Support C# format strings in config

docs/input/docs/reference/configuration.md

@@ -477,6 +477,9 @@ while still updating the `AssemblyFileVersion` and `AssemblyInformationVersion`
 attributes. Valid values: `MajorMinorPatchTag`, `MajorMinorPatch`, `MajorMinor`,
 `Major`, `None`.
 
+For information on using format strings in these properties, see
+[Format Strings](/docs/reference/custom-formatting).
+
 ### assembly-file-versioning-scheme
 
 When updating assembly info, `assembly-file-versioning-scheme` tells GitVersion
@@ -632,7 +635,7 @@ ignore:
     - ^docs\/
 ```
 ##### *Monorepo*
-This ignore config can be used to filter only those commits that belong to a specific project in a monorepo. 
+This ignore config can be used to filter only those commits that belong to a specific project in a monorepo.
 As an example, consider a monorepo consisting of subdirectories for `ProjectA`, `ProjectB` and a shared `LibraryC`. For GitVersion to consider only commits that are part of `projectA` and shared library `LibraryC`, a regex that matches all paths except those starting with `ProjectA` or `LibraryC` can be used. Either one of the following configs would filter out `ProjectB`.
 * Specific match on `/ProjectB/*`:
 ```yaml
@@ -655,7 +658,7 @@ A commit having changes only in `/ProjectB/*` path would be ignored. A commit ha
 * `/ProductA/*` and `/ProductB/*` and `/LibraryC/*`
 
 :::
-Note: The `ignore.paths` configuration is case-sensitive. This can lead to unexpected behavior on case-insensitive file systems, such as Windows. To ensure consistent matching regardless of case, you can prefix your regular expressions with the case-insensitive flag `(?i)`. For example, `(?i)^docs\/` will match both `docs/` and `Docs/`. 
+Note: The `ignore.paths` configuration is case-sensitive. This can lead to unexpected behavior on case-insensitive file systems, such as Windows. To ensure consistent matching regardless of case, you can prefix your regular expressions with the case-insensitive flag `(?i)`. For example, `(?i)^docs\/` will match both `docs/` and `Docs/`.
 :::
 
