# Phase 1: Foundation - Research

**Researched:** 2026-04-02
**Domain:** WPF/.NET 10, MVVM, MSAL authentication, multi-tenant session management, structured logging, localization
**Confidence:** HIGH

---

<user_constraints>
## User Constraints (from CONTEXT.md)

### Locked Decisions

| Decision | Value |
|---|---|
| Runtime | .NET 10 LTS + WPF |
| MVVM framework | CommunityToolkit.Mvvm 8.4.2 |
| SharePoint library | PnP.Framework 1.18.0 |
| Auth | MSAL.NET 4.83.1 + Extensions.Msal 4.83.3 + Desktop 4.82.1 |
| Token cache | MsalCacheHelper — one `IPublicClientApplication` per ClientId |
| DI host | Microsoft.Extensions.Hosting 10.x |
| Logging | Serilog 4.3.1 + rolling file sink → `%AppData%\SharepointToolbox\logs\` |
| JSON | System.Text.Json (built-in) |
| JSON persistence | Write-then-replace (`file.tmp` → validate → `File.Move`) + `SemaphoreSlim(1)` per file |
| Async pattern | `AsyncRelayCommand` everywhere — zero `async void` handlers |
| Trimming | `PublishTrimmed=false` — accept ~150–200 MB EXE |
| Architecture | 4-layer MVVM: View → ViewModel → Service → Infrastructure |
| Cross-VM messaging | `WeakReferenceMessenger` for tenant-switched events |
| Session holder | Singleton `SessionManager` — only class that holds `ClientContext` objects |
| Localization | .resx resource files (EN default, FR overlay) |

### Shell Layout (defaults applied — not re-litigatable)

- `MainWindow` with top `ToolBar`, center `TabControl`, bottom docked `RichTextBox` log panel (150 px, always visible)
- `StatusBar` at very bottom: tenant name | operation status | progress %
- Toolbar (L→R): `ComboBox` (220 px, tenant list) → `Button "Connect"` → `Button "Manage Profiles..."` → separator → `Button "Clear Session"`
- Profile fields: Name, Tenant URL, Client ID — matches `{ name, tenantUrl, clientId }` JSON exactly
- All feature tabs stubbed with "Coming soon" placeholder except Settings (profile management + language)

### Progress + Cancel UX (locked)

- Per-tab: `ProgressBar` + `TextBlock` + `Button "Cancel"` — visible only when `IsRunning`
- `CancellationTokenSource` owned by each ViewModel, recreated per operation
- `IProgress<OperationProgress>` where `OperationProgress = { int Current, int Total, string Message }`
- Log panel writes every meaningful progress event (timestamped)
- `StatusBar` updates from active tab via `WeakReferenceMessenger`

### Error Surface UX (locked)

- Non-fatal: red log panel entry + per-tab status summary — no modal
- Fatal/blocking: `MessageBox.Show` modal + "Copy to Clipboard" button
- No toasts in Phase 1
- Log format: `HH:mm:ss [LEVEL] Message` — green=info, orange=warning, red=error
- Global handlers: `Application.DispatcherUnhandledException` + `TaskScheduler.UnobservedTaskException`
- Empty catch block = build defect; enforced in code review

### JSON Compatibility (locked — live user data)

| File | Schema |
|---|---|
| `Sharepoint_Export_profiles.json` | `{ "profiles": [{ "name": "...", "tenantUrl": "...", "clientId": "..." }] }` |
| `Sharepoint_Settings.json` | `{ "dataFolder": "...", "lang": "en" }` |

### Localization (locked)

- `Strings.resx` (EN/neutral default), `Strings.fr.resx` (FR overlay)
- Key naming mirrors existing PowerShell convention: `tab.perms`, `btn.run.scan`, `menu.language`, etc.
- Dynamic switching: `CultureInfo.CurrentUICulture` swap + `WeakReferenceMessenger` broadcast
- FR strings stubbed with EN fallback in Phase 1

### Infrastructure Patterns (Phase 1 required deliverables)

1. `SharePointPaginationHelper` — static helper wrapping `CamlQuery` + `ListItemCollectionPosition` looping, `RowLimit ≤ 2000`
2. `AsyncRelayCommand` canonical example — `FeatureViewModel` base showing `CancellationTokenSource` + `IsRunning` + `IProgress<OperationProgress>` + `OperationCanceledException` handling
3. `ObservableCollection` threading rule — accumulate in `List<T>` on background, then `Dispatcher.InvokeAsync` with `new ObservableCollection<T>(list)`
4. `ExecuteQueryRetryAsync` wrapper — wraps PnP Framework retry; surfaces retry events as log + progress messages
5. `ClientContext` disposal — always `await using`; unit tests verify `Dispose()` on cancellation

### Deferred Ideas (OUT OF SCOPE for Phase 1)

- Log panel collapsibility
- Dark/light theme toggle
- Toast/notification system
- FR locale completeness (Phase 5)
- User access export, storage charts, simplified permissions view
</user_constraints>

