dotnet-library-api-compat

Binary and source compatibility rules for .NET library authors. Covers which API changes break consumers at the binary level (assembly loading, JIT resolution) versus at the source level (compilation), how to use type forwarders for assembly reorganization without breaking consumers, and how versioning decisions map to SemVer major/minor/patch increments.

Version assumptions: .NET 8.0+ baseline. Compatibility rules apply to all .NET versions but examples target modern SDK-style projects.

Out of scope: HTTP API versioning -- see [skill:dotnet-api-versioning]. NuGet package metadata, signing, and publish workflows -- see [skill:dotnet-nuget-authoring]. Multi-TFM packaging mechanics (polyfill strategy, conditional compilation) -- see [skill:dotnet-multi-targeting]. PublicApiAnalyzers and API surface validation tooling -- see [skill:dotnet-api-surface-validation]. Roslyn analyzer configuration -- see [skill:dotnet-roslyn-analyzers].

Cross-references: [skill:dotnet-api-versioning] for HTTP API versioning, [skill:dotnet-nuget-authoring] for NuGet packaging and SemVer rules, [skill:dotnet-multi-targeting] for multi-TFM packaging and ApiCompat tooling.

Binary Compatibility

Binary compatibility means existing compiled assemblies continue to work at runtime without recompilation. A binary-breaking change causes TypeLoadException, MissingMethodException, MissingFieldException, or TypeInitializationException at runtime.

Safe Changes (Binary Compatible)

Breaking Changes (Binary Incompatible)

Default Interface Members

Default interface members (DIM) added in C# 8 allow adding members to interfaces without breaking existing implementors -- but only at the binary level:

public interface IWidget
{
    string Name { get; }

    // Binary-safe: existing implementors inherit this default
    string DisplayName => Name.ToUpperInvariant();
}

However, if a consumer explicitly casts to the interface and the runtime cannot find the default implementation (older runtime), this fails. All runtimes in the .NET 8.0+ baseline support DIMs.

Source Compatibility

Source compatibility means existing consumer code continues to compile without changes. A source-breaking change causes compiler errors or changes behavior silently (which is worse).

Common Source-Breaking Changes

Overload Resolution Pitfalls

Adding overloads is the most common source of source-breaking changes in libraries. The C# compiler picks the "best" overload at compile time, and a new overload can change which method wins:

// V1 -- only overload
public void Send(object message) { }

// V2 -- new overload; ALL callers passing string now bind here
public void Send(string message) { }

This is source-breaking (callers silently rebind) but binary-compatible (old compiled code still calls the object overload token).

Mitigation: When adding overloads to public APIs, prefer parameter types that do not create implicit conversion paths from existing parameter types. Use [EditorBrowsable(EditorBrowsableState.Never)] on compatibility shims that must remain for binary compatibility but should not appear in IntelliSense.

Extension Method Conflicts

Extension methods resolve at compile time based on imported namespaces. Adding a new extension method can shadow an existing instance method or conflict with extensions from other libraries:

// Library V1 ships in namespace MyLib.Extensions
public static class StringExtensions
{
    public static string Truncate(this string s, int maxLength) =>
        s.Length <= maxLength ? s : s[..maxLength];
}

// Library V2 adds to SAME namespace -- safe
// Library V2 adds to DIFFERENT namespace -- may conflict
// if consumer imports both namespaces

Mitigation: Keep extension methods in the same namespace across versions. Document any namespace additions in release notes.

Type Forwarders

Type forwarders allow moving a public type from one assembly to another without breaking existing compiled references. The original assembly contains a forwarding entry that redirects the runtime type resolver to the new location.

When to Use Type Forwarders

• Splitting a large assembly into smaller, focused assemblies
• Merging assemblies for packaging simplification
• Reorganizing namespaces across assembly boundaries
• Moving types to a shared assembly consumed by multiple packages

Adding Type Forwarders

In the original assembly (the one types are moving FROM), add forwarding attributes after moving the types to the new assembly:

// In the ORIGINAL assembly's AssemblyInfo.cs or a dedicated TypeForwarders.cs
// This tells the runtime: "Widget now lives in MyLib.Core"
using System.Runtime.CompilerServices;

[assembly: TypeForwardedTo(typeof(MyLib.Core.Widget))]
[assembly: TypeForwardedTo(typeof(MyLib.Core.IWidgetFactory))]
[assembly: TypeForwardedTo(typeof(MyLib.Core.WidgetOptions))]

The original assembly must reference the destination assembly so that typeof() resolves correctly.

Receiving Type Forwarders

The destination assembly (the one types are moving TO) contains the actual type definitions. No special attributes are needed on the destination side. The [TypeForwardedFrom] attribute is optional metadata that records where the type originally lived -- useful for serialization compatibility:

// In the DESTINATION assembly -- optional but recommended for
// types that participate in serialization
using System.Runtime.CompilerServices;

namespace MyLib.Core;

[TypeForwardedFrom("MyLib, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null")]
public class Widget
{
    public string Name { get; set; } = string.Empty;
    public decimal Price { get; set; }
}

[TypeForwardedFrom] is critical for types deserialized by BinaryFormatter, DataContractSerializer, or any serializer that encodes assembly-qualified type names. Without it, deserialization of data written by older versions will fail with TypeLoadException.

Type Forwarder Chain

Type forwarders can chain: Assembly A forwards to Assembly B, which forwards to Assembly C. The runtime follows the chain. However, keep chains short (ideally one hop) to minimize assembly loading overhead.