 ::: {.alert .alert-warning}

docs/input/docs/reference/custom-formatting.md

@@ -0,0 +1,252 @@
+---
+title: Format Strings
+description: Using C# format strings in GitVersion configuration
+---
+
+GitVersion supports C# format strings in configuration, allowing you to apply standard .NET formatting and custom transformations to version properties. This enhancement provides more flexibility and control over how version information is displayed and used throughout your build process.
+
+## Overview
+
+The custom formatter functionality introduces several new formatters that can be used in GitVersion configuration files and templates:
+
+- **FormattableFormatter**: Supports standard .NET format strings for numeric values, dates, and implements `IFormattable`
+- **NumericFormatter**: Handles numeric formatting with culture-aware output
+- **DateTimeFormatter**: Provides date and time formatting with standard and custom format specifiers
+- **String Case Formatters**: Provides text case transformations with custom format specifiers
+
+## Standard .NET Format Strings
+
+### Numeric Formatting
+
+You can now use standard .NET numeric format strings with version components:
+
+```yaml
+# GitVersion.yml
+template: "{Major}.{Minor}.{Patch:F2}-{PreReleaseLabel}"
+```
+
+**Supported Numeric Formats:**
+
+- `F` or `f` (Fixed-point): `{Patch:F2}` → `"1.23"`
+- `N` or `n` (Number): `{BuildMetadata:N0}` → `"1,234"`
+- `C` or `c` (Currency): `{Major:C}` → `"¤1.00"`
+- `P` or `p` (Percent): `{CommitsSinceVersionSource:P}` → `"12,345.60 %"`
+- `D` or `d` (Decimal): `{Major:D4}` → `"0001"`
+- `X` or `x` (Hexadecimal): `{Patch:X}` → `"FF"`
+
+### Date and Time Formatting
+
+When working with date-related properties like `CommitDate`:
+
+```yaml
+template: "Build-{SemVer}-{CommitDate:yyyy-MM-dd}"
+```
+
+**Common Date Format Specifiers:**
+
+- `yyyy-MM-dd` → `"2024-03-15"`
+- `HH:mm:ss` → `"14:30:22"`
+- `MMM dd, yyyy` → `"Mar 15, 2024"`
+- `yyyy-MM-dd'T'HH:mm:ss'Z'` → `"2024-03-15T14:30:22Z"`
+
+## Custom String Case Formatters
+
+GitVersion introduces custom format specifiers for string case transformations that can be used in templates:
+
+### Available Case Formats
+
+| Format | Description                                                   | Example Input    | Example Output   |
+|--------|---------------------------------------------------------------|------------------|------------------|
+| `u`    | **Uppercase** - Converts entire string to uppercase           | `feature-branch` | `FEATURE-BRANCH` |
+| `l`    | **Lowercase** - Converts entire string to lowercase           | `Feature-Branch` | `feature-branch` |
+| `t`    | **Title Case** - Capitalizes first letter of each word        | `feature-branch` | `Feature-Branch` |
+| `s`    | **Sentence Case** - Capitalizes only the first letter         | `feature-branch` | `Feature-branch` |
+| `c`    | **PascalCase** - Removes separators and capitalizes each word | `feature-branch` | `FeatureBranch`  |
+
+### Usage Examples
+
+```yaml
+# GitVersion.yml configuration
+branches:
+  feature:
+    label: "{BranchName:c}"  # Converts to PascalCase
+
+template: "{Major}.{Minor}.{Patch}-{PreReleaseLabel:l}.{CommitsSinceVersionSource:0000}"
+```
+
+**Template Usage:**
+
+```yaml
+# Using format strings in templates
+assembly-informational-format: "{Major}.{Minor}.{Patch}-{CommitsSinceVersionSource:0000}"
+template: "{SemVer}-{BranchName:l}"
+```
+
+## Examples
+
+Based on actual test cases from the implementation:
+
+### Zero-Padded Numeric Formatting
+
+```yaml
+# Zero-padded commit count
+assembly-informational-format: "{Major}.{Minor}.{Patch}-{CommitsSinceVersionSource:0000}"
+# Result: "1.2.3-0042"
+```
+
+### String Case Transformations
+
+```yaml
+branches:
+  feature:
+    label: "{BranchName:c}"  # PascalCase: "feature-branch" → "FeatureBranch"
+  hotfix:
+    label: "hotfix-{BranchName:l}"  # Lowercase: "HOTFIX-BRANCH" → "hotfix-branch"
+```
+
+### Date and Time Formatting
+
+```yaml
+template: "{SemVer}-build-{CommitDate:yyyy-MM-dd}"
+# Result: "1.2.3-build-2021-01-01"
+```
+
+### Numeric Formatting
+
+```yaml
+# Currency format (uses InvariantCulture)
+template: "Cost-{Major:C}"  # Result: "Cost-¤1.00"
+
+# Percentage format
+template: "Progress-{Minor:P}"  # Result: "Progress-200.00 %"
+
+# Thousands separator
+template: "Build-{CommitsSinceVersionSource:N0}"  # Result: "Build-1,234"
+```
+
+## Configuration Integration
+
+The format strings are used in GitVersion configuration files through various formatting properties:
+
+### Assembly Version Formatting
+
+```yaml
+# GitVersion.yml
+assembly-informational-format: "{Major}.{Minor}.{Patch}-{CommitsSinceVersionSource:0000}"
+assembly-versioning-format: "{Major}.{Minor}.{Patch}.{env:BUILD_NUMBER}"
+assembly-file-versioning-format: "{MajorMinorPatch}.{CommitsSinceVersionSource}"
+```
+
+### Template-Based Configuration
+
+```yaml
+# Global template for consistent formatting across all variables
+template: "{SemVer}-{BranchName:l}-{ShortSha}"
+
+branches:
+  main:
+    label: ""
+  feature:
+    label: "{BranchName:c}.{CommitsSinceVersionSource}"
+    increment: Minor
+  release:
+    label: "rc-{CommitsSinceVersionSource:000}"
+    increment: None
+```
+
+### Environment Variable Integration
+
+```yaml
+# Using environment variables with fallbacks
+template: "{Major}.{Minor}.{Patch}-{env:RELEASE_STAGE ?? 'dev'}"
+assembly-informational-format: "{SemVer}+{env:BUILD_ID ?? 'local'}"
+```
+
+### Real-World Integration Examples
+
+Based on the actual test implementation:
+
+```yaml
+# Example from VariableProviderTests.cs
+assembly-informational-format: "{Major}.{Minor}.{Patch}-{CommitsSinceVersionSource:0000}"
+# Result: "1.2.3-0042" when CommitsSinceVersionSource = 42
+
+# Branch-specific formatting
+branches:
+  feature:
+    label: "{BranchName:c}"  # PascalCase conversion
+  hotfix:
+    label: "hotfix.{CommitsSinceVersionSource:00}"
+```
+
+## Invariant Culture Formatting
+
+The formatting system uses `CultureInfo.InvariantCulture` by default through the chained `TryFormat` overload implementation. This provides:
+
+- **Consistent results** across all environments and systems
+- **Predictable numeric formatting** with period (.) as decimal separator and comma (,) as thousands separator
+- **Standard date formatting** using English month names and formats
+- **No localization variations** regardless of system locale
+
+```csharp
+// All environments produce the same output:
+// {CommitsSinceVersionSource:N0} → "1,234"
+// {CommitDate:MMM dd, yyyy} → "Mar 15, 2024"
+// {Major:C} → "¤1.00" (generic currency symbol)
+```
+
+This ensures that version strings generated by GitVersion are consistent across different build environments, developer machines, and CI/CD systems.
+
+## Verified Examples
+
+The following examples are verified by actual unit tests in the GitVersion codebase:
+
+### Zero-Padded Numeric Formatting
+
+```yaml
+assembly-informational-format: "{Major}.{Minor}.{Patch}-{CommitsSinceVersionSource:0000}"
+```
+
+**Test**: `VariableProviderTests.Format_Allows_CSharp_FormatStrings()`
+**Input**: `CommitsSinceVersionSource = 42`
+**Output**: `"1.2.3-0042"`
+
+### String Case Transformations
+
+```csharp
+// From StringFormatterTests.cs
+[TestCase("hello world", "c", "HelloWorld")]  // PascalCase
+[TestCase("hello", "u", "HELLO")]             // Uppercase
+[TestCase("HELLO", "l", "hello")]             // Lowercase
+[TestCase("hello world", "t", "Hello World")] // Title Case
+[TestCase("hELLO", "s", "Hello")]             // Sentence Case
+```
+
+### Numeric Format Specifiers
+
+```csharp
+// From NumericFormatterTests.cs
+[TestCase("1234.5678", "n", "1,234.57")]      // Number format
+[TestCase("1234.5678", "f2", "1234.57")]      // Fixed-point format
+[TestCase("1234.5678", "f0", "1235")]         // No decimals
+```
+
+### Date Formatting
+
+```csharp
+// From DateFormatterTests.cs
+[TestCase("2021-01-01", "yyyy-MM-dd", "2021-01-01")]
+[TestCase("2021-01-01T12:00:00Z", "yyyy-MM-ddTHH:mm:ssZ", "2021-01-01T12:00:00Z")]
+```
+
+### Currency and Percentage (InvariantCulture)
+
+```csharp
+// From FormattableFormatterTests.cs
+[TestCase(123.456, "C", "¤123.46")]           // Generic currency symbol
+[TestCase(123.456, "P", "12,345.60 %")]       // Percentage format
+[TestCase(1234567890, "N0", "1,234,567,890")] // Thousands separators
+```
+
+[reference-configuration]: /docs/reference/configuration
+[variables]: /docs/reference/variables

