feat: Implement nbgv path-filters command for monorepo pathFilters automation

docfx/docs/nbgv-cli.md

@@ -250,4 +250,6 @@ usage: nbgv <command> [<args>]
     prepare-release  Prepares a release by creating a release branch for
                      the current version and adjusting the version on the
                      current branch.
+    path-filters     Manages the pathFilters property in version.json files
+                     based on MSBuild project references and imports.
 ```

docfx/docs/path-filters.md

@@ -57,3 +57,138 @@ Multiple path filters may also be specified. The order is irrelevant. After a pa
 | `/root-file.txt`<br>`:/dir/file.txt`                                               | File will be included. Path is absolute (i.e., relative to the root of the repository).                    |
 | `:!bar.txt`<br>`:^../foo/baz.txt`                                                  | File will be excluded. Path is relative to the `version.json` file. `:!` and `:^` prefixes are synonymous. |
 | `:!/root-file.txt`                                                                 | File will be excluded. Path is absolute (i.e., relative to the root of the repository).                    |
+
+## Managing path filters with `nbgv path-filters`
+
+For repositories with multiple projects and version.json files, manually maintaining `pathFilters` can be error-prone. The `nbgv path-filters` command automates this process by analyzing your MSBuild project structure and computing the correct path filters based on project references and shared build files.
+
+### When to use path-filters command
+
+Use the `nbgv path-filters` command in the following scenarios:
+
+- **Monorepo with multiple projects** - You have multiple projects in different directories, each with their own `version.json`
+- **Complex project dependencies** - Projects reference each other, and you need path filters to reflect these dependencies
+- **Shared build files** - You use `Directory.Build.props` or other shared MSBuild imports that should be tracked by multiple projects
+- **Maintaining accuracy** - You want to ensure path filters automatically stay in sync with your project structure
+
+### How it works
+
+The `nbgv path-filters` command uses the MSBuild project graph API to:
+
+1. **Discover project files** - Finds all MSBuild project files (`.csproj`, `.vbproj`, etc.) associated with each `version.json`
+2. **Compute transitive dependencies** - Uses the MSBuild project graph to determine the complete set of projects that each project depends on
+3. **Include shared build files** - Identifies MSBuild imports like `Directory.Build.props` that reside within the repository
+4. **Respect boundaries** - Stops searching for projects at directories containing their own `version.json` files, ensuring clean separation of concerns
+5. **Generate path filters** - Converts the discovered projects and files into appropriate `pathFilters` entries
+
+### Important behaviors
+
+- **Only processes version.json files with projects** - A `version.json` file with no associated MSBuild projects is skipped and left unchanged
+- **Respects version.json hierarchy** - When searching for projects under a `version.json`, the search stops at subdirectories that have their own `version.json` files
+- **Includes project directories** - Path filters include entire project directories (e.g., `/ProjectA`) rather than individual `.csproj` files so that all source changes under those directories result in a new version of the project
+- **Filters ignored files** - Automatically excludes files that match `.gitignore` patterns (including generated directories like `obj/` and `bin/`)
+
+### Usage
+
+#### Check current path filters
+
+To see what path filters should be present without making changes:
+
+```ps1
+nbgv path-filters check
+```
+
+This command will:
+- Compare the computed path filters against what's currently in each `version.json`
+- Display mismatches (missing or extra filters)
+- Exit with non-zero code if any mismatches are found (useful for CI validation)
+
+#### Update path filters
+
+To automatically compute and update all `version.json` files:
+
+```ps1
+nbgv path-filters update
+```
+
+This command will:
+- Compute the correct path filters for each `version.json`
+- Update each file that needs changes
+- Display which files were updated
+- Skip any `version.json` files that have no associated projects
+
+#### Specify which version.json files to process
+
+By default, both commands search from the current directory. You can specify specific paths:
+
+```ps1
+nbgv path-filters check ./src/ProjectA ./src/ProjectB
+```
+
+#### Include additional project file extensions
+
+By default, the tool searches for `.csproj` and `.vbproj` files. You can include other extensions:
+
+```ps1
+nbgv path-filters update --ext .fsproj --ext .csproj
+```
+
+### Example
+
+Consider a monorepo with this structure:
+
+```
+/
+  version.json (version: "1.0")
+  Directory.Build.props
+  /ProjectA
+    version.json (version: "2.0")
+    ProjectA.csproj
+  /ProjectB
+    version.json (version: "3.0")
+    ProjectB.csproj
+    (ProjectB.csproj references ProjectA.csproj)
+```
+
+Running `nbgv path-filters update` would produce:
+
+**Root version.json** - Left unchanged (has no projects directly under it)
+
+**ProjectA/version.json**:
+```json
+{
+  "version": "2.0",
+  "pathFilters": [
+    "/ProjectA",
+    "/Directory.Build.props"
+  ]
+}
+```
+
+**ProjectB/version.json**:
+```json
+{
+  "version": "3.0",
+  "pathFilters": [
+    "/ProjectA",
+    "/ProjectB",
+    "/Directory.Build.props"
+  ]
+}
+```
+
+Note that ProjectB's filters include ProjectA because ProjectB depends on it. Any change to ProjectA's source files will now correctly trigger a version bump for ProjectB as well.
+
+### CI Integration
+
+You can use the `path-filters check` command in your CI pipeline to validate that `pathFilters` are correctly maintained:
+
+```ps1
+nbgv path-filters check
+if ($LASTEXITCODE -ne 0) {
+  Write-Error "Path filters are out of date. Run 'nbgv path-filters update' locally."
+  exit 1
+}
+```
+
+This ensures that developers keep path filters in sync with project structure changes.

src/NerdBank.GitVersioning/GitContext.cs

@@ -228,6 +228,13 @@ public static GitContext Create(string path, string? committish = null, Engine e
         }
     }
 
+    /// <summary>
+    /// Determines whether a file would be ignored by git based on common .gitignore patterns.
+    /// </summary>
+    /// <param name="path">The absolute file path to check.</param>
+    /// <returns>True if the file is ignored by git; false otherwise.</returns>
+    public virtual bool IsIgnored(string path) => false;
+
     /// <inheritdoc />
     public void Dispose()
     {
@@ -294,7 +301,7 @@ internal static bool TryFindGitPaths(string? path, [NotNullWhen(true)] out strin
 
     internal abstract Version GetIdAsVersion(VersionOptions? committedVersion, VersionOptions? workingVersion, int versionHeight);
 
-    internal string GetRepoRelativePath(string absolutePath)
+    internal string GetRepoRelativePath(string absolutePath, bool replaceBackslashes = false)
     {
         string? repoRoot = this.WorkingTreePath.TrimEnd(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
 
@@ -303,8 +310,9 @@ internal string GetRepoRelativePath(string absolutePath)
             throw new ArgumentException($"Path '{absolutePath}' is not within repository '{repoRoot}'", nameof(absolutePath));
         }
 
-        return absolutePath.Substring(repoRoot.Length)
+        string result = absolutePath.Substring(repoRoot.Length)
             .TrimStart(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
+        return replaceBackslashes ? result.Replace('\\', '/') : result;
     }
 
     /// <summary>

src/NerdBank.GitVersioning/LibGit2/LibGit2Context.cs

@@ -88,6 +88,9 @@ public static LibGit2Context Create(string path, string? committish = null)
         };
     }
 
+    /// <inheritdoc />
+    public override bool IsIgnored(string path) => this.Repository.Ignore.IsPathIgnored(this.GetRepoRelativePath(path, replaceBackslashes: true));
+
     /// <inheritdoc />
     public override void ApplyTag(string name) => this.Repository.Tags.Add(name, this.Commit);