rudironsoni dotnet-api-surface-validation
Tools and workflows for validating and tracking the public API surface of .NET libraries. Covers three complementary approaches: PublicApiAnalyzers for text-file tracking of shipped/unshipped APIs with Roslyn diagnostics, the Verify snapshot pattern for reflection-based API surface snapshot testing, and ApiCompat CI enforcement for gating pull requests on API surface changes.
Version assumptions: .NET 8.0+ baseline. PublicApiAnalyzers 3.3+ (ships with Microsoft.CodeAnalysis.Analyzers or
standalone Microsoft.CodeAnalysis.PublicApiAnalyzers). ApiCompat tooling included in .NET 8+ SDK.
Scope
- PublicApiAnalyzers text-file tracking of shipped/unshipped APIs
- Verify snapshot pattern for reflection-based API surface testing
- ApiCompat CI enforcement for gating PRs on breaking changes
- Multi-TFM and monorepo API tracking strategies
- PR labeling and suppression file workflows
Out of scope
- Binary vs source compatibility rules, type forwarders, SemVer impact -- see [skill:dotnet-library-api-compat]
- NuGet packaging,
EnablePackageValidationbasics, and suppression file mechanics -- see [skill:dotnet-nuget-authoring] and [skill:dotnet-multi-targeting] - Verify library fundamentals (setup, scrubbing, converters) -- see [skill:dotnet-snapshot-testing]
- General Roslyn analyzer configuration (EditorConfig, severity levels) -- see [skill:dotnet-roslyn-analyzers]
- HTTP API versioning -- see [skill:dotnet-api-versioning]
Cross-references: [skill:dotnet-library-api-compat] for binary/source compatibility rules,
[skill:dotnet-nuget-authoring] for EnablePackageValidation and NuGet SemVer, [skill:dotnet-multi-targeting] for
multi-TFM ApiCompat tool mechanics, [skill:dotnet-snapshot-testing] for Verify fundamentals,
[skill:dotnet-roslyn-analyzers] for general analyzer configuration, [skill:dotnet-api-versioning] for HTTP API
versioning.
PublicApiAnalyzers
PublicApiAnalyzers tracks every public API member in text files committed to source control. The analyzer enforces that new APIs go through an explicit "unshipped" phase before being marked "shipped," preventing accidental public API exposure and undocumented surface area changes.
Setup
Install the analyzer package:
<ItemGroup>
<PackageReference Include="Microsoft.CodeAnalysis.PublicApiAnalyzers" Version="3.3.*" PrivateAssets="all" />
</ItemGroup>
```xml
Create the two tracking files at the project root (adjacent to the `.csproj`):
```csharp
MyLib/
MyLib.csproj
PublicAPI.Shipped.txt # APIs shipped in released versions
PublicAPI.Unshipped.txt # APIs added since last release
```csharp
Both files must exist, even if empty. Each must contain a header comment:
```text
#nullable enable
```text
The `#nullable enable` header tells the analyzer to track nullable annotations in API signatures. Without it, nullable
context differences are ignored.
### Diagnostic Rules
| Rule | Severity | Meaning |
| ------ | -------- | ------------------------------------------------------------ |
| RS0016 | Warning | Public API member not declared in API tracking files |
| RS0017 | Warning | Public API member removed but still in tracking files |
| RS0024 | Warning | Public API member has wrong nullable annotation |
| RS0025 | Warning | Public API symbol marked shipped but has changed signature |
| RS0026 | Warning | New public API added without `PublicAPI.Unshipped.txt` entry |
| RS0036 | Warning | API file missing `#nullable enable` header |
| RS0037 | Warning | Public API declared but does not exist in source |
**RS0016** is the most common diagnostic. When you add a new `public` or `protected` member, RS0016 fires until you add
the member's signature to `PublicAPI.Unshipped.txt`. Use the code fix (lightbulb) in the IDE to automatically add the
entry.
**RS0017** fires when you remove or rename a `public` member but the old signature still exists in the tracking files.
Remove the stale line from the appropriate file.
### File Format
Each line in the tracking files represents one public API symbol using its documentation comment ID format:
```text
#nullable enable
MyLib.Widget
MyLib.Widget.Widget() -> void
MyLib.Widget.Name.get -> string!
