Allow Behaviors with 0, 1, or 2 generic type parameters

[Copilot]

Behaviors previously required exactly 2 generic type parameters. This restriction is now lifted to support 0, 1, or 2 parameters, allowing behaviors with fixed or partially-specified types.

Changes

Analyzer (BehaviorsAnalyzer.cs)

• Accept behaviors with 0-2 type parameters
• Warn only on 3+ parameters
• Handle non-generic type validation correctly
• New: Added IHR0020 diagnostic to validate behavior type arguments match handler request/response types
• New: Added handler-level validation to detect type mismatches between behaviors and handlers

Generator (TransformBehaviors.cs, ImmediateHandlersGenerator.cs, Models.cs)

• Track type parameter count per behavior
• Extract constraints for 0, 1, or 2 parameter cases
• New: Detect which position (request or response) is parameterized in single-parameter behaviors
• Generate correct type arguments based on parameter count
• Behaviors with 3+ parameters are filtered out and produce no code

Template (Handler.sbntxt)

• Conditional type parameter generation based on TypeParameterCount
• Support Behavior, Behavior<TRequest>, and Behavior<TRequest, TResponse> signatures

Example

// Non-generic: fixed to specific types
[assembly: Behaviors(typeof(AuditBehavior))]
public class AuditBehavior : Behavior<MyRequest, MyResponse> { }

// Single parameter: request type varies, response fixed
[assembly: Behaviors(typeof(ValidationBehavior<>))]
public class ValidationBehavior<TRequest> : Behavior<TRequest, Unit> { }

// Single parameter: response type varies, request fixed
[assembly: Behaviors(typeof(LoggingBehavior<>))]
public class LoggingBehavior<TResponse> : Behavior<int, TResponse> { }

// Two parameters: both vary (existing behavior)
[assembly: Behaviors(typeof(LoggingBehavior<,>))]
public class LoggingBehavior<TRequest, TResponse> : Behavior<TRequest, TResponse> { }

Generator constructs behavior instances with handler's request/response types, filtering incompatible behaviors via type matching in the pipeline. The analyzer now validates that behaviors with fixed type arguments match the handler's request/response types, reporting IHR0020 errors for mismatches.

Tests

Analyzer Tests (Tests.BehaviorTypeWithOneGenericParameter.cs)

• BehaviorTypeWithCorrectTRequest_DoesNotAlert: Validates single-parameter behaviors with correct request type don't trigger errors
• BehaviorTypeWithIncorrectTRequest_Alerts: Validates IHR0020 diagnostic for request type mismatches
• BehaviorTypeWithCorrectTResponse_DoesNotAlert: Validates single-parameter behaviors with correct response type don't trigger errors
• BehaviorTypeWithIncorrectTResponse_Alerts: Validates IHR0020 diagnostic for response type mismatches

Generator Tests (BehaviorTests.cs, InvalidBehaviorsTests.cs)

• ZeroTypeParameterBehavior: Validates non-generic behaviors generate correct handler code
• RequestTypeParameterBehavior: Validates single-parameter behaviors (request position) generate correct code
• ResponseTypeParameterBehavior: Validates single-parameter behaviors (response position) generate correct code
• ThreeGenericParameterBehaviorShouldProduceNothing: Validates behaviors with 3+ parameters produce no code

Functional Tests (PartiallySpecifiedBehaviorTests.cs)

• Proves non-generic behaviors execute correctly in isolation
• Proves single-parameter behaviors execute correctly in isolation
• Proves both types can be chained together in a pipeline

Status

Work in Progress: Single-parameter behavior code generation is partially implemented. The analyzer and type detection logic are complete, but the template generation needs additional work to properly construct single-parameter behaviors with the correct type argument position.

