Fix SendKeys sequential immediate modifier(^a^c, +a+c, %f%t) parsing regression

[LeafShi1]

Fixes #14690

Root Cause

In PR #3621, int[] haveKeys (a reference type with implicit by-reference semantics) was refactored to a value tuple. Without adding ref, the modifier state mutations inside AddSimpleKey were silently lost, breaking sequential immediate modifier sequences.

Proposed changes

• Pass modifier state by reference when calling AddSimpleKey, so cancellation updates are preserved in ParseKeys state.
• Update AddSimpleKey signature to accept modifier tuple by ref.
• Add regression unit tests for sequential immediate modifier patterns:^a^c, +a+c, %f%t

Customer Impact

• Apps migrated from .NET Framework that use sequential immediate modifier sequences in SendKeys (e.g. ^a^c) will no longer throw an unexpected

Regression?

• Yes

Risk

• Minimal

Screenshots

Sample project WinFormsApp8.zip

Before

System.ArgumentException: SendKeys string "^a^c" is not valid.

After

^a^c was parsed as Ctrl+A followed by Ctrl+C, consistent with .NET Framework behavior.

Test methodology

• Manually and unit test

Test environment(s)

• .net 11.0.0-preview.7.26330.112

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Fixes a regression in System.Windows.Forms.SendKeys parsing where sequential immediate modifier sequences (e.g. ^a^c, +a+c, %f%t) incorrectly throw ArgumentException on modern .NET, restoring behavior consistent with .NET Framework.

Changes:

• Passes the modifier-state tuple by ref into AddSimpleKey so modifier cancellation updates persist in ParseKeys.
• Updates AddSimpleKey signature and call sites to use ref haveKeys.
• Adds a regression unit test covering sequential immediate modifier patterns.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/test/unit/System.Windows.Forms/System/Windows/Forms/SendKeysTests.cs	Adds regression coverage for sequential immediate modifier parsing.
src/System.Windows.Forms/System/Windows/Forms/SendKeys/SendKeys.cs	Fixes modifier-state propagation by passing the tuple by ref.

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[KlausLoeffelmann]

Looks good!