MyLib.Widget.Name.set -> void
MyLib.Widget.Calculate(int count) -> decimal
MyLib.Widget.CalculateAsync(int count, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task<decimal>!
MyLib.IWidgetFactory
MyLib.IWidgetFactory.Create(string! name) -> MyLib.Widget!
MyLib.WidgetOptions
MyLib.WidgetOptions.WidgetOptions() -> void
MyLib.WidgetOptions.MaxRetries.get -> int
MyLib.WidgetOptions.MaxRetries.set -> void
```text
Key formatting rules:
- The `!` suffix denotes a non-nullable reference type in nullable-enabled context
- The `?` suffix denotes a nullable reference type or nullable value type
- Constructors use the type name (e.g., `Widget.Widget() -> void`)
- Properties expand to `.get` and `.set` entries
- Default parameter values are included in the signature
### Shipped/Unshipped Lifecycle
The workflow across release cycles:
**During development (between releases):**
1. Add new public API member to source code
2. RS0016 fires -- member not tracked
3. Use code fix or manually add to `PublicAPI.Unshipped.txt`
4. RS0016 clears
**At release time:**
1. Move all entries from `PublicAPI.Unshipped.txt` to `PublicAPI.Shipped.txt`
2. Clear `PublicAPI.Unshipped.txt` back to just the `#nullable enable` header
3. Commit both files as part of the release PR
4. Tag the release
**When removing a previously shipped API (major version):**
1. Remove the member from source code
2. Remove the entry from `PublicAPI.Shipped.txt`
3. RS0017 clears (if it fired)
4. Document the removal in release notes
**When removing an unshipped API (before release):**
1. Remove the member from source code
2. Remove the entry from `PublicAPI.Unshipped.txt`
3. No SemVer impact -- the API was never released
### Multi-TFM Projects
For multi-targeted projects, PublicApiAnalyzers supports per-TFM tracking files when the API surface differs across
targets:
```text
MyLib/
MyLib.csproj
PublicAPI.Shipped.txt # Shared across all TFMs
PublicAPI.Unshipped.txt # Shared across all TFMs
PublicAPI.Shipped.net8.0.txt # net8.0-specific APIs
PublicAPI.Unshipped.net8.0.txt # net8.0-specific APIs
PublicAPI.Shipped.net10.0.txt # net10.0-specific APIs
PublicAPI.Unshipped.net10.0.txt # net10.0-specific APIs
```text
The shared files contain APIs common to all TFMs. The TFM-specific files contain APIs that only exist on that target.
The analyzer merges them at build time.
To enable per-TFM files, add to the `.csproj`:
```xml
<PropertyGroup>
<RoslynPublicApiPerTfm>true</RoslynPublicApiPerTfm>
</PropertyGroup>
```xml
See [skill:dotnet-multi-targeting] for multi-TFM packaging mechanics.
### Integrating with CI
PublicApiAnalyzers runs as part of the standard build. To enforce it in CI, ensure warnings are treated as errors for
the RS-series rules:
```xml
<!-- In Directory.Build.props or the library .csproj -->
<PropertyGroup>
<WarningsAsErrors>$(WarningsAsErrors);RS0016;RS0017;RS0036;RS0037</WarningsAsErrors>
</PropertyGroup>
```csharp
This gates CI builds on any undeclared public API changes. Developers must explicitly update the tracking files before
the build passes.
---
## Verify API Surface Snapshot Pattern
Use the Verify library to snapshot-test the entire public API surface of an assembly. This approach uses reflection to
enumerate all public types and members, producing a human-readable snapshot that is committed to source control and
compared on every test run. Any change to the public API surface causes a test failure until the snapshot is explicitly
approved.
This pattern complements PublicApiAnalyzers -- the analyzer catches changes at build time within the project, while the
Verify snapshot catches changes from the perspective of a compiled assembly consumer.
For Verify fundamentals (setup, scrubbing, converters, diff tool integration, CI configuration), see
[skill:dotnet-snapshot-testing].
