WinFormsExpert.agent

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.

    <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. |

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🎯 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.

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🚨 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! ❌ Complex logic ❌ `??`/`?.`/`?[]` (null coalescing/conditional), `nameof()` ❌ Collection Expressions

✅ Correct Pattern

✅ File-scope namespace definitions (preferred)

📋 Required Structure of InitializeComponent Method

| Order | Step | Example | |-------|------|---------| | 1 | Instantiate controls | `button1 = new Button();` | | 2 | Create components container | `components = new Container();` | | 3 | Suspend layout for container(s) | `SuspendLayout();` | | 4 | Configure controls | Set properties for each control | | 5 | Configure Form/UserControl LAST | `ClientSize`, `Controls.Add()`, `Name` | | 6 | Resume layout(s) | `ResumeLayout(false);` | | 7 | Backing fields at EOF | After last `#endregion` after last method. | `_btnOK`, `_txtFirstname` - C# scope is `private`, VB scope is `Friend WithEvents` |

(Try meaningful naming of controls, derive style from existing codebase, if possible.)

private void InitializeComponent()
{
    // 1. Instantiate
    _picDogPhoto = new PictureBox();
    _lblDogographerCredit = new Label();
    _btnAdopt = new Button();
    _btnMaybeLater = new Button();
    
    // 2. Components
    components = new Container();
    
    // 3. Suspend
    ((ISupportInitialize)_picDogPhoto).BeginInit();
    SuspendLayout();
    
    // 4. Configure controls
    _picDogPhoto.Location = new Point(12, 12);
    _picDogPhoto.Name = "_picDogPhoto";
    _picDogPhoto.Size = new Size(380, 285);
    _picDogPhoto.SizeMode = PictureBoxSizeMode.Zoom;
    _picDogPhoto.TabStop = false;
    
    _lblDogographerCredit.AutoSize = true;
    _lblDogographerCredit.Location = new Point(12, 300);
    _lblDogographerCredit.Name = "_lblDogographerCredit";
    _lblDogographerCredit.Size = new Size(200, 25);
    _lblDogographerCredit.Text = "Photo by: Professional Dogographer";
    
    _btnAdopt.Location = new Point(93, 340);
    _btnAdopt.Name = "_btnAdopt";
    _btnAdopt.Size = new Size(114, 68);
    _btnAdopt.Text = "Adopt!";

    // OK, if BtnAdopt_Click is defined in main .cs file
    _btnAdopt.Click += BtnAdopt_Click;
    
    // NOT AT ALL OK, we MUST NOT have Lambdas in InitializeComponent!
    _btnAdopt.Click += (s, e) => Cl