Api Design

Public API Design and Compatibility

When to Use This Skill

Use this skill when:

• Designing public APIs for NuGet packages or libraries
• Making changes to existing public APIs
• Planning wire format changes for distributed systems
• Implementing versioning strategies
• Reviewing pull requests for breaking changes

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The Three Types of Compatibility

| Type | Definition | Scope | |------|------------|-------| | API/Source | Code compiles against newer version | Public method signatures, types | | Binary | Compiled code runs against newer version | Assembly layout, method tokens | | Wire | Serialized data readable by other versions | Network protocols, persistence formats |

Breaking any of these creates upgrade friction for users.

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Extend-Only Design

The foundation of stable APIs: never remove or modify, only extend.

Three Pillars

1. Previous functionality is immutable - Once released, behavior and signatures are locked
2. New functionality through new constructs - Add overloads, new types, opt-in features
3. Removal only after deprecation period - Years, not releases

Benefits

• Old code continues working in new versions
• New and old pathways coexist
• Upgrades are non-breaking by default
• Users upgrade on their schedule

Resources:

• Extend-Only Design
• OSS Compatibility Standards

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API Change Guidelines

Safe Changes (Any Release)

// SAFE: Add NEW overload methods that delegate to existing methods
// Existing method - do not modify its signature
public void Process(Order order) { ... }
// New overload - safe to add
public void Process(Order order, CancellationToken ct)
{
    // implementation that handles cancellation
}

// SAFE: Add NEW overloads for additional functionality
// Existing method - do not modify
public void Send(Message msg) { ... }
// New overload - safe to add
public void Send(Message msg, Priority priority)
{
    // implementation that handles priority
}

// ADD new types, interfaces, enums
public interface IOrderValidator { }
public enum OrderStatus { Pending, Complete, Cancelled }

// ADD new members to existing types
public class Order
{
    public DateTimeOffset? ShippedAt { get; init; }  // NEW
}

Unsafe Changes (Never or Major Version Only)

// REMOVE or RENAME public members
public void ProcessOrder(Order order);  // Was: Process()

// CHANGE parameter types or order
public void Process(int orderId);  // Was: Process(Order order)

// CHANGE return types
public Order? GetOrder(string id);  // Was: public Order GetOrder()

// CHANGE access modifiers
internal class OrderProcessor { }  // Was: public

// ADD optional parameters to EXISTING methods (binary incompatible!)
// The compiled IL method signature changes - callers compiled against
// the old signature will get MissingMethodException at runtime.
// Optional parameter defaults are baked into the CALLER's assembly at compile time.
public void Process(Order order, CancellationToken ct = default);  // Breaks binary compat!
public void Send(Message msg, Priority priority = Priority.Normal);  // Breaks binary compat!
// Correct approach: add a NEW overload method instead (see Safe Changes above)

// ADD required parameters without defaults
public void Process(Order order, ILogger logger);  // Breaks callers!

Deprecation Pattern

// Step 1: Mark as obsolete with version (any release)
[Obsolete("Obsolete since v1.5.0. Use ProcessAsync instead.")]
public void Process(Order order) { }

// Step 2: Add new recommended API (same release)
public Task ProcessAsync(Order order, CancellationToken ct = default);

// Step 3: Remove in next major version (v2.0+)
// Only after users have had time to migrate

---

API Approval Testing

Prevent accidental breaking changes with automated API surface testing.

Using ApiApprover + Verify

dotnet add package PublicApiGenerator
dotnet add package Verify.Xunit

[Fact]
public Task ApprovePublicApi()
{
    var api = typeof(MyLibrary.PublicClass).Assembly.GeneratePublicApi();
    return Verify(api);
}

Creates ApprovePublicApi.verified.txt:

namespace MyLibrary
{
    public class OrderProcessor
    {
        public OrderProcessor() { }
        public void Process(Order order) { }
        public Task ProcessAsync(Order order, CancellationToken ct = default) { }
    }
}

Any API change fails the test - reviewer must explicitly approve changes.

PR Review Process

1. PR includes changes to *.verified.txt files
2. Reviewers see exact API surface changes in diff
3. Breaking changes are immediately visible
4. Conscious decision required to approve

---

Wire Compatibility

For distributed systems, serialized data must be readable across versions.

Requirements

| Direction | Requirement | |-----------|-------------| | Backward | Old writers → New readers (current version reads old data) | | Forward | New writers → Old readers (old version reads new data) |

Both are required for zero-downtime rolling upgrades.

