dotnet-winforms-basics

dotnet-winforms-basics

WinForms on .NET 8+: updated project templates with Host builder and DI, high-DPI support with PerMonitorV2, dark mode via Application.SetColorMode (experimental in .NET 9, targeting finalization in .NET 11), when to use WinForms, modernization tips for migrating from .NET Framework, and common agent pitfalls.

Version assumptions: .NET 8.0+ baseline (current LTS). TFM net8.0-windows. .NET 9 features (dark mode experimental) explicitly marked. .NET 11 finalization targets noted.

Scope boundary: This skill owns WinForms on modern .NET patterns: project setup, high-DPI, dark mode, DI, when to use, modernization. Migration from .NET Framework to .NET 8+ is owned by [skill:dotnet-wpf-migration]. Desktop testing is owned by [skill:dotnet-ui-testing-core].

Out of scope: WinForms .NET Framework patterns (legacy) -- this skill covers .NET 8+ only. Migration guidance -- see [skill:dotnet-wpf-migration]. Desktop testing -- see [skill:dotnet-ui-testing-core]. General Native AOT patterns -- see [skill:dotnet-native-aot]. UI framework selection -- see [skill:dotnet-ui-chooser].

Cross-references: [skill:dotnet-ui-testing-core] for desktop testing, [skill:dotnet-wpf-modern] for WPF patterns, [skill:dotnet-winui] for WinUI 3 patterns, [skill:dotnet-wpf-migration] for migration guidance, [skill:dotnet-native-aot] for general AOT, [skill:dotnet-ui-chooser] for framework selection.

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.NET 8+ Differences

WinForms on .NET 8+ is a significant modernization from .NET Framework WinForms, with an SDK-style project format, DI support, and updated APIs.

New Project Template

<!-- MyWinFormsApp.csproj (SDK-style) -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net8.0-windows</TargetFramework>
    <UseWindowsForms>true</UseWindowsForms>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Hosting" Version="8.*" />
  </ItemGroup>
</Project>

Key differences from .NET Framework WinForms:

• SDK-style .csproj (no packages.config, no AssemblyInfo.cs)
• Nullable reference types enabled by default
• Implicit usings enabled
• NuGet PackageReference format
• Program.cs uses top-level statements
• dotnet publish produces a single deployment artifact
• Side-by-side .NET installation (no machine-wide framework dependency)

Host Builder Pattern

Modern WinForms apps use the generic host for dependency injection:

// Program.cs
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

ApplicationConfiguration.Initialize();

var host = Host.CreateDefaultBuilder()
    .ConfigureServices((context, services) =>
    {
        // Services
        services.AddSingleton<IProductService, ProductService>();
        services.AddSingleton<ISettingsService, SettingsService>();

        // HTTP client
        services.AddHttpClient("api", client =>
        {
            client.BaseAddress = new Uri("https://api.example.com");
        });

        // Forms
        services.AddTransient<MainForm>();
        services.AddTransient<ProductDetailForm>();
    })
    .Build();

var mainForm = host.Services.GetRequiredService<MainForm>();
Application.Run(mainForm);

// MainForm.cs -- constructor injection
public partial class MainForm : Form
{
    private readonly IProductService _productService;
    private readonly IServiceProvider _serviceProvider;

    public MainForm(IProductService productService, IServiceProvider serviceProvider)
    {
        _productService = productService;
        _serviceProvider = serviceProvider;
        InitializeComponent();
    }

    private async void btnLoad_Click(object sender, EventArgs e)
    {
        var products = await _productService.GetProductsAsync();
        dataGridProducts.DataSource = products.ToList();
    }

    private void btnDetails_Click(object sender, EventArgs e)
    {
        var detailForm = _serviceProvider.GetRequiredService<ProductDetailForm>();
        detailForm.ShowDialog();
    }
}

ApplicationConfiguration.Initialize

.NET 8+ WinForms uses ApplicationConfiguration.Initialize() as the entry point, which consolidates multiple legacy configuration calls:

// ApplicationConfiguration.Initialize() is equivalent to:
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.SetHighDpiMode(HighDpiMode.SystemAware);  // default; override below for PerMonitorV2

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High-DPI

WinForms on .NET 8+ has significantly improved high-DPI support. The recommended mode is PerMonitorV2, which handles per-monitor DPI changes automatically.

Enabling PerMonitorV2

// Program.cs -- set before ApplicationConfiguration.Initialize()
Application.SetHighDpiMode(HighDpiMode.PerMonitorV2);
ApplicationConfiguration.Initialize();
// Note: SetHighDpiMode() called before Initialize() takes precedence
// over the default SystemAware mode set by Initialize().