---

<phase_requirements>
## Phase Requirements

| ID | Description | Research Support |
|----|-------------|-----------------|
| FOUND-01 | Application built with C#/WPF (.NET 10 LTS) using MVVM architecture | Generic Host DI pattern, CommunityToolkit.Mvvm ObservableObject/RelayCommand stack confirmed |
| FOUND-02 | Multi-tenant profile registry — create, rename, delete, switch tenant profiles | ProfileService using System.Text.Json + write-then-replace pattern; ComboBox bound to ObservableCollection |
| FOUND-03 | Multi-tenant session caching — stay authenticated across tenant switches | MsalCacheHelper per ClientId (one IPublicClientApplication per tenant), AcquireTokenSilent flow |
| FOUND-04 | Interactive Azure AD OAuth login via browser — no client secrets | MSAL PublicClientApplicationBuilder + AcquireTokenInteractive; PnP AuthenticationManager.CreateWithInteractiveLogin |
| FOUND-05 | All long-running operations report progress to the UI in real-time | IProgress<OperationProgress> + Progress<T> (marshals to UI thread automatically) |
| FOUND-06 | User can cancel any long-running operation mid-execution | CancellationTokenSource per ViewModel; AsyncRelayCommand.Cancel(); OperationCanceledException handling |
| FOUND-07 | All errors surface to the user with actionable messages — no silent failures | Global DispatcherUnhandledException + TaskScheduler.UnobservedTaskException; empty-catch policy |
| FOUND-08 | Structured logging for diagnostics | Serilog 4.3.1 + Serilog.Sinks.File (rolling daily) → %AppData%\SharepointToolbox\logs\ |
| FOUND-09 | Localization system supporting English and French with dynamic language switching | Strings.resx + Strings.fr.resx; singleton TranslationSource + WeakReferenceMessenger broadcast |
| FOUND-10 | JSON-based local storage compatible with current app format | System.Text.Json; existing field names preserved exactly; write-then-replace with SemaphoreSlim(1) |
| FOUND-12 | Configurable data output folder for exports | SettingsService reads/writes `Sharepoint_Settings.json`; FolderBrowserDialog in Settings tab |
</phase_requirements>

---

## Summary

Phase 1 establishes the entire skeleton on which all feature phases build. The technical choices are fully locked and research-validated. The stack (.NET 10 + WPF + CommunityToolkit.Mvvm + MSAL + PnP.Framework + Serilog + System.Text.Json) is internally consistent, widely documented, and has no version conflicts identified.

The three highest-risk areas for planning are: (1) WPF + Generic Host integration — the WPF STA threading model requires explicit plumbing that is not in the default Host template; (2) MSAL per-tenant token cache scoping — the `MsalCacheHelper` must be instantiated with a unique cache file name per `ClientId`, and the `IPublicClientApplication` instance must be kept alive in `SessionManager` for `AcquireTokenSilent` to work across tenant switches; (3) Dynamic localization without a restart — WPF's standard `x:Static` bindings to generated `.resx` classes are evaluated at startup only, so a `TranslationSource` singleton bound to `INotifyPropertyChanged` (or `MarkupExtension` returning a `Binding`) is required for runtime culture switching.

**Primary recommendation:** Build the Generic Host wiring, `SessionManager`, and `TranslationSource` in Wave 1 of the plan. All other components depend on DI being up and the culture system being in place.

---

## Standard Stack

### Core

| Library | Version | Purpose | Why Standard |
|---------|---------|---------|--------------|
| CommunityToolkit.Mvvm | 8.4.2 | ObservableObject, RelayCommand, AsyncRelayCommand, WeakReferenceMessenger | Microsoft-maintained; source generator MVVM; replaces MVVM Light |
| Microsoft.Extensions.Hosting | 10.x | Generic Host — DI container, lifetime, configuration | Official .NET hosting model; Serilog integrates via UseSerilog() |
| MSAL.NET (Microsoft.Identity.Client) | 4.83.1 | Public client OAuth2 interactive login | Official Microsoft identity library for desktop |
| Microsoft.Identity.Client.Extensions.Msal | 4.83.3 | MsalCacheHelper — cross-platform encrypted file token cache | Required for persistent token cache on desktop |
| Microsoft.Identity.Client.Broker | 4.82.1 | WAM (Windows Auth Manager) broker support | Better Windows 11 SSO; falls back gracefully |
| PnP.Framework | 1.18.0 | AuthenticationManager, ClientContext, CSOM operations | Only library containing PnP Provisioning Engine |
| Serilog | 4.3.1 | Structured logging | De-facto .NET logging library |
| Serilog.Sinks.File | (latest) | Rolling daily log file | The modern replacement for deprecated Serilog.Sinks.RollingFile |
| Serilog.Extensions.Hosting | (latest) | host.UseSerilog() integration | Wires Serilog into ILogger<T> DI |
| System.Text.Json | built-in (.NET 10) | JSON serialization/deserialization | Zero dependency; sufficient for flat profile/settings schemas |

### Supporting

| Library | Version | Purpose | When to Use |
|---------|---------|---------|-------------|
| Microsoft.Extensions.DependencyInjection | 10.x | DI abstractions (bundled with Hosting) | Service registration and resolution |
| xUnit | 2.x | Unit testing | ViewModel and service layer tests |
| Moq or NSubstitute | latest | Mocking in tests | Isolate services in ViewModel tests |

### Alternatives Considered

| Instead of | Could Use | Tradeoff |
|------------|-----------|----------|
| CommunityToolkit.Mvvm | Prism | Prism is heavier, module-oriented; overkill for single-assembly app |
| Serilog.Sinks.File | NLog or log4net | Serilog integrates cleanly with Generic Host; NLog would work but adds config file complexity |
| System.Text.Json | Newtonsoft.Json | Newtonsoft handles more edge cases but is unnecessary for the flat schemas here |

**Installation:**

```bash
dotnet add package CommunityToolkit.Mvvm --version 8.4.2
dotnet add package Microsoft.Extensions.Hosting
dotnet add package Microsoft.Identity.Client --version 4.83.1
dotnet add package Microsoft.Identity.Client.Extensions.Msal --version 4.83.3
dotnet add package Microsoft.Identity.Client.Broker --version 4.82.1
dotnet add package PnP.Framework --version 1.18.0
dotnet add package Serilog
dotnet add package Serilog.Sinks.File
dotnet add package Serilog.Extensions.Hosting
```