docs/input/docs/reference/mdsource/configuration.source.md

@@ -78,6 +78,9 @@ while still updating the `AssemblyFileVersion` and `AssemblyInformationVersion`
 attributes. Valid values: `MajorMinorPatchTag`, `MajorMinorPatch`, `MajorMinor`,
 `Major`, `None`.
 
+For information on using format strings in these properties, see
+[Format Strings](/docs/reference/custom-formatting).
+
 ### assembly-file-versioning-scheme
 
 When updating assembly info, `assembly-file-versioning-scheme` tells GitVersion
@@ -233,7 +236,7 @@ ignore:
     - ^docs\/
 ```
 ##### *Monorepo*
-This ignore config can be used to filter only those commits that belong to a specific project in a monorepo. 
+This ignore config can be used to filter only those commits that belong to a specific project in a monorepo.
 As an example, consider a monorepo consisting of subdirectories for `ProjectA`, `ProjectB` and a shared `LibraryC`. For GitVersion to consider only commits that are part of `projectA` and shared library `LibraryC`, a regex that matches all paths except those starting with `ProjectA` or `LibraryC` can be used. Either one of the following configs would filter out `ProjectB`.
 * Specific match on `/ProjectB/*`:
 ```yaml
@@ -256,7 +259,7 @@ A commit having changes only in `/ProjectB/*` path would be ignored. A commit ha
 * `/ProductA/*` and `/ProductB/*` and `/LibraryC/*`
 
