Add prerelease property to inheriting version.json files

docfx/docs/versionJson.md

@@ -65,7 +65,8 @@ The content of the version.json file is a JSON serialized object with these prop
     "versionIncrement" : "minor",
     "firstUnstableTag" : "alpha"
   },
-  "inherit": false // optional. Set to true in secondary version.json files used to tweak settings for subsets of projects.
+  "inherit": false, // optional. Set to true in secondary version.json files used to tweak settings for subsets of projects.
+  "prerelease": "beta" // optional. Only valid when inherit is true. Adds or overrides the prerelease tag of the inherited version.
 }
 ```
 
@@ -109,4 +110,80 @@ In this example, the offset of 100 will be applied as long as the version remain
 > [!NOTE]
 > This feature is particularly useful when a `version.json` file uses `"inherit": true` to get the version from a parent `version.json` file higher in the source tree. In such cases, you can set `versionHeightOffset` and `versionHeightOffsetAppliesTo` in the inheriting file without having to update it when the parent version changes. The offset will automatically stop applying when the inherited version no longer matches `versionHeightOffsetAppliesTo`.
 
+## Prerelease Tag in Inheriting Files
+
+The `prerelease` property allows a version.json file that inherits from a parent to add or override the prerelease tag without duplicating the version number. This is particularly useful when you want to publish an "unstable" prerelease from a stable branch, or when one package is still considered unstable while its peers are already stable.
+
+### Usage Rules
+
+- The `prerelease` property can **only** be used when `"inherit": true` is set.
+- The `version` property in the inheriting file must **not** include a prerelease tag (the `-suffix` part).
+- Setting `"prerelease": "beta"` will append `-beta` to the inherited version.
+- Setting `"prerelease": ""` (empty string) will explicitly **suppress** any prerelease tag inherited from the parent.
+- Omitting the `prerelease` property will inherit the prerelease tag as-is from the parent.
+
+### Examples
+
+**Example 1: Adding a prerelease tag to a stable inherited version**
+
+Parent `version.json` at repository root:
+```json
+{
+  "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/main/src/NerdBank.GitVersioning/version.schema.json",
+  "version": "1.2"
+}
+```
+
+Child `version.json` in a subdirectory (e.g., `src/ExperimentalPackage/version.json`):
+```json
+{
+  "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/main/src/NerdBank.GitVersioning/version.schema.json",
+  "inherit": true,
+  "prerelease": "beta"
+}
+```
+
+Result: The subdirectory will use version `1.2-beta` while the rest of the repository uses `1.2`.
+
+**Example 2: Suppressing an inherited prerelease tag**
+
+Parent `version.json` at repository root:
+```json
+{
+  "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/main/src/NerdBank.GitVersioning/version.schema.json",
+  "version": "1.2-alpha"
+}
+```
+
+Child `version.json` in a subdirectory (e.g., `src/StablePackage/version.json`):
+```json
+{
+  "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/main/src/NerdBank.GitVersioning/version.schema.json",
+  "inherit": true,
+  "prerelease": ""
+}
+```
+
+Result: The subdirectory will use version `1.2` (stable) while the rest of the repository uses `1.2-alpha`.
+
+**Example 3: Inheriting the prerelease tag as-is**
+
+Parent `version.json` at repository root:
+```json
+{
+  "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/main/src/NerdBank.GitVersioning/version.schema.json",
+  "version": "1.2-rc"
+}
+```
+
+Child `version.json` in a subdirectory:
+```json
+{
+  "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/main/src/NerdBank.GitVersioning/version.schema.json",
+  "inherit": true
+}
+```
+
+Result: The subdirectory will inherit and use version `1.2-rc` from the parent.
+
 [Learn more about pathFilters](path-filters.md).

src/NerdBank.GitVersioning/LibGit2/LibGit2VersionFile.cs

@@ -126,6 +126,7 @@ internal LibGit2VersionFile(LibGit2Context context)
                             }
 
                             JsonConvert.PopulateObject(versionJsonContent, result, VersionOptions.GetJsonSettings(repoRelativeBaseDirectory: searchDirectory));
+                            ApplyPrereleaseProperty(result);
                             result.Inherit = false;
                         }
                     }

src/NerdBank.GitVersioning/Managed/ManagedVersionFile.cs

@@ -141,6 +141,7 @@ public ManagedVersionFile(GitContext context)
                             }
 
                             JsonConvert.PopulateObject(versionJsonContent, result, VersionOptions.GetJsonSettings(repoRelativeBaseDirectory: searchDirectory));
+                            ApplyPrereleaseProperty(result);
                             result.Inherit = false;
                         }
                         else

src/NerdBank.GitVersioning/VersionFile.cs

@@ -197,6 +197,70 @@ protected static bool VersionOptionsSatisfyRequirements(VersionOptions? options,
         return true;
     }
 
+    /// <summary>
+    /// Applies the standalone <see cref="VersionOptions.Prerelease"/> property to the <see cref="VersionOptions.Version"/> property.
+    /// </summary>
+    /// <param name="options">The version options to modify.</param>
+    /// <remarks>
+    /// This method should be called after merging a child version.json with a parent version.json.
+    /// If the child specified a <see cref="VersionOptions.Prerelease"/> property, this method will apply it to the version.