---

## Architecture Patterns

### Recommended Project Structure

```
SharepointToolbox/
├── App.xaml / App.xaml.cs         # Generic Host entry point, global exception handlers
├── Core/
│   ├── Models/
│   │   ├── TenantProfile.cs       # { Name, TenantUrl, ClientId }
│   │   └── OperationProgress.cs   # record { int Current, int Total, string Message }
│   ├── Messages/
│   │   ├── TenantSwitchedMessage.cs
│   │   └── LanguageChangedMessage.cs
│   └── Helpers/
│       ├── SharePointPaginationHelper.cs
│       └── ExecuteQueryRetryHelper.cs
├── Infrastructure/
│   ├── Persistence/
│   │   ├── ProfileRepository.cs   # write-then-replace + SemaphoreSlim(1)
│   │   └── SettingsRepository.cs
│   ├── Auth/
│   │   └── MsalClientFactory.cs   # creates and caches IPublicClientApplication per ClientId
│   └── Logging/
│       └── LogPanelSink.cs        # custom Serilog sink → RichTextBox
├── Services/
│   ├── SessionManager.cs          # singleton, owns all ClientContext instances
│   ├── ProfileService.cs
│   └── SettingsService.cs
├── Localization/
│   ├── TranslationSource.cs       # singleton INotifyPropertyChanged; ResourceManager wrapper
│   ├── Strings.resx               # EN (neutral default)
│   └── Strings.fr.resx            # FR overlay
├── ViewModels/
│   ├── MainWindowViewModel.cs
│   ├── ProfileManagementViewModel.cs
│   ├── FeatureViewModelBase.cs    # canonical async pattern: CTS + IsRunning + IProgress
│   └── Tabs/
│       └── SettingsViewModel.cs
└── Views/
    ├── MainWindow.xaml
    ├── Dialogs/
    │   └── ProfileManagementDialog.xaml
    └── Tabs/
        └── SettingsView.xaml
```

### Pattern 1: Generic Host + WPF Wiring

**What:** Replace WPF's default `StartupUri`-based startup with a `static Main` that builds a Generic Host, then resolves `MainWindow` from DI.

**When to use:** Required for all DI-injected ViewModels and services in WPF.

**Example:**

```csharp
// App.xaml.cs
// Source: https://formatexception.com/2024/02/adding-host-to-wpf-for-dependency-injection/
public partial class App : Application
{
    [STAThread]
    public static void Main(string[] args)
    {
        using IHost host = Host.CreateDefaultBuilder(args)
            .UseSerilog((ctx, cfg) => cfg
                .WriteTo.File(
                    Path.Combine(
                        Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
                        "SharepointToolbox", "logs", "app-.log"),
                    rollingInterval: RollingInterval.Day,
                    retainedFileCountLimit: 30))
            .ConfigureServices(RegisterServices)
            .Build();

        host.Start();

        App app = new();
        app.InitializeComponent();
        app.MainWindow = host.Services.GetRequiredService<MainWindow>();
        app.MainWindow.Visibility = Visibility.Visible;
        app.Run();
    }

    private static void RegisterServices(HostBuilderContext ctx, IServiceCollection services)
    {
        services.AddSingleton<SessionManager>();
        services.AddSingleton<ProfileService>();
        services.AddSingleton<SettingsService>();
        services.AddSingleton<MsalClientFactory>();
        services.AddSingleton<MainWindowViewModel>();
        services.AddTransient<ProfileManagementViewModel>();
        services.AddSingleton<MainWindow>();
    }
}
```

```xml
<!-- App.xaml: remove StartupUri, keep x:Class -->
<Application x:Class="SharepointToolbox.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Application.Resources/>
</Application>
```

```xml
<!-- SharepointToolbox.csproj: override StartupObject, demote App.xaml from ApplicationDefinition -->
<PropertyGroup>
    <StartupObject>SharepointToolbox.App</StartupObject>
</PropertyGroup>
<ItemGroup>
    <ApplicationDefinition Remove="App.xaml" />
    <Page Include="App.xaml" />
</ItemGroup>
```

### Pattern 2: AsyncRelayCommand Canonical Pattern (FeatureViewModelBase)

**What:** Base class for all feature ViewModels demonstrating CancellationTokenSource lifecycle, IsRunning binding, IProgress<OperationProgress> wiring, and graceful OperationCanceledException handling.

**When to use:** Every feature tab ViewModel inherits from this or replicates the pattern.

```csharp
// Source: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/asyncrelaycommand
public abstract class FeatureViewModelBase : ObservableRecipient
{
    private CancellationTokenSource? _cts;

    [ObservableProperty]
    private bool _isRunning;

    [ObservableProperty]
    private string _statusMessage = string.Empty;

    [ObservableProperty]
    private int _progressValue;

    public IAsyncRelayCommand RunCommand { get; }
    public RelayCommand CancelCommand { get; }

    protected FeatureViewModelBase()
    {
        RunCommand = new AsyncRelayCommand(ExecuteAsync, () => !IsRunning);
        CancelCommand = new RelayCommand(() => _cts?.Cancel(), () => IsRunning);
    }

    private async Task ExecuteAsync()
    {
        _cts = new CancellationTokenSource();
        IsRunning = true;
        StatusMessage = string.Empty;
        try
        {
            var progress = new Progress<OperationProgress>(p =>
            {
                ProgressValue = p.Total > 0 ? (int)(100.0 * p.Current / p.Total) : 0;
                StatusMessage = p.Message;
            });
            await RunOperationAsync(_cts.Token, progress);
        }
        catch (OperationCanceledException)
        {
            StatusMessage = "Operation cancelled.";
        }
        catch (Exception ex)
        {
            StatusMessage = $"Error: {ex.Message}";
            // Log via Serilog ILogger injected into derived class
        }
        finally
        {
            IsRunning = false;
            _cts.Dispose();
            _cts = null;
        }
    }

    protected abstract Task RunOperationAsync(CancellationToken ct, IProgress<OperationProgress> progress);
}
```

### Pattern 3: MSAL Per-Tenant Token Cache

**What:** One `IPublicClientApplication` per ClientId, backed by a per-ClientId `MsalCacheHelper` file. `SessionManager` (singleton) holds the dictionary and performs `AcquireTokenSilent` before falling back to `AcquireTokenInteractive`.