### Extracting the Public API Surface
Create a helper method that reflects over an assembly to produce a stable, sorted representation of all public types and
their members:
```csharp
using System.Reflection;
using System.Text;
public static class PublicApiExtractor
{
public static string GetPublicApi(Assembly assembly)
{
var sb = new StringBuilder();
var publicTypes = assembly
.GetTypes()
.Where(t => t.IsPublic || t.IsNestedPublic)
.OrderBy(t => t.FullName, StringComparer.Ordinal);
foreach (var type in publicTypes)
{
AppendType(sb, type);
}
return sb.ToString();
}
private static void AppendType(StringBuilder sb, Type type)
{
var kind = type switch
{
{ IsEnum: true } => "enum",
{ IsValueType: true } => "struct",
{ IsInterface: true } => "interface",
{ IsAbstract: true, IsSealed: true } => "static class",
{ IsAbstract: true } => "abstract class",
{ IsSealed: true } => "sealed class",
_ => "class"
};
sb.AppendLine($"{kind} {type.FullName}");
var members = type
.GetMembers(BindingFlags.Public | BindingFlags.Instance
| BindingFlags.Static | BindingFlags.DeclaredOnly)
.OrderBy(m => m.MemberType)
.ThenBy(m => m.Name, StringComparer.Ordinal)
.ThenBy(m => m.ToString(), StringComparer.Ordinal);
foreach (var member in members)
{
sb.AppendLine($" {FormatMember(member)}");
}
sb.AppendLine();
}
private static string FormatMember(MemberInfo member) =>
member switch
{
ConstructorInfo c => $".ctor({FormatParameters(c.GetParameters())})",
MethodInfo m when !m.IsSpecialName =>
$"{m.ReturnType.Name} {m.Name}({FormatParameters(m.GetParameters())})",
PropertyInfo p => $"{p.PropertyType.Name} {p.Name} {{ {GetAccessors(p)} }}",
FieldInfo f => $"{f.FieldType.Name} {f.Name}",
EventInfo e => $"event {e.EventHandlerType?.Name} {e.Name}",
_ => member.ToString() ?? string.Empty
};
private static string FormatParameters(ParameterInfo[] parameters) =>
string.Join(", ", parameters.Select(p => $"{p.ParameterType.Name} {p.Name}"));
private static string GetAccessors(PropertyInfo prop)
{
var parts = new List<string>();
if (prop.GetMethod?.IsPublic == true) parts.Add("get;");
if (prop.SetMethod?.IsPublic == true) parts.Add("set;");
return string.Join(" ", parts);
}
}
```text
### Writing the Snapshot Test
```csharp
[UsesVerify]
public class PublicApiSurfaceTests
{
[Fact]
public Task PublicApi_ShouldMatchApprovedSurface()
{
var assembly = typeof(Widget).Assembly;
var publicApi = PublicApiExtractor.GetPublicApi(assembly);
return Verify(publicApi);
}
}
```text
On first run, this creates a `.verified.txt` file containing the full public API listing. Subsequent runs compare the
current API surface against the approved snapshot. Any addition, removal, or modification of public members causes a
test failure with a clear diff.
### Reviewing API Surface Changes
When the snapshot test fails:
1. Verify generates a `.received.txt` file showing the new API surface
2. Diff the `.received.txt` against `.verified.txt` to review changes
3. If the changes are intentional, accept the new snapshot with `verify accept`
4. If the changes are accidental, revert the code changes
This creates a code-review checkpoint where every public API change must be explicitly approved by someone reviewing the
snapshot diff in the pull request.
### Combining with PublicApiAnalyzers
The two approaches serve different purposes:
| Concern | PublicApiAnalyzers | Verify Snapshot |
| -------------------- | ----------------------------------- | ---------------------------- |
| Detection timing | Build time (in-IDE) | Test time (post-compile) |
| Granularity | Per-member signatures | Assembly-wide surface |
| Nullable annotations | Tracked via `#nullable enable` | Requires explicit reflection |
| Approval workflow | Edit text files (shipped/unshipped) | Accept snapshot diffs |
| Multi-TFM | Per-TFM files | Per-TFM test targets |
| CI gating | Warnings-as-errors | Test failures |
Use both for maximum coverage: PublicApiAnalyzers catches changes during development, while Verify snapshots provide an
end-to-end assembly-level validation in the test suite.
---
## ApiCompat CI Enforcement
ApiCompat compares two assemblies (or a baseline NuGet package against the current build) and reports API differences.
When integrated into CI, it gates pull requests on API surface changes -- any breaking change produces a build error
that the author must explicitly acknowledge.
For `EnablePackageValidation` basics and suppression file mechanics, see [skill:dotnet-nuget-authoring] and
[skill:dotnet-multi-targeting].
