test: DialogService and FileSystemService baseline coverage

docs/knowledgebase/gui-architecture-deferred.md

@@ -62,6 +62,7 @@ Plans: `docs/plans/2026-06-03-012`, `021`–`029`, `033`–`057`. Surface refere
 | Directory picker init/sync → `SettingsService` | Done | plan `075`, PR #123 |
 | Mod context menu + global flyout → `MenuBuilderService` | Done | plan `072`, PR #130 |
 
+**Headless tests:** `dotnet test src/ModSync.Tests/ModSync.Tests.csproj --filter SettingsService` (plan `077`); `--filter MenuBuilderService` (plan `072`); `--filter DialogServiceTests` / `FileSystemServiceTests` (plan `113`).
 **Headless tests:** `dotnet test src/ModSync.Tests/ModSync.Tests.csproj --filter SettingsService` (plan `077`); `--filter MenuBuilderService` (plan `072`); `--filter FilterItemModel` (plan `112`).
 **Headless tests:** `dotnet test src/ModSync.Tests/ModSync.Tests.csproj --filter SettingsService` (plan `077`); `--filter MenuBuilderService` (plan `072`); `--filter FileTreeNode` (plan `111`).
 **Headless tests:** `dotnet test src/ModSync.Tests/ModSync.Tests.csproj --filter SettingsService` (plan `077`); `--filter MenuBuilderService` (plan `072`); `--filter ArchiveEnumerationService` (plan `110`).

docs/plans/2026-06-04-113-test-dialog-filesystem-service-plan.md

@@ -0,0 +1,19 @@
+---
+title: "test: DialogService and FileSystemService basics"
+status: shipped
+pr: pending
+---
+
+# test: DialogService and FileSystemService basics
+
+## Problem
+
+`DialogService` path-folder helper and `FileSystemService` watcher/dispose lifecycle had no dedicated tests.
+
+## Solution
+
+Add `DialogServiceTests` and `FileSystemServiceTests` for constructor validation, empty-path handling, and safe dispose/stop paths.
+
+## Verification
+
+`dotnet test src/ModSync.Tests/ModSync.Tests.csproj --filter "FullyQualifiedName~DialogServiceTests|FullyQualifiedName~FileSystemServiceTests"

src/ModSync.Tests/DialogServiceTests.cs

@@ -0,0 +1,32 @@
+// Copyright 2021-2025 ModSync
+// Licensed under the Business Source License 1.1 (BSL 1.1).
+// See LICENSE.txt file in the project root for full license information.
+
+using System;
+using System.Threading.Tasks;
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using ModSync.Services;
+using Xunit;
+
+namespace ModSync.Tests
+{
+    [Collection(HeadlessTestApp.CollectionName)]
+    public sealed class DialogServiceTests
+    {
+        [Fact(DisplayName = "Constructor rejects null parent window")]
+        public void Constructor_NullParentWindow_Throws()
+        {
+            Assert.Throws<ArgumentNullException>(() => new DialogService(null));
+        }
+
+        [AvaloniaFact(DisplayName = "GetStorageFolderFromPathAsync returns null for empty path")]
+        public async Task GetStorageFolderFromPathAsync_EmptyPath_ReturnsNull()
+        {
+            var service = new DialogService(new Window());
+
+            Assert.Null(await service.GetStorageFolderFromPathAsync(string.Empty));
+            Assert.Null(await service.GetStorageFolderFromPathAsync(null));
+        }
+    }
+}

src/ModSync.Tests/FileSystemServiceTests.cs

@@ -0,0 +1,33 @@
+// Copyright 2021-2025 ModSync
+// Licensed under the Business Source License 1.1 (BSL 1.1).
+// See LICENSE.txt file in the project root for full license information.
+
+using ModSync.Services;
+using Xunit;
+
+namespace ModSync.Tests
+{
+    public sealed class FileSystemServiceTests
+    {
+        [Fact(DisplayName = "SetupModDirectoryWatcher does not throw when watcher disabled")]
+        public void SetupModDirectoryWatcher_Disabled_DoesNotThrow()
+        {
+            FileSystemService.SetupModDirectoryWatcher("/tmp", _ => { });
+        }
+
+        [Fact(DisplayName = "StopWatcher does not throw on fresh service")]
+        public void StopWatcher_FreshService_DoesNotThrow()
+        {
+            using var service = new FileSystemService();
+            service.StopWatcher();
+        }
+
+        [Fact(DisplayName = "Dispose can be called multiple times")]
+        public void Dispose_MultipleCalls_DoesNotThrow()
+        {
+            var service = new FileSystemService();
+            service.Dispose();
+            service.Dispose();
+        }
+    }
+}