**When to use:** Every SharePoint authentication flow.

```csharp
// Source: https://learn.microsoft.com/en-us/entra/msal/dotnet/how-to/token-cache-serialization
public class MsalClientFactory
{
    private readonly Dictionary<string, IPublicClientApplication> _clients = new();
    private readonly SemaphoreSlim _lock = new(1, 1);
    private readonly string _cacheDir = Path.Combine(
        Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
        "SharepointToolbox", "auth");

    public async Task<IPublicClientApplication> GetOrCreateAsync(string clientId)
    {
        await _lock.WaitAsync();
        try
        {
            if (_clients.TryGetValue(clientId, out var existing))
                return existing;

            var storageProps = new StorageCreationPropertiesBuilder(
                    $"msal_{clientId}.cache", _cacheDir)
                .Build();

            var pca = PublicClientApplicationBuilder
                .Create(clientId)
                .WithDefaultRedirectUri()
                .WithLegacyCacheCompatibility(false)
                .Build();

            var helper = await MsalCacheHelper.CreateAsync(storageProps);
            helper.RegisterCache(pca.UserTokenCache);

            _clients[clientId] = pca;
            return pca;
        }
        finally { _lock.Release(); }
    }
}
```

### Pattern 4: Dynamic Localization (TranslationSource + MarkupExtension)

**What:** A singleton `TranslationSource` implements `INotifyPropertyChanged`. XAML binds to it via an indexer `[key]`. When `CurrentCulture` changes, `PropertyChanged` fires for all keys simultaneously, refreshing every bound string in the UI — no restart required.

**When to use:** All localizable strings in XAML.

```csharp
// TranslationSource.cs — singleton, INotifyPropertyChanged
public class TranslationSource : INotifyPropertyChanged
{
    public static readonly TranslationSource Instance = new();
    private ResourceManager _resourceManager = Strings.ResourceManager;
    private CultureInfo _currentCulture = CultureInfo.CurrentUICulture;

    public string this[string key] =>
        _resourceManager.GetString(key, _currentCulture) ?? $"[{key}]";

    public CultureInfo CurrentCulture
    {
        get => _currentCulture;
        set
        {
            if (_currentCulture == value) return;
            _currentCulture = value;
            Thread.CurrentThread.CurrentUICulture = value;
            // Raise PropertyChanged with null/empty = "all properties changed"
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(string.Empty));
        }
    }

    public event PropertyChangedEventHandler? PropertyChanged;
}
```

```xml
<!-- XAML usage — no restart needed -->
<TextBlock Text="{Binding Source={x:Static loc:TranslationSource.Instance},
                           Path=[tab.perms]}" />
```

```csharp
// Language switch handler (in SettingsViewModel)
// Broadcasts so StatusBar and other VMs reset any cached strings
TranslationSource.Instance.CurrentCulture = new CultureInfo("fr");
WeakReferenceMessenger.Default.Send(new LanguageChangedMessage("fr"));
```

### Pattern 5: WeakReferenceMessenger for Tenant Switching

**What:** When the user selects a different tenant, `MainWindowViewModel` sends a `TenantSwitchedMessage`. All feature ViewModels that inherit `ObservableRecipient` register for this message and reset their state.

```csharp
// Message definition (in Core/Messages/)
public sealed class TenantSwitchedMessage : ValueChangedMessage<TenantProfile>
{
    public TenantSwitchedMessage(TenantProfile profile) : base(profile) { }
}

// MainWindowViewModel sends on ComboBox selection change
WeakReferenceMessenger.Default.Send(new TenantSwitchedMessage(selectedProfile));

// FeatureViewModelBase registers in OnActivated (ObservableRecipient lifecycle)
protected override void OnActivated()
{
    Messenger.Register<TenantSwitchedMessage>(this, (r, m) =>
        r.OnTenantSwitched(m.Value));
}
```

### Pattern 6: JSON Write-Then-Replace

**What:** Prevents corrupt files on crash during write. Validate JSON before replacing.

```csharp
// ProfileRepository.cs
private readonly SemaphoreSlim _writeLock = new(1, 1);

public async Task SaveAsync(IReadOnlyList<TenantProfile> profiles)
{
    await _writeLock.WaitAsync();
    try
    {
        var json = JsonSerializer.Serialize(
            new { profiles },
            new JsonSerializerOptions { WriteIndented = true });

        var tmpPath = _filePath + ".tmp";
        await File.WriteAllTextAsync(tmpPath, json, Encoding.UTF8);

        // Validate round-trip before replacing
        JsonDocument.Parse(await File.ReadAllTextAsync(tmpPath)).Dispose();

        File.Move(tmpPath, _filePath, overwrite: true);
    }
    finally { _writeLock.Release(); }
}
```

