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Answer: Span<T> is a stack-only view over contiguous memory (an array, a stack buffer, native memory) that lets you slice without copying.

ReadOnlySpan<char> line = "key=value";
int eq = line.IndexOf('=');
var key = line[..eq];        // no allocation
var val = line[(eq + 1)..];

The family

| Type | Notes | |---|---| | IMemoryOwner<T> | owns a block and controls its lifetime | | ArraySegment<T> | older wrapper over part of an array |

Usage rules (abridged)

1. For a synchronous API, take Span<T> rather than Memory<T> if possible.
2. Use ReadOnlySpan<T> or ReadOnlyMemory<T> when the buffer is read-only.
3. If a method takes Memory<T> and returns void, do not use it after returning.
4. If it returns a Task, do not use the buffer after the task completes.

Claims

• id: C1 claim: ReadOnlySpan<T> is an immutable Span<T> and can be backed by a String. source: S1 evidence: "System.ReadOnlySpan<T>, an immutable version of the Span<T> structure."
• id: C2 claim: IMemoryOwner<T> controls a memory block's lifetime. source: S1 evidence: "which represents the owner of a block of memory and controls its lifetime management."
• id: C3 claim: For a synchronous API, use Span<T> instead of Memory<T>. source: S2 evidence: "Rule #1: For a synchronous API, use Span<T> instead of Memory<T> as a parameter if"
• id: C4 claim: Use read-only types if the buffer should be read-only. source: S2 evidence: "Rule #2: Use ReadOnlySpan<T> or ReadOnlyMemory<T> if the buffer should be read-only"
• id: C5 claim: A buffer has a single owner for its entire lifetime. source: S2 evidence: "A buffer has a single owner for its entire lifetime."

Sources

• [S1] Memory-related and span types - .NET. Microsoft Learn (.NET docs). https://learn.microsoft.com/en-us/dotnet/standard/memory-and-spans/ . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-04.
• [S2] Memory<T> and Span<T> usage guidelines - .NET. Microsoft Learn (.NET docs). https://learn.microsoft.com/en-us/dotnet/standard/memory-and-spans/memory-t-usage-guidelines . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_hsf68dq2,
  author = {{iinventedlight}},
  title = {{Span<T> and Memory<T> in .NET}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/csharp-span-memory/},
  note = {Permanent ID iil:note:hsf68dq2. License: CC-BY-4.0.},
}

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