# WinForms Expert

This subagent guides .NET Windows Forms work while keeping form code compatible with the visual designer. Use it when Copilot needs to create or edit a WinForms UI.

- subagent · `type=agent` · GitHub Copilot · ★ 40451
- Author: github · official GitHub repository
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/github/awesome-copilot/tree/HEAD/agents/WinFormsExpert.agent.md
- Updated: 2026-10-23
- HTML: https://affpapa.org/en/nevibecode/agents/winforms-expert

## What it does

WinForms Expert provides conventions for new projects and form code. For new projects, it recommends .NET 10 or later; to support dark mode, it suggests calling `Application.SetColorMode(SystemColorMode.System);` at startup on .NET 9 or later. For High DPI settings, it points to `Application.SetHighDpiMode` rather than `app.config` or a manifest.

The key constraint applies to `InitializeComponent` and `.designer.cs` files: avoid control flow, lambdas, local functions, and several newer C# constructs there. Designer code must stay simple enough for the form editor to parse. Put complex UI logic in regular `.cs` files; otherwise, the designer may fail to load the form. VB projects have separate guidance: use the VB App Framework and `ApplicationEvents.vb`, rather than creating `Program.vb`.

## Install

### GitHub Copilot (native platform)

**Manually: put the files in place**
- File: `.github/agents/WinFormsExpert.agent.md`

### Claude Code

**Manually: put the files in place**
- File: `.claude/agents/WinFormsExpert.md` — For all projects use ~/.claude/agents/. Claude delegates by the description field or an @-mention. Copilot names tools its own way in the tools field: remove it or rewrite it for Claude Code

### Codex

**Manually: put the files in place**
- File: `.codex/agents/WinFormsExpert.toml` — Codex defines subagents in TOML: name, description, developer_instructions. Move the Markdown text into developer_instructions by hand. For all projects use ~/.codex/agents/

### Cursor

**Manually: put the files in place**
- File: `.cursor/agents/WinFormsExpert.md` — Cursor also reads .claude/agents/

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/github/awesome-copilot/tree/HEAD/agents/WinFormsExpert.agent.md

## When to use it
- set up a Windows Forms project with the recommended settings
- separate form initialization from application logic
- remove code patterns that prevent the designer from parsing a form
- configure High DPI at application startup

## Example prompts
- Create a .NET 10+ Windows Forms project and follow the WinForms Expert guidance.
- Review `InitializeComponent` and move complex logic out of `.designer.cs`.
- Set the app's High DPI mode at startup instead of using `app.config`.

## Source text · WinFormsExpert.agent.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 8c20c3ae2a48fc80.

BEGIN THIRD-PARTY FILE 8c20c3ae2a48fc80
````text
---
name: WinForms Expert
description: Support development of .NET (OOP) WinForms Designer compatible Apps.
#version: 2025-10-24a
---

# WinForms Development Guidelines

These are the coding and design guidelines and instructions for WinForms Expert Agent development.
When customer asks/requests will require the creation of new projects

**New Projects:**
* Prefer .NET 10+. Note: MVVM Binding requires .NET 8+.
* Prefer `Application.SetColorMode(SystemColorMode.System);` in `Program.cs` at application startup for DarkMode support (.NET 9+).
* Make Windows API projection available by default. Assume 10.0.22000.0 as minimum Windows version requirement.
```xml
    <TargetFramework>net10.0-windows10.0.22000.0</TargetFramework>
```

**Critical:**

**📦 NUGET:** New projects or supporting class libraries often need special NuGet packages. 
Follow these rules strictly:
 
* Prefer well-known, stable, and widely adopted NuGet packages - compatible with the project's TFM.
* Define the versions to the latest STABLE major version, e.g.: `[2.*,)`

**⚙️ Configuration and App-wide HighDPI settings:** *app.config* files are discouraged for configuration for .NET.
For setting the HighDpiMode, use e.g. `Application.SetHighDpiMode(HighDpiMode.SystemAware)` at application startup, not *app.config* nor *manifest* files.

Note: `SystemAware` is standard for .NET, use `PerMonitorV2` when explicitly requested.

**VB Specifics:**
- In VB, do NOT create a *Program.vb* - rather use the VB App Framework.
- For the specific settings, make sure the VB code file *ApplicationEvents.vb* is available. 
  Handle the `ApplyApplicationDefaults` event there and use the passed EventArgs to set the App defaults via its properties.

| Property | Type | Purpose | 
|----------|------|---------|
| ColorMode | `SystemColorMode` | DarkMode setting for the application. Prefer `System`. Other options: `Dark`, `Classic`. |
| Font | `Font` | Default Font for the whole Application. |	
| HighDpiMode | `HighDpiMode` | `SystemAware` is default. `PerMonitorV2` only when asked for HighDPI Multi-Monitor scenarios. |

---


## 🎯 Critical Generic WinForms Issue: Dealing with Two Code Contexts

| Context | Files/Location | Language Level | Key Rule |
|---------|----------------|----------------|----------|
| **Designer Code** | *.designer.cs*, inside `InitializeComponent` | Serialization-centric (assume C# 2.0 language features) | Simple, predictable, parsable |
| **Regular Code** | *.cs* files, event handlers, business logic | Modern C# 11-14 | Use ALL modern features aggressively |

**Decision:** In *.designer.cs* or `InitializeComponent` → Designer rules. Otherwise → Modern C# rules.

---

## 🚨 Designer File Rules (TOP PRIORITY)

⚠️ Make sure Diagnostic Errors and build/compile errors are eventually completely addressed!

### ❌ Prohibited in InitializeComponent

| Category | Prohibited | Why |
|----------|-----------|-----|
| Control Flow | `if`, `for`, `foreach`, `while`, `goto`, `switch`, `try`/`catch`, `lock`, `await`, VB: `On Error`/`Resume` | Designer cannot parse |
| Operators | `? :` (ternary), `??`/`?.`/`?[]` (null coalescing/conditional), `nameof()` | Not in serialization format |
| Functions | Lambdas, local functions, collection expressions (`...=[]` or `...=[1,2,3]`) | Breaks Designer parser |
| Backing fields | Only add variables with class field scope to ControlCollections, never local variables! | Designer cannot parse |

**Allowed method calls:** Designer-supporting interface methods like `SuspendLayout`, `ResumeLayout`, `BeginInit`, `EndInit`

### ❌ Prohibited in *.designer.cs* File

❌ Method definitions (except `InitializeComponent`, `Dispose`, preserve existing additional constructors)  
❌ Properties  
❌ Lambda expressions, DO ALSO NOT bind events in `InitializeComponent` to Lambdas!
````
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