 :::
-Note: The `ignore.paths` configuration is case-sensitive. This can lead to unexpected behavior on case-insensitive file systems, such as Windows. To ensure consistent matching regardless of case, you can prefix your regular expressions with the case-insensitive flag `(?i)`. For example, `(?i)^docs\/` will match both `docs/` and `Docs/`. 
+Note: The `ignore.paths` configuration is case-sensitive. This can lead to unexpected behavior on case-insensitive file systems, such as Windows. To ensure consistent matching regardless of case, you can prefix your regular expressions with the case-insensitive flag `(?i)`. For example, `(?i)^docs\/` will match both `docs/` and `Docs/`.
 :::
 
 ::: {.alert .alert-warning}

docs/input/docs/reference/variables.md

@@ -74,4 +74,8 @@ within a [supported build server][build-servers]), the above version variables
 may be exposed automatically as **environment variables** in the format
 `GitVersion_FullSemVer`.
 
+## Formatting Variables
+
+GitVersion variables can be formatted using C# format strings. See [Format Strings](/docs/reference/custom-formatting) for details.
+
 [build-servers]: ./build-servers/

docs/input/docs/usage/cli/arguments.md

@@ -38,6 +38,7 @@ GitVersion [path]
                     - will output `1.2.3+beta.4`
     /format         Used in conjunction with /output json, will output a format
                     containing version variables.
+                    Supports C# format strings - see [Format Strings](/docs/reference/custom-formatting) for details.
                     E.g. /output json /format {SemVer} - will output `1.2.3+beta.4`
                          /output json /format {Major}.{Minor} - will output `1.2`
     /l              Path to logfile.

docs/input/docs/usage/msbuild.md

@@ -98,6 +98,8 @@ Now, when you build:
   appended to it.
 * `AssemblyInformationalVersion` will be set to the `InformationalVersion` variable.
 
+Assembly version formatting can use C# format strings. See [Format Strings](/docs/reference/custom-formatting) for available options.
+
 #### Other injected Variables
 
 All other [variables](/docs/reference/variables) will be injected into an

new-cli/GitVersion.Common/GitVersion.Common.csproj

@@ -12,6 +12,7 @@
         <Compile Include="..\..\src\GitVersion.Core\Extensions\DictionaryExtensions.cs" Link="%(Filename)%(Extension)" />
         <Compile Include="..\..\src\GitVersion.Core\Extensions\StringExtensions.cs" Link="Extensions\StringExtensions.cs" />
         <Compile Include="..\..\src\GitVersion.Core\Extensions\CommonExtensions.cs" Link="Extensions\CommonExtensions.cs" />
+        <Compile Include="..\..\src\GitVersion.Core\Formatting\*.cs" Link="Formatting\%(Filename)%(Extension)" />
         <Compile Include="..\..\src\GitVersion.Core\Helpers\*.cs" Link="Helpers\%(Filename)%(Extension)" />
         <Compile Include="..\..\src\GitVersion.Core\Git\*.cs" Link="Git\%(Filename)%(Extension)" />
         <Compile Include="..\..\src\GitVersion.Core\SemVer\*.cs" Link="SemVer\%(Filename)%(Extension)" />

src/GitVersion.Core.Tests/Extensions/ShouldlyExtensions.cs

@@ -0,0 +1,24 @@
+namespace GitVersion.Core.Tests.Extensions;
+
+public static class ShouldlyExtensions
+{
+    /// <summary>
+    /// Asserts that the action throws an exception of type TException
+    /// with the expected message.
+    /// </summary>
+    public static void ShouldThrowWithMessage<TException>(this Action action, string expectedMessage) where TException : Exception
+    {
+        var ex = Should.Throw<TException>(action);
+        ex.Message.ShouldBe(expectedMessage);
+    }
+
+    /// <summary>
+    /// Asserts that the action throws an exception of type TException,
+    /// and allows further assertion on the exception instance.
+    /// </summary>
+    public static void ShouldThrow<TException>(this Action action, Action<TException> additionalAssertions) where TException : Exception
+    {
+        var ex = Should.Throw<TException>(action);
+        additionalAssertions(ex);
+    }
+}