[sumit-2007-git]

Contributes to #90400

This PR enables nullable reference types and annotates System.IO.Ports:

• Removed <Nullable>disable</Nullable> and <NoWarn>$(NoWarn);nullable</NoWarn> from ref/System.IO.Ports.csproj and src/System.IO.Ports.csproj.
• Annotated ref/System.IO.Ports.cs events as nullable delegates (DataReceived, ErrorReceived, PinChanged).
• Annotated SerialPort, SerialStream, and platform-specific implementations (Windows and Unix) with appropriate nullable reference types and attributes ([MemberNotNullWhen], [MemberNotNull]).

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[Copilot]

🟡 Changes recommended

The nullable contracts misrepresent runtime behavior: GetPortNames() can return null elements after a registry value disappears, while WriteLine’s non-null parameter annotation warns on valid null arguments.

2 open issues

• Filter registry values that disappear during port enumeration · New
• Mark WriteLine parameter nullable in the API contract · New

What changed in this PR

This PR enables nullable reference types in System.IO.Ports, contributing to the remaining-library annotation work tracked by #90400.

Changes:

• Removes nullable opt-outs from the implementation and reference projects.
• Annotates public events and internal state across Windows and Unix implementations.
• Adds flow-analysis attributes to handle checks and throwing helpers.

File	Description
src/​libraries/​System.IO.Ports/​src/​System/​IO/​Ports/​SerialStream.Windows.cs	Annotates handles, events, callbacks, and asynchronous state; adds null-safe disposal.
src/​libraries/​System.IO.Ports/​src/​System/​IO/​Ports/​SerialStream.Unix.cs	Annotates handles, events, queued requests, and timeout helpers.
src/​libraries/​System.IO.Ports/​src/​System/​IO/​Ports/​SerialStream.cs	Adds nullable event and handle-validation attributes.
src/​libraries/​System.IO.Ports/​src/​System/​IO/​Ports/​SerialPort.Windows.cs	Annotates registry access used for port enumeration.
src/​libraries/​System.IO.Ports/​src/​System/​IO/​Ports/​SerialPort.cs	Annotates events, internal state, and the open-state contract.
src/​libraries/​System.IO.Ports/​src/​System/​IO/​Ports/​InternalResources.cs	Marks throwing helpers with DoesNotReturn.
src/​libraries/​System.IO.Ports/​src/​System.IO.Ports.csproj	Enables nullable analysis for the implementation.
src/​libraries/​System.IO.Ports/​ref/​System.IO.Ports.csproj	Enables nullable annotations for the reference assembly.
src/​libraries/​System.IO.Ports/​ref/​System.IO.Ports.cs	Publishes nullable event delegate annotations.

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

The change introduces an API behavior regression by making Write(null) silently succeed on an open port instead of throwing ArgumentNullException.

2 open issues

• Restore Write null check and non-nullable signature · New
• Mark WriteLine parameter nullable in the API contract

1 resolved since last review

• Filter registry values that disappear during port enumeration

_{Review effort: Balanced}

[jkotas]

/ba-g opened #136892 on the test failure

[sumit-2007-git]

Thank you so much @jkotas for reviewing and merging this PR! Really appreciate your time, patient guidance, and support throughout.