### Package Validation in CI
The simplest enforcement uses `EnablePackageValidation` during `dotnet pack`:
```xml
<PropertyGroup>
<EnablePackageValidation>true</EnablePackageValidation>
<PackageValidationBaselineVersion>1.2.0</PackageValidationBaselineVersion>
</PropertyGroup>
```text
In a CI pipeline, `dotnet pack` runs package validation automatically:
```yaml
# GitHub Actions -- gate PRs on API compatibility
name: API Compatibility Check
on:
pull_request:
paths:
- 'src/**'
- '*.props'
- '*.targets'
jobs:
api-compat:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- uses: actions/setup-dotnet@v4
with:
dotnet-version: '8.0.x'
- name: Restore
run: dotnet restore
- name: Build
run: dotnet build --configuration Release --no-restore
- name: Pack with API validation
run: dotnet pack --configuration Release --no-build
# EnablePackageValidation runs during pack and fails
# the build if breaking changes are detected
```text
### Standalone ApiCompat Tool for Assembly Comparison
When you need to compare assemblies without packing (e.g., comparing a feature branch build against the main branch
build), use the standalone ApiCompat tool:
```yaml
# GitHub Actions -- compare assemblies directly
name: API Diff Check
on:
pull_request:
jobs:
api-diff:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
with:
fetch-depth: 0
- uses: actions/setup-dotnet@v4
with:
dotnet-version: '8.0.x'
- name: Install ApiCompat tool
run: dotnet tool install --global Microsoft.DotNet.ApiCompat.Tool
- name: Build current branch
run: dotnet build src/MyLib/MyLib.csproj -c Release -o artifacts/current
- name: Build baseline (main branch)
run: |
git stash
git checkout origin/main -- src/MyLib/
dotnet build src/MyLib/MyLib.csproj -c Release -o artifacts/baseline
git checkout - -- src/MyLib/
git stash pop || true
- name: Compare APIs
run: |
apicompat --left-assembly artifacts/baseline/MyLib.dll \
--right-assembly artifacts/current/MyLib.dll
```text
### PR Labeling for API Changes
Combine ApiCompat with PR labeling to surface API changes to reviewers:
```yaml
- name: Check for API changes
id: api-check
continue-on-error: true
run: |
apicompat --left-assembly artifacts/baseline/MyLib.dll \
--right-assembly artifacts/current/MyLib.dll 2>&1 | tee api-diff.txt
echo "has_changes=$([[ -s api-diff.txt ]] && echo true || echo false)" >> "$GITHUB_OUTPUT"
- name: Label PR with API changes
if: steps.api-check.outputs.has_changes == 'true'
run: gh pr edit "${{ github.event.pull_request.number }}" --add-label "api-change"
env:
GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
```text
### Handling Intentional Breaking Changes
When a breaking change is intentional (new major version), generate a suppression file:
```bash
dotnet pack /p:GenerateCompatibilitySuppressionFile=true
```bash
This creates `CompatibilitySuppressions.xml` in the project directory. Reference it explicitly if stored elsewhere:
```xml
<ItemGroup>
<ApiCompatSuppressionFile Include="CompatibilitySuppressions.xml" />
</ItemGroup>
```xml
Note: `ApiCompatSuppressionFile` is an **ItemGroup item**, not a PropertyGroup property. Using PropertyGroup syntax
silently does nothing.
The suppression file documents the specific breaking changes that are accepted:
```xml
<?xml version="1.0" encoding="utf-8"?>
<Suppressions xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<Suppression>
<DiagnosticId>CP0002</DiagnosticId>
<Target>M:MyLib.Widget.Calculate</Target>
<Left>lib/net8.0/MyLib.dll</Left>
<Right>lib/net8.0/MyLib.dll</Right>
</Suppression>
</Suppressions>
```text
Commit suppression files to source control. Reviewers can inspect the file to verify that breaking changes are
documented and intentional.