+    /// </remarks>
+    protected static void ApplyPrereleaseProperty(VersionOptions options)
+    {
+        Requires.NotNull(options, nameof(options));
+
+        // Only apply if the Prerelease property was explicitly set
+        if (options.Prerelease is null)
+        {
+            return;
+        }
+
+        // The version must exist to apply a prerelease tag
+        if (options.Version is null)
+        {
+            throw new InvalidOperationException("The 'prerelease' property cannot be used without a 'version' property.");
+        }
+
+        // If prerelease is an empty string, it suppresses any inherited prerelease tag
+        if (options.Prerelease == string.Empty)
+        {
+            // Remove any existing prerelease tag
+            if (!string.IsNullOrEmpty(options.Version.Prerelease))
+            {
+                options.Version = new SemanticVersion(
+                    options.Version.Version,
+                    null,
+                    options.Version.BuildMetadata);
+            }
+
+            options.Prerelease = null;
+            return;
+        }
+
+        // Validate that the version doesn't already have a prerelease tag (non-empty prerelease being applied)
+        if (!string.IsNullOrEmpty(options.Version.Prerelease))
+        {
+            throw new InvalidOperationException("The 'prerelease' property cannot be used when the 'version' property already includes a prerelease tag.");
+        }
+
+        // Apply the prerelease tag to the version
+        string prereleaseTag = options.Prerelease;
+        if (!prereleaseTag.StartsWith("-", StringComparison.Ordinal))
+        {
+            // Add the hyphen prefix if not present
+            prereleaseTag = "-" + prereleaseTag;
+        }
+
+        // Create a new SemanticVersion with the prerelease tag applied
+        options.Version = new SemanticVersion(
+            options.Version.Version,
+            prereleaseTag,
+            options.Version.BuildMetadata);
+
+        // Clear the Prerelease property since it has been applied
+        options.Prerelease = null;
+    }
+
     protected static string TrimTrailingPathSeparator(string path)
         => path.Length > 0 && (path[^1] == Path.DirectorySeparatorChar || path[^1] == Path.AltDirectorySeparatorChar) ? path[..^1] : path;
 
@@ -319,6 +383,7 @@ protected void ApplyLocations(VersionOptions? options, string currentLocation, r
                                 versionJsonContent,
                                 result,
                                 VersionOptions.GetJsonSettings(repoRelativeBaseDirectory: repoRelativeBaseDirectory));
+                            ApplyPrereleaseProperty(result);
                             result.Inherit = false;
                         }
                     }

src/NerdBank.GitVersioning/VersionOptions.cs

@@ -133,6 +133,12 @@ public class VersionOptions : IEquatable<VersionOptions>
     [DebuggerBrowsable(DebuggerBrowsableState.Never)]
     private bool inherit;
 
+    /// <summary>
+    /// Backing field for the <see cref="Prerelease"/> property.
+    /// </summary>
+    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
+    private string? prerelease;
+
     /// <summary>
     /// Initializes a new instance of the <see cref="VersionOptions"/> class.
     /// </summary>
@@ -161,6 +167,7 @@ public VersionOptions(VersionOptions copyFrom)
         this.cloudBuild = copyFrom.cloudBuild is object ? new CloudBuildOptions(copyFrom.cloudBuild) : null;
         this.release = copyFrom.release is object ? new ReleaseOptions(copyFrom.release) : null;
         this.pathFilters = copyFrom.pathFilters?.ToList();
+        this.prerelease = copyFrom.prerelease;
     }
 
     /// <summary>
@@ -556,6 +563,27 @@ public bool Inherit
         set => this.SetIfNotReadOnly(ref this.inherit, value);
     }
 
+    /// <summary>
+    /// Gets or sets a prerelease tag to append to an inherited version.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This property is only valid when <see cref="Inherit"/> is <see langword="true"/> and the <see cref="Version"/> property
+    /// does not already include a prerelease tag. When set, this prerelease tag will be appended to the version number
+    /// inherited from the parent version.json file.
+    /// </para>
+    /// <para>
+    /// Setting this to an empty string explicitly suppresses any prerelease tag that might be inherited.
+    /// Omitting this property (leaving it as <see langword="null"/>) means the prerelease tag will be inherited as-is from the parent.
+    /// </para>
+    /// </remarks>
+    [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]
+    public string? Prerelease
+    {
+        get => this.prerelease;
+        set => this.SetIfNotReadOnly(ref this.prerelease, value);
+    }
+
     /// <summary>
     /// Gets a value indicating whether this instance rejects all attempts to mutate it.
     /// </summary>

src/NerdBank.GitVersioning/version.schema.json

@@ -16,6 +16,7 @@
     "inheritingFile": {
       "allOf": [
         { "$ref": "#/definitions/allProperties" },
+        { "$ref": "#/definitions/inheritingProperties" },
         { "required": [ "inherit" ] },
         {
           "properties": {
@@ -217,6 +218,15 @@
         }
       }
     },
+    "inheritingProperties": {
+      "properties": {
+        "prerelease": {
+          "type": "string",
+          "description": "A prerelease tag to append to the version inherited from a parent version.json file. This property can only be used when 'inherit' is true and the 'version' property does not include a prerelease tag. Set to an empty string to explicitly suppress an inherited prerelease tag. Omit this property to inherit the prerelease tag as-is from the parent.",
+          "pattern": "^[\\da-zA-Z\\-]*$"
+        }
+      }
+    },
     "twoToFourComponentVersion": {
       "type": "string",
       "description": "A major.minor[.build[.revision]] version (2-4 version components).",