0013 — Spans on .NET Framework come from System.Memory

Status: Accepted. Amends 0006.

Context

The managed PDF writer and the font and image parsers are byte-level code where Span<T>, ReadOnlySpan<T> and ArrayPool<T> decide whether they allocate per glyph or not at all. .NET 10 provides these types in the runtime. netstandard2.0 does not: on .NET Framework they come from Microsoft's System.Memory package, which cannot be polyfilled from source because the runtime and other libraries must agree on the type identity.

The alternative — writing every parser twice, or against arrays and offsets only — would make the modern target slower to match the old one, or double the code that most needs to be right.

Decision

The core references System.Memory for its netstandard2.0 build only. The net10.0 build still has no dependencies at all. No other runtime dependency is allowed under this exception.

Consequences

• One implementation of every parser and writer, allocation-conscious on both targets.
• .NET Framework consumers receive System.Memory and its two small companions — packages already present in most .NET Framework applications through other libraries.