Or configure via runtimeconfig.json:

{
  "runtimeOptions": {
    "configProperties": {
      "System.Windows.Forms.ApplicationHighDpiMode": 3
    }
  }
}

High-DPI modes:

Mode	Value	Behavior
DpiUnaware	0	No scaling; system bitmap-stretches the window
SystemAware	1	Scales to primary monitor DPI at startup (default in .NET 8)
PerMonitor	2	Adjusts when moved between monitors (basic)
PerMonitorV2	3	Full per-monitor scaling with non-client area support (recommended)
DpiUnawareGdiScaled	4	DPI-unaware but GDI+ text renders at native resolution

DPI-Unaware Designer Mode (.NET 9+)

.NET 9 introduces a DPI-unaware designer mode that prevents layout scaling issues in the Visual Studio WinForms designer. The designer renders at 96 DPI regardless of system DPI, preventing corrupted .Designer.cs files.

<!-- .csproj: opt in to DPI-unaware designer (.NET 9+) -->
<PropertyGroup>
  <ForceDesignerDPIUnaware>true</ForceDesignerDPIUnaware>
</PropertyGroup>

Scaling Gotchas

• Do not use absolute pixel sizes for controls. Use AutoScaleMode.Dpi on forms and let the layout engine scale controls automatically.
• Anchor and Dock layouts scale better than absolute positioning. TableLayoutPanel and FlowLayoutPanel handle DPI changes more reliably than fixed-position controls.
• Custom drawing (OnPaint) must use DPI-aware coordinates. Scale drawing coordinates by DeviceDpi / 96.0f in OnPaint overrides.
• Image resources need multiple resolutions. Provide 1x, 1.5x, and 2x versions of icons and images, or use SVG-based rendering.

// DPI-aware custom drawing
protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e);
    float scale = DeviceDpi / 96.0f;
    float fontSize = 12.0f * scale;
    using var font = new Font("Segoe UI", fontSize);
    e.Graphics.DrawString("Scaled text", font, Brushes.Black, 10 * scale, 10 * scale);
}

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Dark Mode

WinForms dark mode is experimental in .NET 9 and is targeting finalization in .NET 11. It provides system-integrated dark mode for WinForms controls using the Windows dark mode APIs.

Enabling Dark Mode (.NET 9+ Experimental)

// Program.cs -- set before ApplicationConfiguration.Initialize()
Application.SetColorMode(SystemColorMode.Dark);
ApplicationConfiguration.Initialize();

Or follow system theme:

// Follow system light/dark preference
Application.SetColorMode(SystemColorMode.System);

SystemColorMode values:

Mode	Behavior
Classic	Standard WinForms colors (no dark mode)
System	Follow Windows system light/dark theme setting
Dark	Force dark mode

Dark Mode Caveats

• Experimental status: The API surface may change before .NET 11 finalization. Do not depend on specific color values or rendering behavior in production.
• Control coverage: Not all controls support dark mode in .NET 9. Standard controls (Button, TextBox, Label, ListBox, DataGridView) have dark mode support. Third-party and custom-drawn controls may not render correctly.
• Owner-drawn controls: Controls using DrawMode.OwnerDrawFixed or custom OnPaint overrides must manually read SystemColors to respond to dark mode. They do not automatically inherit dark mode colors.
• Windows version: Dark mode requires Windows 10 version 1809 (build 17763) or later.
• .NET 11 target: Microsoft has indicated that WinForms visual styles (including dark mode) are targeting finalization in .NET 11. Plan for API stability after that release.

// Owner-drawn controls must use SystemColors for dark mode compatibility
protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e);
    // Use SystemColors instead of hardcoded colors
    using var textBrush = new SolidBrush(SystemColors.ControlText);
    using var bgBrush = new SolidBrush(SystemColors.Control);
    e.Graphics.FillRectangle(bgBrush, ClientRectangle);
    e.Graphics.DrawString("Text", Font, textBrush, 10, 10);
}

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When to Use

WinForms is the right choice for specific scenarios. It is not a general-purpose UI framework for new customer-facing applications.

Good Fit

• Rapid prototyping: Drag-and-drop designer for quick internal tools and proof-of-concept UIs
• Internal enterprise tools: Line-of-business forms, data entry, CRUD applications with DataGridView
• Simple Windows-only utilities: System tray apps, configuration tools, diagnostics dashboards
• Existing WinForms maintenance: Modernizing existing .NET Framework WinForms apps to .NET 8+
• Data-heavy tabular UIs: DataGridView with virtual mode handles millions of rows efficiently

Not a Good Fit

• New customer-facing applications: Use WPF (rich Windows desktop), WinUI 3 (modern Windows), MAUI (cross-platform), or Blazor (web)
• Complex custom UI: WinForms controls are limited in styling; WPF or WinUI provide rich templating
• Cross-platform requirements: WinForms is Windows-only; use MAUI or Uno Platform
• Accessibility-first applications: WPF and WinUI have better accessibility APIs and screen reader support
• Touch-optimized interfaces: WinForms was designed for mouse/keyboard; WinUI or MAUI handle touch better

Decision Guidance

Scenario	Recommended Framework
Quick internal tool	WinForms
Data entry form (Windows)	WinForms or WPF
Modern Windows desktop app	WinUI 3 or WPF (.NET 9+ Fluent)
Cross-platform mobile + desktop	MAUI or Uno Platform
Cross-platform + web	Uno Platform or Blazor
Existing WinForms modernization	WinForms on .NET 8+

For the full framework decision tree, see [skill:dotnet-ui-chooser].