### Pattern 7: ObservableCollection Threading Rule

**What:** Never modify an `ObservableCollection<T>` from a `Task.Run` background thread. The bound `ItemsControl` will throw or silently malfunction.

```csharp
// In FeatureViewModel — collect on background, assign on UI thread
var results = new List<SiteItem>();
await Task.Run(async () =>
{
    // ... enumerate, add to results ...
}, ct);

// Switch back to UI thread for collection assignment
await Application.Current.Dispatcher.InvokeAsync(() =>
{
    Items = new ObservableCollection<SiteItem>(results);
});
```

### Anti-Patterns to Avoid

- **`async void` event handlers:** Use `AsyncRelayCommand` instead. `async void` swallows exceptions silently and is untestable.
- **Direct `ObservableCollection.Add()` from background thread:** Causes cross-thread `InvalidOperationException`. Always use the dispatcher + `new ObservableCollection<T>(list)` pattern.
- **Single `IPublicClientApplication` for all tenants:** MSAL's token cache is scoped per app instance. Sharing one instance for multiple ClientIds causes tenant bleed. Each ClientId must have its own PCA.
- **Holding `ClientContext` in ViewModels:** `ClientContext` is expensive and not thread-safe. Only `SessionManager` holds it; ViewModels call a service method that takes the URL and returns results.
- **`x:Static` bindings to generated resx class:** `Properties.Strings.SomeKey` is resolved once at startup. It will not update when `CurrentUICulture` changes. Use `TranslationSource` binding instead.
- **`await using` on `ClientContext` without cancellation check:** PnP CSOM operations do not respect `CancellationToken` at the HTTP level in all paths. Check `ct.ThrowIfCancellationRequested()` before each `ExecuteQuery` call.

---

## Don't Hand-Roll

| Problem | Don't Build | Use Instead | Why |
|---------|-------------|-------------|-----|
| Token cache file encryption on Windows | Custom DPAPI wrapper | `MsalCacheHelper` (Extensions.Msal) | Handles DPAPI, Mac keychain, Linux SecretService, and fallback; concurrent access safe |
| Async command with cancellation | Custom `ICommand` implementation | `AsyncRelayCommand` from CommunityToolkit.Mvvm | Handles re-entrancy, `IsRunning`, `CanExecute` propagation, source-generated attributes |
| Cross-VM broadcast | Events on a static class | `WeakReferenceMessenger.Default` | Prevents memory leaks; no strong reference from sender to recipient |
| Retry on SharePoint throttle | Custom retry loop | Wrap PnP Framework's built-in retry in `ExecuteQueryRetryAsync` | PnP already handles 429 backoff; wrapper just exposes events for progress reporting |
| CSOM list pagination | Manual rowlimit + while loop | `SharePointPaginationHelper` (built in Phase 1) | Forgetting `ListItemCollectionPosition` on large lists causes silent data truncation at 5000 items |
| Rolling log file | Custom `ILogger` sink | `Serilog.Sinks.File` with `rollingInterval: RollingInterval.Day` | Note: `Serilog.Sinks.RollingFile` is deprecated — use `Serilog.Sinks.File` |

**Key insight:** The highest-value "don't hand-roll" is `SharePointPaginationHelper`. The existing PowerShell app likely has silent list threshold failures. Building this helper correctly in Phase 1 is what prevents PERM-07 and every other list-enumeration feature from hitting the 5,000-item wall.

---

## Common Pitfalls

### Pitfall 1: WPF STA Thread + Generic Host Conflict

**What goes wrong:** `Host.CreateDefaultBuilder` creates a multi-threaded environment. WPF requires the UI thread to be STA. If `Main` is not explicitly marked `[STAThread]`, or if `Application.Run()` is called from the wrong thread, the application crashes at startup with a threading exception.

**Why it happens:** The default `Program.cs` generated by the WPF template uses `[STAThread]` on `Main` and calls `Application.Run()` directly. When replacing with Generic Host, the entry point changes and the STA attribute must be manually preserved.

**How to avoid:** Mark `static void Main` with `[STAThread]`. Remove `StartupUri` from `App.xaml`. Add `<StartupObject>` to the csproj. Demote `App.xaml` from `ApplicationDefinition` to `Page`.

**Warning signs:** `InvalidOperationException: The calling thread must be STA` at startup.

### Pitfall 2: MSAL Token Cache Sharing Across Tenants

**What goes wrong:** One `IPublicClientApplication` is created and reused for all tenants. Tokens from tenant A contaminate the cache for tenant B, causing silent auth failures or incorrect user context.

**Why it happens:** `IPublicClientApplication` has one `UserTokenCache`. The cache keys internally include the ClientId; if multiple tenants use the same ClientId (which is possible in multi-tenant Azure AD apps), the cache is shared and `AcquireTokenSilent` may return a token for the wrong tenant account.

**How to avoid:** Create one `IPublicClientApplication` per `ClientId`, backed by a cache file named `msal_{clientId}.cache`. If two profiles share a ClientId, they share the PCA (same ClientId = same app registration), but switching requires calling `AcquireTokenSilent` with the correct account from `pca.GetAccountsAsync()`.

**Warning signs:** User is authenticated as wrong tenant after switch; `MsalUiRequiredException` on switch despite being previously logged in.

### Pitfall 3: Dynamic Localization Not Updating All Strings

**What goes wrong:** Language is switched via `CultureInfo`, but 30–40% of strings in the UI still show the old language. Specifically, strings bound via `x:Static` to the generated resource class accessor (e.g., `{x:Static p:Strings.SaveButton}`) are resolved at load time and never re-queried.

**Why it happens:** The WPF design-time resource designer generates static string properties. `x:Static` retrieves the value once. No `INotifyPropertyChanged` mechanism re-fires.

**How to avoid:** Use `TranslationSource.Instance[key]` binding pattern for all strings. Never use `x:Static` on the generated Strings class for UI text. The `TranslationSource.PropertyChanged` with an empty string key triggers WPF to re-evaluate all bindings on the source object simultaneously.

