<!-- 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>

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

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

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

// 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);
}

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

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

// 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);
}

// 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;
    }
}

// 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();
    }
}

// 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;
    }
}

undefined

**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

// 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") ?? [];
}