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Modernization Tips

Tips for modernizing existing .NET Framework WinForms applications to .NET 8+.

Add Dependency Injection

Replace static references and singletons with constructor injection via Host builder (see .NET 8+ Differences section above).

Before (legacy pattern):

// Anti-pattern: static service references
public partial class MainForm : Form
{
    private void btnLoad_Click(object sender, EventArgs e)
    {
        var products = ProductService.Instance.GetProducts();
        dataGridProducts.DataSource = products;
    }
}

After (modern pattern):

// Modern: constructor injection
public partial class MainForm : Form
{
    private readonly IProductService _productService;

    public MainForm(IProductService productService)
    {
        _productService = productService;
        InitializeComponent();
    }

    private async void btnLoad_Click(object sender, EventArgs e)
    {
        var products = await _productService.GetProductsAsync();
        dataGridProducts.DataSource = products.ToList();
    }
}

Use Async Patterns

Replace synchronous blocking calls with async/await to keep the UI responsive:

// Before: blocks UI thread
private void btnSave_Click(object sender, EventArgs e)
{
    var client = new HttpClient();
    var result = client.PostAsync(url, content).Result; // BLOCKS UI
    MessageBox.Show("Saved!");
}

// After: async keeps UI responsive
private async void btnSave_Click(object sender, EventArgs e)
{
    btnSave.Enabled = false;
    try
    {
        var result = await _httpClient.PostAsync(url, content);
        result.EnsureSuccessStatusCode();
        MessageBox.Show("Saved!");
    }
    catch (HttpRequestException ex)
    {
        MessageBox.Show($"Error: {ex.Message}");
    }
    finally
    {
        btnSave.Enabled = true;
    }
}

Convert to .NET 8+

Use the .NET Upgrade Assistant for automated migration:

# Install upgrade assistant
dotnet tool install -g upgrade-assistant

# Analyze the project
upgrade-assistant analyze MyWinFormsApp.csproj

# Upgrade the project
upgrade-assistant upgrade MyWinFormsApp.csproj

Common migration issues:

• App.config settings need manual migration to appsettings.json or Host builder configuration
• My.Settings (VB.NET) and Settings.settings need manual migration
• Third-party controls may not have .NET 8 compatible versions
• Designer-generated code in .Designer.cs files usually migrates cleanly
• COM interop (System.Runtime.InteropServices) syntax may differ

Adopt Modern C# Features

// File-scoped namespaces
namespace MyApp.Forms;

// Null-conditional event invocation
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Value)));

// Collection expressions
var columns = new[] { "Name", "Price", "Category" };

// Primary constructors for services (C# 12)
public class ProductService(HttpClient httpClient) : IProductService
{
    public async Task<List<Product>> GetProductsAsync()
        => await httpClient.GetFromJsonAsync<List<Product>>("/products") ?? [];
}

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Agent Gotchas

1. Do not recommend WinForms for new customer-facing applications. WinForms is appropriate for internal tools, rapid prototyping, and data-centric utilities. For customer-facing apps, recommend WPF, WinUI 3, MAUI, or Blazor depending on requirements.
2. Do not use deprecated WinForms APIs. Menu (use MenuStrip), MainMenu (use MenuStrip), ContextMenu (use ContextMenuStrip), StatusBar (use StatusStrip), ToolBar (use ToolStrip), DataGrid (use DataGridView).
3. Do not assume dark mode is production-ready. Dark mode via Application.SetColorMode is experimental in .NET 9 and targeting finalization in .NET 11. API surface and rendering may change.
4. Do not use HighDpiMode.SystemAware without testing multi-monitor scenarios. PerMonitorV2 is recommended for apps used on multi-monitor setups with different DPI settings.
5. Do not block the UI thread with synchronous calls. Use async void for event handlers and async Task for all other async methods. Never use .Result or .Wait() on the UI thread.
6. Do not use Control.Invoke when await suffices. In .NET 8+ WinForms, await automatically marshals back to the UI thread via SynchronizationContext. Manual Invoke/BeginInvoke is only needed when called from non-async code (timers, COM callbacks).
7. Do not hardcode colors when dark mode is enabled. Use SystemColors properties (e.g., SystemColors.ControlText, SystemColors.Control) in custom drawing and owner-drawn controls to respond correctly to theme changes.
8. Do not forget to call ApplicationConfiguration.Initialize() before Application.Run. Omitting it disables visual styles and high-DPI configuration.

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Prerequisites

• .NET 8.0+ with Windows desktop workload
• TFM: net8.0-windows (no Windows SDK version needed for WinForms)
• Visual Studio 2022+ with Windows desktop workload (for designer support)
• For dark mode: .NET 9+ (experimental), Windows 10 version 1809+
• For DPI-unaware designer: .NET 9+

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References

• WinForms on .NET Documentation
• High DPI Support in WinForms
• .NET Upgrade Assistant
• WinForms Dark Mode (.NET 9)
• Microsoft.Extensions.Hosting