AdvancedGenericTypeConstraints.Abstractions 0.2.1

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="0.2.1" />
  <PackageReference Include="AdvancedGenericTypeConstraints.Analyzers" Version="0.2.1" 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
• MustBeOpenGenericTypeAttribute

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 related types is violated
• AGTC007: MustMatchAssemblyNameOf references an invalid related parameter
• AGTC008: a Type argument is not an open generic type definition

Matching semantics

The analyzer validates both concrete type arguments and forwarded generic type parameters.

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

If a generic method or type forwards one of its own type parameters into another constrained generic API, the forwarded type parameter also counts as a match when it already declares an equivalent or stricter MustImplementOpenGenericAttribute constraint.

Attribute checks

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

Forwarded generic type parameters are also accepted when they already declare the same MustHaveAttributeAttribute constraint.

Assembly naming checks

MustMatchAssemblyNameOfAttribute compares simple assembly names.

You can apply it to:

• a generic type parameter
• a method parameter of type System.Type

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"

The same rule also works for Type-based overloads when the call site passes statically analyzable values such as typeof(SomeType):

void RegisterInProcessApi(
    [MustMatchAssemblyNameOf(nameof(implementationType), suffix: ".Contracts")] Type serviceType,
    Type implementationType);

registry.RegisterInProcessApi(typeof(Feature.Contracts.IService), typeof(Feature.ServiceImplementation));

You can also configure:

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

Forwarded generic type parameters are also accepted when they already declare an equivalent or stricter MustMatchAssemblyNameOfAttribute constraint. This allows delegating overloads and explicit interface implementations to pass constrained generic parameters through without needing #pragma warning disable AGTC006.

The same applies when a generic overload forwards into a Type-based overload via typeof(TService) and typeof(TImplementation).

Open generic Type checks

MustBeOpenGenericTypeAttribute applies to method parameters of type System.Type.

It requires statically analyzable call sites to pass an open generic type definition such as typeof(IFoo<>) or typeof(Foo<>).

Example:

void RegisterInProcessApi(
    [MustBeOpenGenericType] Type serviceType,
    [MustBeOpenGenericType] Type implementationType);

featureRegistry.RegisterInProcessApi(
    serviceType: typeof(ISendResetSalesOrderSurchargesCommandService<>),
    implementationType: typeof(SendResetSalesOrderSurchargesCommandService<>));

Example:

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

Forwarding example:

public interface IFeatureRegistry
{
    IFeatureRegistry RegisterInProcessApi<
        [MustMatchAssemblyNameOf(nameof(TImplementation), suffix: ".Contracts")] TService,
        TImplementation>()
        where TService : class
        where TImplementation : class, TService;
}

public sealed class ConfiguredFeatureRegistry : IFeatureRegistry
{
    public ConfiguredFeatureRegistry RegisterInProcessApi<
        [MustMatchAssemblyNameOf(nameof(TImplementation), suffix: ".Contracts")] TService,
        TImplementation>()
        where TService : class
        where TImplementation : class, TService
    {
        return this;
    }

    IFeatureRegistry IFeatureRegistry.RegisterInProcessApi<
        [MustMatchAssemblyNameOf(nameof(TImplementation), suffix: ".Contracts")] TService,
        TImplementation>()
    {
        return RegisterInProcessApi<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.