**Warning signs:** Some strings update on language switch, others don't; exactly the strings using `x:Static` are the ones that don't update.

### Pitfall 4: Empty `catch` Swallows SharePoint Exceptions

**What goes wrong:** A `catch (Exception)` block with no body (or only a comment) causes SharePoint operations to silently fail. The user sees a blank result grid with no error message, and the log shows nothing.

**Why it happens:** PnP CSOM throws `ServerException` with SharePoint error codes. Developers add broad `catch` blocks during development to "handle errors later" and forget to complete them.

**How to avoid:** Enforce the project policy from day one: every `catch` block must log-and-recover, log-and-rethrow, or log-and-surface. Code review rejects any empty or comment-only catch. Serilog's structured logging makes logging trivial.

**Warning signs:** Operations complete in ~0ms, return zero results, log shows no entry.

### Pitfall 5: `ClientContext` Not Disposed on Cancellation

**What goes wrong:** `ClientContext` holds an HTTP connection to SharePoint. If cancellation is requested and the `ClientContext` is abandoned rather than disposed, connections accumulate. Long-running sessions leak sockets.

**Why it happens:** The `await using` pattern is dropped when developers switch from the canonical pattern to a try/catch block and forget to add the `finally { ctx.Dispose(); }`.

**How to avoid:** Enforce `await using` in all code touching `ClientContext`. Unit tests verify `Dispose()` is called even when `OperationCanceledException` is thrown (mock `ClientContext` and assert `Dispose` call count).

**Warning signs:** `SocketException` or connection timeout errors appearing after the application has been running for several hours; memory growth over a long session.

### Pitfall 6: `ObservableCollection` Modified from Background Thread

**What goes wrong:** `Add()` or `Clear()` called on an `ObservableCollection` from inside `Task.Run`. WPF's `CollectionView` throws `NotSupportedException: This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread`.

**Why it happens:** Developers call `Items.Add(item)` inside a `for` loop that runs on a background thread, which feels natural but violates WPF's cross-thread collection rule.

**How to avoid:** Accumulate results in a plain `List<T>` on the background thread. When the operation completes (or at batch boundaries), `await Application.Current.Dispatcher.InvokeAsync(() => Items = new ObservableCollection<T>(list))`.

**Warning signs:** `InvalidOperationException` or `NotSupportedException` with "Dispatcher thread" in the message, occurring only when the result set is large enough to trigger background processing.

---

## Code Examples

Verified patterns from official sources:

### MsalCacheHelper Desktop Setup

```csharp
// Source: https://learn.microsoft.com/en-us/entra/msal/dotnet/how-to/token-cache-serialization
var storageProperties = new StorageCreationPropertiesBuilder(
        $"msal_{clientId}.cache",
        Path.Combine(Environment.GetFolderPath(
            Environment.SpecialFolder.ApplicationData), "SharepointToolbox", "auth"))
    .Build();

var pca = PublicClientApplicationBuilder
    .Create(clientId)
    .WithDefaultRedirectUri()
    .WithLegacyCacheCompatibility(false)
    .Build();

var cacheHelper = await MsalCacheHelper.CreateAsync(storageProperties);
cacheHelper.RegisterCache(pca.UserTokenCache);
```

### AcquireTokenSilent with Interactive Fallback

```csharp
// Source: MSAL.NET documentation pattern
public async Task<string> GetAccessTokenAsync(
    IPublicClientApplication pca,
    string[] scopes,
    CancellationToken ct)
{
    var accounts = await pca.GetAccountsAsync();
    AuthenticationResult result;
    try
    {
        result = await pca.AcquireTokenSilent(scopes, accounts.FirstOrDefault())
            .ExecuteAsync(ct);
    }
    catch (MsalUiRequiredException)
    {
        result = await pca.AcquireTokenInteractive(scopes)
            .WithUseEmbeddedWebView(false)
            .ExecuteAsync(ct);
    }
    return result.AccessToken;
}
```

### PnP AuthenticationManager Interactive Login

```csharp
// Source: https://pnp.github.io/pnpframework/api/PnP.Framework.AuthenticationManager.html
var authManager = AuthenticationManager.CreateWithInteractiveLogin(
    clientId: profile.ClientId,
    tenantId: null,              // null = common endpoint (multi-tenant)
    redirectUrl: "http://localhost");

await using var ctx = await authManager.GetContextAsync(profile.TenantUrl, ct);
// ctx is a SharePoint CSOM ClientContext ready for ExecuteQueryAsync
```

### WeakReferenceMessenger Send + Register

```csharp
// Source: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/messenger

// Define message
public sealed class TenantSwitchedMessage : ValueChangedMessage<TenantProfile>
{
    public TenantSwitchedMessage(TenantProfile p) : base(p) { }
}

// Send (in MainWindowViewModel)
WeakReferenceMessenger.Default.Send(new TenantSwitchedMessage(selected));

// Register (in ObservableRecipient-derived ViewModel)
protected override void OnActivated()
{
    Messenger.Register<TenantSwitchedMessage>(this, (r, m) =>
        r.HandleTenantSwitch(m.Value));
}
```

### Serilog Setup with Rolling File

```csharp
// Source: https://github.com/serilog/serilog-sinks-file
// NOTE: Use Serilog.Sinks.File — Serilog.Sinks.RollingFile is DEPRECATED
Log.Logger = new LoggerConfiguration()
    .MinimumLevel.Debug()
    .WriteTo.File(
        path: Path.Combine(
            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
            "SharepointToolbox", "logs", "app-.log"),
        rollingInterval: RollingInterval.Day,
        retainedFileCountLimit: 30,
        outputTemplate: "{Timestamp:HH:mm:ss} [{Level:u3}] {Message:lj}{NewLine}{Exception}")
    .CreateLogger();
```