### Enforcing PublicApiAnalyzers Files in CI
Combine PublicApiAnalyzers warnings-as-errors with a CI step that verifies tracking files are not stale:
```yaml
- name: Build with API tracking enforcement
run: dotnet build -c Release /p:TreatWarningsAsErrors=true /warnaserror:RS0016,RS0017,RS0036,RS0037
- name: Verify PublicAPI files are committed
run: |
if git diff --name-only | grep -q 'PublicAPI'; then
echo "::error::PublicAPI tracking files have uncommitted changes"
git diff -- '**/PublicAPI.*.txt'
exit 1
fi
```text
### Multi-Library Monorepo Enforcement
For repositories with multiple libraries, apply API validation at the solution level:
```xml
<!-- Directory.Build.props -- applied to all library projects -->
<Project>
<PropertyGroup Condition="'$(IsPackable)' == 'true'">
<EnablePackageValidation>true</EnablePackageValidation>
<WarningsAsErrors>$(WarningsAsErrors);RS0016;RS0017;RS0036;RS0037</WarningsAsErrors>
</PropertyGroup>
<ItemGroup Condition="'$(IsPackable)' == 'true'">
<PackageReference Include="Microsoft.CodeAnalysis.PublicApiAnalyzers"
Version="3.3.*" PrivateAssets="all" />
</ItemGroup>
</Project>
```text
This ensures every packable project in the repository has both PublicApiAnalyzers and package validation enabled without
duplicating configuration.
---
## Agent Gotchas
1. **Do not forget to create both `PublicAPI.Shipped.txt` and `PublicAPI.Unshipped.txt`** -- PublicApiAnalyzers requires
both files to exist, even if empty. Missing files cause RS0037 warnings on every public member.
2. **Do not omit the `#nullable enable` header from PublicAPI tracking files** -- without it (RS0036), the analyzer
ignores nullable annotation differences, missing real API surface changes in nullable-enabled libraries.
3. **Do not put `ApiCompatSuppressionFile` in a PropertyGroup** -- it is an ItemGroup item
(`<ApiCompatSuppressionFile Include="..." />`). PropertyGroup syntax is silently ignored, and suppression will not
work.
4. **Do not move entries from `PublicAPI.Unshipped.txt` to `PublicAPI.Shipped.txt` mid-development** -- move entries
only at release time. Premature shipping makes it impossible to cleanly revert unreleased API additions.
5. **Do not use the Verify API surface snapshot as the sole validation mechanism** -- it runs at test time, after
compilation. Use PublicApiAnalyzers for immediate build-time feedback and ApiCompat for baseline comparison; add
Verify snapshots as an additional safety net.
6. **Do not hardcode TFM-specific paths in CI ApiCompat workflows** -- use MSBuild output path variables or parameterize
the TFM to avoid breakage when TFMs are added or changed.
7. **Do not suppress RS0016 globally with `<NoWarn>`** -- this silently disables all public API tracking. Instead, add
the missing API entries to the tracking files. If an API is intentionally internal but must be `public` (e.g., for
`InternalsVisibleTo` alternatives), use `[EditorBrowsable(EditorBrowsableState.Never)]` and add it to the tracking
files.
8. **Do not generate the suppression file with `GenerateCompatibilitySuppressionFile=true` and forget to review it** --
the file may suppress more changes than intended. Always review the generated XML before committing.
---
## Prerequisites
- .NET 8.0+ SDK
- `Microsoft.CodeAnalysis.PublicApiAnalyzers` NuGet package (for RS0016/RS0017 diagnostics)
- `EnablePackageValidation` MSBuild property (for baseline API comparison during `dotnet pack`)
- `Microsoft.DotNet.ApiCompat.Tool` (optional, for standalone assembly comparison outside of `dotnet pack`)
- Verify test library and test framework integration package (for API surface snapshot testing) -- see
[skill:dotnet-snapshot-testing] for setup
- Understanding of binary vs source compatibility rules -- see [skill:dotnet-library-api-compat]
---
## References
- [Microsoft.CodeAnalysis.PublicApiAnalyzers](https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md)
- [Microsoft Learn: Package validation](https://learn.microsoft.com/dotnet/fundamentals/apicompat/package-validation/overview)
- [Microsoft Learn: API compatibility](https://learn.microsoft.com/dotnet/fundamentals/apicompat/overview)
- [Microsoft.DotNet.ApiCompat.Tool](https://www.nuget.org/packages/Microsoft.DotNet.ApiCompat.Tool)
- [Verify library](https://github.com/VerifyTests/Verify) -- snapshot testing framework
- [PublicApiAnalyzers diagnostics reference](https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/api-design-rules)