Safely Evolving Wire Formats

Phase 1: Add read-side support (opt-in)

// New message type - readers deployed first
public sealed record HeartbeatV2(
    Address From,
    long SequenceNr,
    long CreationTimeMs);  // NEW field

// Deserializer handles both old and new
public object Deserialize(byte[] data, string manifest) => manifest switch
{
    "Heartbeat" => DeserializeHeartbeatV1(data),   // Old format
    "HeartbeatV2" => DeserializeHeartbeatV2(data), // New format
    _ => throw new NotSupportedException()
};

Phase 2: Enable write-side (opt-out, next minor version)

// Config to enable new format (off by default initially)
akka.cluster.use-heartbeat-v2 = on

Phase 3: Make default (future version)

After install base has absorbed read-side code.

Schema-Based Serialization

Prefer schema-based formats over reflection-based:

| Format | Type | Wire Compatibility | |--------|------|-------------------| | Protocol Buffers | Schema-based | Excellent - explicit field numbers | | MessagePack | Schema-based | Good - with contracts | | System.Text.Json | Schema-based (with source gen) | Good - explicit properties | | Newtonsoft.Json | Reflection-based | Poor - type names in payload | | BinaryFormatter | Reflection-based | Terrible - never use |

See dotnet/serialization skill for details.

---

Encapsulation Patterns

Internal APIs

Mark non-public APIs explicitly:

// Attribute for documentation
[InternalApi]
public class ActorSystemImpl { }

// Namespace convention
namespace MyLibrary.Internal
{
    public class InternalHelper { }  // Public for extensibility, not for users
}

Document clearly:

> Types in .Internal namespaces or marked with [InternalApi] may change between any releases without notice.

Sealing Classes

// DO: Seal classes not designed for inheritance
public sealed class OrderProcessor { }

// DON'T: Leave unsealed by accident
public class OrderProcessor { }  // Users might inherit, blocking changes

Interface Segregation

// DO: Small, focused interfaces
public interface IOrderReader
{
    Order? GetById(OrderId id);
}

public interface IOrderWriter
{
    Task SaveAsync(Order order);
}

// DON'T: Monolithic interfaces (can't add methods without breaking)
public interface IOrderRepository
{
    Order? GetById(OrderId id);
    Task SaveAsync(Order order);
    // Adding new methods breaks all implementations!
}

---

Versioning Strategy

Semantic Versioning (Practical)

| Version | Changes Allowed | |---------|----------------| | Patch (1.0.x) | Bug fixes, security patches | | Minor (1.x.0) | New features, deprecations, obsolete removal | | Major (x.0.0) | Breaking changes, old API removal |

Key Principles

1. No surprise breaks - Even major versions should be announced and planned
2. Extensions anytime - New APIs can ship in any release
3. Deprecate before remove - [Obsolete] for at least one minor version
4. Communicate timelines - Users need to plan upgrades

Chesterton's Fence

> Before removing or changing something, understand why it exists.

Assume every public API is used by someone. If you want to change it:

1. Socialize the proposal on GitHub
2. Document migration path
3. Provide deprecation period
4. Ship in planned release

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Pull Request Checklist

When reviewing PRs that touch public APIs:

• [ ] No removed public members (use [Obsolete] instead)
• [ ] No changed signatures (add overloads instead)
• [ ] No new required parameters (use defaults)
• [ ] API approval test updated (.verified.txt changes reviewed)
• [ ] Wire format changes are opt-in (read-side first)
• [ ] Breaking changes documented (release notes, migration guide)

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Anti-Patterns

Breaking Changes Disguised as Fixes

// "Bug fix" that breaks users
public async Task GetOrderAsync(OrderId id)  // Was sync!
{
    // "Fixed" to be async - but breaks all callers
}

// Correct: Add new method, deprecate old
[Obsolete("Use GetOrderAsync instead")]
public Order GetOrder(OrderId id) => GetOrderAsync(id).Result;

public async Task GetOrderAsync(OrderId id) { }

Silent Behavior Changes

// Changing defaults breaks users who relied on old behavior
public void Configure(bool enableCaching = true)  // Was: false!

// Correct: New parameter with new name
public void Configure(
    bool enableCaching = false,  // Original default preserved
    bool enableNewCaching = true)  // New behavior opt-in

Polymorphic Serialization

// AVOID: Type names in wire format
{ "$type": "MyApp.Order, MyApp", "Id": 123 }

// Renaming Order class = wire break!

// PREFER: Explicit discriminators
{ "type": "order", "id": 123 }

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Resources

• Making Public API Changes
• Wire Format Changes
• Extend-Only Design
• OSS Compatibility Standards
• Semantic Versioning
• PublicApiGenerator

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Aaronontheweb
• Source: Aaronontheweb/dotnet-skills
• License: MIT

Install and usage instructions live in the source repository linked above.