### Global Exception Handlers in App.xaml.cs

```csharp
// App.xaml.cs — wire in Application constructor or OnStartup
Application.Current.DispatcherUnhandledException += (_, e) =>
{
    Log.Fatal(e.Exception, "Unhandled dispatcher exception");
    MessageBox.Show(
        $"An unexpected error occurred:\n\n{e.Exception.Message}\n\n" +
        "Check the log file for details.",
        "Unexpected Error",
        MessageBoxButton.OK, MessageBoxImage.Error);
    e.Handled = true; // prevent crash; or set false to let it crash
};

TaskScheduler.UnobservedTaskException += (_, e) =>
{
    Log.Error(e.Exception, "Unobserved task exception");
    e.SetObserved(); // prevent process termination
};
```

---

## State of the Art

| Old Approach | Current Approach | When Changed | Impact |
|--------------|------------------|--------------|--------|
| Serilog.Sinks.RollingFile NuGet | Serilog.Sinks.File with `rollingInterval` param | ~2018 | Rolling file is deprecated; same behavior, different package |
| MSAL v2 `TokenCacheCallback` | `MsalCacheHelper.RegisterCache()` | MSAL 4.x | Much simpler; handles encryption and cross-platform automatically |
| ADAL.NET | MSAL.NET | 2020+ | ADAL is end-of-life; all new auth must use MSAL |
| `async void` event handlers | `AsyncRelayCommand` | CommunityToolkit.Mvvm era | `async void` is an anti-pattern; toolkit makes the right thing easy |
| `x:Static` on resx | `TranslationSource` binding | No standard date | Required for runtime culture switch without restart |
| WPF app without DI | Generic Host + WPF | .NET Core 3+ | Enables testability, Serilog wiring, and lifetime management |

**Deprecated/outdated:**

- `Serilog.Sinks.RollingFile`: Deprecated; replaced by `Serilog.Sinks.File`. Do not add this package.
- `Microsoft.Toolkit.Mvvm` (old namespace): Superseded by `CommunityToolkit.Mvvm`. Same toolkit, new package ID.
- `ADAL.NET` (Microsoft.IdentityModel.Clients.ActiveDirectory): End-of-life. Use MSAL only.
- `MvvmLight` (GalaSoft): Unmaintained. CommunityToolkit.Mvvm is the successor.

---

## Open Questions

1. **PnP AuthenticationManager vs raw MSAL for token acquisition**
   - What we know: `PnP.Framework.AuthenticationManager.CreateWithInteractiveLogin` wraps MSAL internally and produces a `ClientContext`. There is also a constructor accepting an external `IAuthenticationProvider`.
   - What's unclear: Whether passing an externally-managed `IPublicClientApplication` (from `MsalClientFactory`) into `AuthenticationManager` is officially supported in PnP.Framework 1.18, or if we must create a new PCA inside `AuthenticationManager` and bypass `MsalClientFactory`.
   - Recommendation: In Wave 1, spike with `CreateWithInteractiveLogin(clientId, ...)` — accept that PnP creates its own internal PCA. If we need to share the token cache with a separately-created PCA, use the `IAuthenticationProvider` constructor overload.

2. **WAM Broker behavior on Windows 10 LTSC**
   - What we know: `Microsoft.Identity.Client.Broker` enables WAM on Windows 11. The locked runtime decision includes it.
   - What's unclear: Behavior on the user's Windows 10 IoT LTSC environment. WAM may not be available or may fall back silently.
   - Recommendation: Configure MSAL with `.WithDefaultRedirectUri()` as fallback and do not hard-require WAM. Test on Windows 10 LTSC before shipping.

---

## Validation Architecture

### Test Framework

| Property | Value |
|----------|-------|
| Framework | xUnit 2.x |
| Config file | none — see Wave 0 |
| Quick run command | `dotnet test --filter "Category=Unit" --no-build` |
| Full suite command | `dotnet test --no-build` |

### Phase Requirements → Test Map

| Req ID | Behavior | Test Type | Automated Command | File Exists? |
|--------|----------|-----------|-------------------|-------------|
| FOUND-01 | App starts, MainWindow resolves from DI | smoke | `dotnet test --filter "FullyQualifiedName~AppStartupTests" -x` | ❌ Wave 0 |
| FOUND-02 | ProfileService: create/rename/delete/load profiles; JSON written correctly | unit | `dotnet test --filter "FullyQualifiedName~ProfileServiceTests" -x` | ❌ Wave 0 |
| FOUND-03 | MsalClientFactory: unique PCA per ClientId; same ClientId returns cached instance | unit | `dotnet test --filter "FullyQualifiedName~MsalClientFactoryTests" -x` | ❌ Wave 0 |
| FOUND-04 | SessionManager: AcquireTokenSilent called before Interactive; MsalUiRequiredException triggers interactive | unit (mock MSAL) | `dotnet test --filter "FullyQualifiedName~SessionManagerTests" -x` | ❌ Wave 0 |
| FOUND-05 | FeatureViewModelBase: IProgress<OperationProgress> updates ProgressValue and StatusMessage on UI thread | unit | `dotnet test --filter "FullyQualifiedName~FeatureViewModelBaseTests" -x` | ❌ Wave 0 |
| FOUND-06 | FeatureViewModelBase: CancelCommand calls CTS.Cancel(); operation stops; IsRunning resets to false | unit | `dotnet test --filter "FullyQualifiedName~FeatureViewModelBaseTests" -x` | ❌ Wave 0 |
| FOUND-07 | Global exception handlers log and surface (verify log written + MessageBox shown) | integration | manual-only (UI dialog) | — |
| FOUND-08 | Serilog writes to rolling file in %AppData%\SharepointToolbox\logs\ | integration | `dotnet test --filter "FullyQualifiedName~LoggingIntegrationTests" -x` | ❌ Wave 0 |
| FOUND-09 | TranslationSource: switching CurrentCulture fires PropertyChanged with empty key; string lookup uses new culture | unit | `dotnet test --filter "FullyQualifiedName~TranslationSourceTests" -x` | ❌ Wave 0 |
| FOUND-10 | ProfileRepository: write-then-replace atomicity; SemaphoreSlim prevents concurrent writes; corrupt JSON on tmp does not replace original | unit | `dotnet test --filter "FullyQualifiedName~ProfileRepositoryTests" -x` | ❌ Wave 0 |
| FOUND-12 | SettingsService: reads/writes Sharepoint_Settings.json; dataFolder field round-trips correctly | unit | `dotnet test --filter "FullyQualifiedName~SettingsServiceTests" -x` | ❌ Wave 0 |

