New Control Api

1.2 Environment-aware issue management

• Copilot CLI / Copilot Web-UI (CCA): You have permission to interact with the GitHub API. Follow the rules below to check, create, or update issues.
• VS Code / Visual Studio: Do not attempt to create or modify issues yourself — the necessary permissions may not be available. Instead, remind the user to verify and manage the API issue manually.

1.3 Creating or updating the API issue

⚠️ NEVER create a new issue in the upstream repo (dotnet/winforms).

1. If the issue exists upstream and you can modify it (you are acting on behalf of a WinForms team member with sufficient permissions), update it in-place.
2. If you cannot modify the upstream issue, check whether a corresponding issue exists in the origin fork. If yes, update it there. If no, create it there.
3. If no issue exists at all, create it in the origin fork using the format described below.

1.4 Required issue content

The API issue must contain all of the following sections. If any information is missing, stop and ask the user before proceeding with implementation.

Section 1 — Background and motivation: Why is this API needed? What scenario does it enable?

Section 2 — API Proposal (C# code block with full public signature, no method bodies):

namespace System.Windows.Forms;

public partial class ExistingControl
{
    public Color NewProperty { get; set; }
    public event EventHandler? NewPropertyChanged;
    protected virtual void OnNewPropertyChanged(EventArgs e);
}

Section 3 — API Usage (C# code block showing consumption):

var control = new ExistingControl();
control.NewProperty = Color.Red;
control.NewPropertyChanged += (s, e) => Console.WriteLine("Changed!");

Section 4 — Alternative Designs: Other approaches considered.

Section 5 — Risks: Breaking changes, perf regressions, etc.

Section 6 — Will this feature affect UI controls? Designer support, accessibility impact, localization needs.

2. PublicAPI File Tracking

All new public and protected members must be recorded in the PublicAPI text files so the Roslyn analyzer can enforce API compatibility.

2.1 During development — PublicAPI.Unshipped.txt

Add entries for every new API surface to the Unshipped file of the project that contains the new code. For System.Windows.Forms controls this