Multi-TFM Type Forwarder Pattern

When restructuring assemblies in a multi-TFM library, the forwarding assembly must target all TFMs that consumers might use. A common pattern:

<!-- Original assembly (MyLib.csproj) -- now just a forwarding shim -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>net8.0;netstandard2.0</TargetFrameworks>
  </PropertyGroup>
  <ItemGroup>
    <ProjectReference Include="../MyLib.Core/MyLib.Core.csproj" />
  </ItemGroup>
</Project>

See [skill:dotnet-multi-targeting] for multi-TFM packaging mechanics and [skill:dotnet-nuget-authoring] for NuGet packaging of forwarding shims.

SemVer Impact Summary

Map API changes to Semantic Versioning increments. For full SemVer rules and NuGet versioning strategies, see [skill:dotnet-nuget-authoring].

Deprecation Lifecycle with [Obsolete]

The standard workflow for removing public API members across major versions:

// v2.1 -- warn consumers
[Obsolete("Use CalculateAsync() instead. This method will be removed in v3.0.")]
public int Calculate() => CalculateAsync().GetAwaiter().GetResult();

// v2.3 -- block new compilation against this member
[Obsolete("Use CalculateAsync() instead. This method will be removed in v3.0.", error: true)]
public int Calculate() => CalculateAsync().GetAwaiter().GetResult();

// v3.0 -- remove the member (Major version bump)

Always include the replacement API and the planned removal version in the obsolete message so both humans and agents can migrate proactively.

Multi-TFM Binary Compatibility

Adding or removing target frameworks affects binary compatibility for consumers:

• Adding a new TFM (e.g., adding net9.0 to an existing net8.0 package): Minor version bump. Existing consumers on net8.0 are unaffected; new consumers on net9.0 gain optimized code paths.
• Removing a TFM (e.g., dropping netstandard2.0): Major version bump. Consumers targeting the removed TFM can no longer resolve a compatible assembly.
• Changing the lowest supported TFM (e.g., net6.0 to net8.0): Major version bump. Consumers on the dropped TFM lose compatibility.

See [skill:dotnet-multi-targeting] for practical guidance on managing TFM additions and removals.

Compatibility Verification

Use EnablePackageValidation in your .csproj to automatically compare the current build against the previously shipped package and detect binary/source-breaking changes:

<PropertyGroup>
  <EnablePackageValidation>true</EnablePackageValidation>
  <!-- Compare against the last shipped version -->
  <PackageValidationBaselineVersion>1.2.0</PackageValidationBaselineVersion>
</PropertyGroup>

Build output flags breaking changes:

error CP0002: Member 'MyLib.Widget.Calculate()' was removed
error CP0006: Cannot change return type of 'MyLib.Widget.GetName()'

To suppress known intentional breaks, generate a suppression file:

dotnet pack /p:GenerateCompatibilitySuppressionFile=true

This produces a CompatibilitySuppressions.xml file that can be checked in. If unspecified, the SDK reads CompatibilitySuppressions.xml from the project directory automatically. To specify explicit suppression files:

<ItemGroup>
  <ApiCompatSuppressionFile Include="CompatibilitySuppressions.xml" />
</ItemGroup>

Note: ApiCompatSuppressionFile is an ItemGroup item, not a PropertyGroup property. Multiple suppression files can be included.

For deeper API surface tracking with PublicApiAnalyzers and CI enforcement workflows, see [skill:dotnet-api-surface-validation].

Agent Gotchas

1. Do not assume adding an overload is always safe -- it is binary-compatible but can be source-breaking due to overload resolution changes. Always check for implicit conversion paths between existing and new parameter types.
2. Do not remove public members without a major version bump -- even [Obsolete] members must be preserved until the next major version to maintain binary compatibility.
3. Do not forget type forwarders when moving types between assemblies -- without [TypeForwardedTo], consumers get TypeLoadException at runtime. Always add forwarders in the original assembly.
4. Do not change optional parameter default values in patch releases -- this silently changes behavior for recompiled consumers while old binaries retain the old default, creating version-dependent behavior divergence.
5. Do not confuse binary compatibility with source compatibility -- a change can be binary-safe but source-breaking (new overload) or source-safe but binary-breaking (changing return type from int to long). Test both.
6. Do not skip [TypeForwardedFrom] on serializable types -- serializers that encode assembly-qualified type names (DataContractSerializer, legacy BinaryFormatter) will fail to deserialize data written by older versions.
7. Do not put ApiCompatSuppressionFile in a PropertyGroup -- it is an ItemGroup item (<ApiCompatSuppressionFile Include="..." />), not a property. Using PropertyGroup syntax silently does nothing.
8. Do not remove a TFM from a library package without a major version bump -- consumers on the removed TFM lose compatibility with no fallback.

Prerequisites

• .NET 8.0+ SDK
• EnablePackageValidation MSBuild property for automated compatibility checking
• Understanding of SemVer 2.0 conventions (see [skill:dotnet-nuget-authoring])
• Familiarity with assembly loading and binding (strong naming concepts)

References

• Microsoft Learn: Breaking changes
• Microsoft Learn: Type forwarding in the CLR
• Microsoft Learn: EnablePackageValidation
• .NET API compatibility analyzer
• SemVer 2.0 Specification
• Library guidance: Breaking changes