### Sampling Rate

- **Per task commit:** `dotnet test --filter "Category=Unit" --no-build`
- **Per wave merge:** `dotnet test --no-build`
- **Phase gate:** Full suite green before `/gsd:verify-work`

### Wave 0 Gaps

- [ ] `SharepointToolbox.Tests/SharepointToolbox.Tests.csproj` — xUnit test project, add packages: xunit, xunit.runner.visualstudio, Moq (or NSubstitute), Microsoft.NET.Test.Sdk
- [ ] `SharepointToolbox.Tests/Services/ProfileServiceTests.cs` — covers FOUND-02, FOUND-10
- [ ] `SharepointToolbox.Tests/Services/SettingsServiceTests.cs` — covers FOUND-12
- [ ] `SharepointToolbox.Tests/Auth/MsalClientFactoryTests.cs` — covers FOUND-03
- [ ] `SharepointToolbox.Tests/Auth/SessionManagerTests.cs` — covers FOUND-04
- [ ] `SharepointToolbox.Tests/ViewModels/FeatureViewModelBaseTests.cs` — covers FOUND-05, FOUND-06
- [ ] `SharepointToolbox.Tests/Localization/TranslationSourceTests.cs` — covers FOUND-09
- [ ] `SharepointToolbox.Tests/Integration/LoggingIntegrationTests.cs` — covers FOUND-08

---

## Sources

### Primary (HIGH confidence)

- [AsyncRelayCommand — Microsoft Learn (CommunityToolkit)](https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/asyncrelaycommand) — AsyncRelayCommand API, IsRunning, CancellationToken, IProgress patterns
- [Messenger — Microsoft Learn (CommunityToolkit)](https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/messenger) — WeakReferenceMessenger, Send/Register patterns, ValueChangedMessage
- [Token Cache Serialization — Microsoft Learn (MSAL.NET)](https://learn.microsoft.com/en-us/entra/msal/dotnet/how-to/token-cache-serialization) — MsalCacheHelper desktop setup, StorageCreationPropertiesBuilder, per-user cache
- [PnP Framework AuthenticationManager API](https://pnp.github.io/pnpframework/api/PnP.Framework.AuthenticationManager.html) — CreateWithInteractiveLogin overloads, GetContextAsync
- [Serilog.Sinks.File GitHub](https://github.com/serilog/serilog-sinks-file) — modern rolling file sink (RollingFile deprecated)
- Existing project files: `Sharepoint_Settings.json`, `lang/fr.json`, `Sharepoint_ToolBox.ps1:1-152` — exact JSON schemas and localization keys confirmed

### Secondary (MEDIUM confidence)

- [Adding Host to WPF for DI — FormatException (2024)](https://formatexception.com/2024/02/adding-host-to-wpf-for-dependency-injection/) — Generic Host + WPF wiring pattern (verified against Generic Host official docs)
- [Custom Resource MarkupExtension — Microsoft DevBlogs](https://devblogs.microsoft.com/ifdef-windows/use-a-custom-resource-markup-extension-to-succeed-at-ui-string-globalization/) — MarkupExtension for resx (verified pattern approach)
- [NuGet: CommunityToolkit.Mvvm 8.4.2](https://www.nuget.org/packages/CommunityToolkit.Mvvm/) — version confirmed

### Tertiary (LOW confidence)

- Multiple WebSearch results on WPF localization patterns (2012–2020 vintage, not 2025-specific). The `TranslationSource` singleton pattern is consistent across sources but no single authoritative 2025 doc was found. Implementation is straightforward enough to treat as MEDIUM.

---

## Metadata

**Confidence breakdown:**

- Standard stack: HIGH — all packages are official, versions verified on NuGet, no version conflicts identified
- Architecture: HIGH — Generic Host + WPF pattern is well-documented for .NET Core+; MSAL per-tenant pattern verified against official MSAL docs
- Pitfalls: HIGH — pitfalls 1–4 are documented in official sources; pitfalls 5–6 are well-known WPF threading behaviors with extensive community documentation
- Localization (TranslationSource): MEDIUM — the `INotifyPropertyChanged` singleton approach is the standard community pattern for dynamic resx switching; no single authoritative Microsoft doc covers it end-to-end
- PnP Framework auth integration: MEDIUM — `AuthenticationManager.CreateWithInteractiveLogin` API is documented; exact behavior when combining with external `MsalClientFactory` needs a validation spike

**Research date:** 2026-04-02
**Valid until:** 2026-05-02 (30 days — stable libraries, conservative estimate)