Document InlineArrayAttribute is the correct approach for fixed-size buffers

[Copilot]

Fixes dotnet/runtime#112104

Users attempting to use StructLayoutAttribute.Size to create fixed-size buffers encounter memory corruption because the Size property only affects unmanaged layout, not managed memory allocation for non-blittable types.

Changes

• customize-struct-marshalling.md: Added "Fixed-size buffers and inline arrays" section

  • Warns against using StructLayoutAttribute.Size for fixed-size buffers
  • Explains technical limitation: only affects managed layout for blittable types
  • Provides dangerous vs. safe example using InlineArrayAttribute
  • Cross-references C# language documentation

• apply-interop-attributes.md: Added cross-reference to InlineArrayAttribute in the StructLayoutAttribute table entry

Example

// ❌ Dangerous: only allocates 1 byte, corrupts memory beyond
[StructLayout(LayoutKind.Explicit, Size = 100)]
struct UnsafeBuffer
{
    [FieldOffset(0)]
    private byte b;
}

// ✔️ Safe: allocates full 100 bytes
struct SafeBuffer
{
    private Buffer100 buffer;
}

[InlineArray(100)]
internal struct Buffer100
{
    private byte _element0;
}

Original prompt

Please implement the suggestion from the user in the last common on this page: dotnet/runtime#112104

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Internal previews

📄 File	🔗 Preview link
docs/standard/native-interop/apply-interop-attributes.md	Applying Interop Attributes
docs/standard/native-interop/customize-struct-marshalling.md	Customizing structure marshalling - .NET

[AaronRobinsonMSFT]

/cc @gewarren @jkotas @elinor-fung

[Copilot]

Pull request overview

This PR updates the native interop documentation to steer readers away from using StructLayoutAttribute.Size as a managed fixed-size buffer mechanism, and to point them to C# inline arrays (InlineArrayAttribute) as the correct approach.

Changes:

• Added a new “Fixed-size buffers and inline arrays” section to explain why StructLayoutAttribute.Size can be unsafe for managed fixed-size buffers and to recommend InlineArrayAttribute.
• Updated the interop attributes reference table to cross-reference InlineArrayAttribute from the StructLayoutAttribute entry.

Show a summary per file

File	Description
docs/standard/native-interop/customize-struct-marshalling.md	Adds guidance and examples for fixed-size buffers, recommending inline arrays over StructLayoutAttribute.Size.
docs/standard/native-interop/apply-interop-attributes.md	Updates the StructLayoutAttribute table entry to point readers toward inline arrays for fixed-size buffers.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 3