System.Memory Design Guidelines

System.Memory is a collection of types and features that make working with buffers and raw memory more efficient while remaining type safe. The feature specs can be found here:

• Span<T>
• Memory<T>

Overview

• ReadOnlySpan<T> is effectively the universal receiver, in that T[], T*, Memory<T>, ReadOnlyMemory<T>, Span<T>, ArraySegment<T> can all be converted to it. So if you can declare your API to accept a ReadOnlySpan<T> and behave efficiently, that's best, as any of these inputs can be used with your method.
• Similarly for Span<T>, if you need write access in the implementation.
• It allows building safe public APIs that can operate on unmanaged memory without forcing all consumers to use pointers (and thus becoming unsafe). The implementation can still extract a raw pointer, therefore getting equivalent performance if necessary.
• It's generally best for a synchronous method to accept Span<T> or ReadOnlySpan<T>. However, since ReadOnlySpan<T>/Span<T> are stack-only [1], this may be too limiting for the implementation. In particular, if the implementation needs to be able to store the argument for later usage, such as with an asynchronous method or an iterator, ReadOnlySpan<T>/Span<T> is inappropriate. ReadOnlyMemory<T>/Memory<T> should be used in such situations.

[1] stack-only isn't the best way to put it. Strictly speaking, these types are called ref-like types. These types must be structs, cannot be fields in classes, cannot be boxed, and cannot be used to instantiate generic types. Value types containing fields of ref-like types must themselves be ref-like types.

Guidance

• DO NOT use pointers for methods operating on buffers. Instead, use appropriate type from below. In performance critical code where bounds checking is unacceptable, the method's implementation can still pin the span and get the raw pointer if necessary. The key is that you don't spread the pointer through the public API.
  • Synchronous, read-only access needed: ReadOnlySpan<T>
  • Synchronous, writable access needed: Span<T>
  • Asynchronous, read-only access needed: ReadOnlyMemory<T>
  • Asynchronous, writable access needed: Memory<T>
• CONSIDER using stackalloc with Span<T> when you need small temporary storage but you need to avoid allocations and associated life-time management.
• AVOID providing overloads for both ReadOnlySpan<T> and Span<T> as Span<T> can be implicitly converted to ReadOnlySpan<T>.
• AVOID providing overloads for both ReadOnlySpan<T>/Span<T> as well as pointers and arrays as those can be implicitly converted to ReadOnlySpan<T>/Span<T>.