AdvancedGenericTypeConstraints.Abstractions 0.2.0

AdvancedGenericTypeConstraints

AdvancedGenericTypeConstraints enables compile-time-like validation for generic type rules in C# by combining lightweight attributes with a Roslyn analyzer.

Why this exists

Native C# generic constraints cannot express rules like:

• the supplied type must implement IHandleMessages<T> for some T
• the supplied type must carry a specific attribute
• one generic type argument must come from an assembly whose name is derived from another type argument's assembly

This project closes that gap with:

• a small abstractions package that exposes declarative attributes
• a Roslyn analyzer package that validates supplied type arguments at compile time

Packages

Install both packages in the consuming project:

<ItemGroup>
  <PackageReference Include="AdvancedGenericTypeConstraints.Abstractions" Version="0.2.0" />
  <PackageReference Include="AdvancedGenericTypeConstraints.Analyzers" Version="0.2.0" PrivateAssets="all" />
</ItemGroup>

Example

using AdvancedGenericTypeConstraints;

[AttributeUsage(AttributeTargets.Class)]
public sealed class ServiceAttribute : Attribute;

public interface IHandleMessages<TMessage>;

public interface IFeatureRegistry
{
    void RegisterMessageHandler<
        [MustImplementOpenGeneric(typeof(IHandleMessages<>))]
        [MustHaveAttribute(typeof(ServiceAttribute))]
        TMessageHandler>();

    void RegisterServiceContract<
        [MustMatchAssemblyNameOf(nameof(TImplementation), suffix: ".Contracts")]
        TService,
        TImplementation>();
}

Available checks

The current API supports:

• MustImplementOpenGenericAttribute
• MustImplementOpenGenericAttribute(Type openGenericType, bool exactlyOne)
• MustNotImplementOpenGenericAttribute
• MustHaveAttributeAttribute
• MustMatchAssemblyNameOfAttribute

Diagnostic IDs

The analyzer currently emits these diagnostics:

• AGTC001: required open generic type is missing
• AGTC002: forbidden open generic type is present
• AGTC003: required open generic type is not matched exactly once
• AGTC004: invalid MustImplementOpenGeneric configuration on a generic parameter
• AGTC005: required attribute is missing
• AGTC006: assembly naming rule between two type arguments is violated
• AGTC007: MustMatchAssemblyNameOf references an invalid generic parameter

Matching semantics

Open generic checks

The analyzer compares open generic type definitions, not closed constructed types.

A type counts as a match when the configured open generic type definition appears on:

• the concrete type argument itself
• any base type in its inheritance chain
• any implemented interface

Attribute checks

MustHaveAttributeAttribute checks whether the supplied type argument is directly annotated with the configured attribute type. Derived attributes also satisfy the rule.

Assembly naming checks

MustMatchAssemblyNameOfAttribute compares simple assembly names.

For a declaration like:

void RegisterServiceContract<
    [MustMatchAssemblyNameOf(nameof(TImplementation), suffix: ".Contracts")]
    TService,
    TImplementation>();

the analyzer requires TService to come from an assembly named:

{AssemblyOf(TImplementation)} + ".Contracts"

You can also configure:

• prefix
• suffix
• AllowedTypes as an explicit whitelist for legacy exceptions

Example:

void RegisterServiceContract<
    [MustMatchAssemblyNameOf(
        nameof(TImplementation),
        suffix: ".Contracts",
        AllowedTypes = new Type[] { typeof(ICelestialPostService), typeof(IOrbitalEchoStore) })]
    TService,
    TImplementation>();

Non-goals

• no changes to the C# type system
• no runtime validation
• no replacement for native generic constraints

The solution is intentionally based on static analysis only.