions
{
    public static ReadOnlySpan<char> AsSpan(this string? text, Range range);
}

API Usage

ReadOnlySpan<char> span = string.AsSpan(range);

Alternative Designs

No response

Risks

No response

[ghost]

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Issue Details

---

Background and motivation

Some APIs return Range instances to specify subsets. You can use such a Range with a string like str[range] to get the substring, but that allocates a string. MemoryExtensions exposes AsSpan methods that take Ranges, but only T[] and ArraySegment<T>, not string. It does have a Range-based string extension, but only AsMemory.

API Proposal

namespace System;

public static class MemoryExtensions
{
    public static ReadOnlySpan<char> AsSpan(this string? text, Range range);
}

API Usage

ReadOnlySpan<char> span = string.AsSpan(range);

Alternative Designs

No response

Risks

No response

Author:	stephentoub
Assignees:	-
Labels:	area-System.Runtime, api-ready-for-review
Milestone:	8.0.0

[stephentoub]

(I have a memory of discussing this at some point, and deciding that str.AsSpan()[range] was sufficient, but I can't find that now, and if that was the decision, the presence of the array-based span methods and string-based memory method is strangely inconsistent.)

[Tornhoof]

Why is the string nullable in your proposal?

[stephentoub]

Why is the string nullable in your proposal?

For consistency with the other such methods, where null is permitted in certain circumstances, e.g.

runtime/src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs

Line 137 in 6bae351

public static ReadOnlySpan<char> AsSpan(this string? text, int start, int length)

[eerhardt]

There are some other minor inconsistencies with MemoryExtensions.

runtime/src/libraries/System.Memory/ref/System.Memory.cs

Lines 208 to 220 in e1081df

	public static System.ReadOnlySpan<char> AsSpan(this string? text) { throw null; }
	public static System.ReadOnlySpan<char> AsSpan(this string? text, int start) { throw null; }
	public static System.ReadOnlySpan<char> AsSpan(this string? text, int start, int length) { throw null; }
	public static System.Span<T> AsSpan<T>(this System.ArraySegment<T> segment) { throw null; }
	public static System.Span<T> AsSpan<T>(this System.ArraySegment<T> segment, System.Index startIndex) { throw null; }
	public static System.Span<T> AsSpan<T>(this System.ArraySegment<T> segment, int start) { throw null; }
	public static System.Span<T> AsSpan<T>(this System.ArraySegment<T> segment, int start, int length) { throw null; }
	public static System.Span<T> AsSpan<T>(this System.ArraySegment<T> segment, System.Range range) { throw null; }
	public static System.Span<T> AsSpan<T>(this T[]? array) { throw null; }
	public static System.Span<T> AsSpan<T>(this T[]? array, System.Index startIndex) { throw null; }
	public static System.Span<T> AsSpan<T>(this T[]? array, int start) { throw null; }
	public static System.Span<T> AsSpan<T>(this T[]? array, int start, int length) { throw null; }
	public static System.Span<T> AsSpan<T>(this T[]? array, System.Range range) { throw null; }

• The Array and ArraySegment overloads also have System.Index startIndex overloads along with the System.Range overload. Should we add a System.Index overload for string as well?

runtime/src/libraries/System.Memory/ref/System.Memory.cs

Lines 195 to 207 in e1081df

	public static System.ReadOnlyMemory<char> AsMemory(this string? text) { throw null; }
	public static System.ReadOnlyMemory<char> AsMemory(this string? text, System.Index startIndex) { throw null; }
	public static System.ReadOnlyMemory<char> AsMemory(this string? text, int start) { throw null; }
	public static System.ReadOnlyMemory<char> AsMemory(this string? text, int start, int length) { throw null; }
	public static System.ReadOnlyMemory<char> AsMemory(this string? text, System.Range range) { throw null; }
	public static System.Memory<T> AsMemory<T>(this System.ArraySegment<T> segment) { throw null; }
	public static System.Memory<T> AsMemory<T>(this System.ArraySegment<T> segment, int start) { throw null; }
	public static System.Memory<T> AsMemory<T>(this System.ArraySegment<T> segment, int start, int length) { throw null; }
	public static System.Memory<T> AsMemory<T>(this T[]? array) { throw null; }
	public static System.Memory<T> AsMemory<T>(this T[]? array, System.Index startIndex) { throw null; }
	public static System.Memory<T> AsMemory<T>(this T[]? array, int start) { throw null; }
	public static System.Memory<T> AsMemory<T>(this T[]? array, int start, int length) { throw null; }
	public static System.Memory<T> AsMemory<T>(this T[]? array, System.Range range) { throw null; }

• The ArraySegment AsMemory methods don't have Index or Range overloads, like we have for string and T[]. Should we add them as well to make these 2 sets of methods be consistent?

[bartonjs]

Since there's already an AsMemory overload with the same shape, this seems like it's just properly squaring off the feature.

Since AsSpan(string?, Index) isn't present, but AsSpan(T[], Index) and AsSpan(ArraySegment<T>, Index) do, we've added that to further square this off.

namespace System;

public static partial class MemoryExtensions
{
    public static ReadOnlySpan<char> AsSpan(this string? text, Range range);
    public static ReadOnlySpan<char> AsSpan(this string? text, Index startIndex);
}