Current test status: 9 tests failing, all related to single-parameter behavior code generation.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Allow Behaviors to be partially or fully specified types</issue_title>
<issue_description>Allow:

public sealed class ShortCircuitBehavior : Behavior<GetCachedOrder, Order>
{
	private readonly Order _cachedOrder = new(999, 49.99m, DateTime.UtcNow);

	public override async ValueTask<TResponse> HandleAsync(TRequest request, CancellationToken cancellationToken)
	{
		// Always short-circuit with cached result - simulates cache hit scenario
		return (TResponse)_cachedOrder;
	}
}
</issue_description>

<agent_instructions>* Update analyzer to support zero, one, or two generic parameters. Still warn on three or more parameters
* Update generator to match implemented type's generic arguments with handler request or response types</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Allow Behaviors to be partially or fully specified types #288

---

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[viceroypenguin]

• Test changed in InvalidBehaviorsTests should be changed to demonstrate that the generator does not generate any code when a behavior with 3 generic parameters is specified.
• Add tests in BehaviorTests for both zero and one type-parameter behaviors
• Add new functional tests under Immediate.Handlers.FunctionalTests; BehaviorTests.cs to prove code builds and runs with zero and one type-parameter behaviors

[Copilot]

• Test changed in InvalidBehaviorsTests should be changed to demonstrate that the generator does not generate any code when a behavior with 3 generic parameters is specified.
• Add tests in `Behavio...

Completed in commit c0eab83:

1. InvalidBehaviorsTests: Changed test to ThreeGenericParameterBehaviorShouldProduceNothing which demonstrates that behaviors with 3 generic parameters generate no code
2. BehaviorTests: Added ZeroTypeParameterBehavior and OneTypeParameterBehavior tests with verified snapshots
3. Functional tests: Added PartiallySpecifiedBehaviorTests.cs with 3 tests that prove zero and one type-parameter behaviors compile and execute correctly, including behavior chaining

All 131 tests pass.

[viceroypenguin]

Check new tests and update analyzer and generator codes to make tests pass

[Copilot]

Check new tests and update analyzer and generator codes to make tests pass

Started working on this in commit 8f2f6e6. I've:

1. Added IHR0020 diagnostic to detect behavior type argument mismatches
2. Updated the analyzer to validate behaviors against handler types
3. Updated the generator to detect which position (request/response) is parameterized in single-parameter behaviors

Still working on fixing the template generation logic - the tests expect behaviors where a single type parameter can map to either the request OR response position, which requires more sophisticated template handling than the current implementation. The generator needs to construct behaviors like RequestLoggingBehavior<string> when the behavior's single parameter is in the response position.

Current status: 9 tests failing, all related to single-parameter behavior code generation.

[Copilot]

Pull request overview

Expands the Immediate.Handlers behavior model so behaviors can be declared with 0, 1, or 2 generic type parameters (instead of exactly 2), updates the generator/analyzers accordingly, and adds tests/functional coverage for partially specified behaviors.

Changes:

• Analyzer: allow 0–2 generic parameters for behaviors; add IHR0020 (behavior request/response compatibility) and IHR0021 (invalid behavior constraints).
• Generator: compute behavior constraint info and generate correct behavior type arguments for 0/1/2-parameter behaviors; filter out incompatible behaviors.
• Tests: add analyzer, generator snapshot, and functional tests for non-generic and single-generic behaviors.

Reviewed changes

Copilot reviewed 52 out of 52 changed files in this pull request and generated 16 comments.

Show a summary per file

File	Description
tests/Immediate.Handlers.Tests/GeneratorTests/InvalidBehaviorsTests.cs	Updates generator invalid-behavior test inputs (e.g., static classes become partial); removes cases that are now valid.
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.cs	Adds generator tests for 0/1 type-parameter behaviors and constraint-filtering scenarios.
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.ZeroTypeParameterBehavior_assemblies=Normal#IH.Dummy.GetUsersQuery.g.verified.cs	New verified output snapshot for non-generic behavior pipeline generation (Normal).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.ZeroTypeParameterBehavior_assemblies=Msdi#IH.ServiceCollectionExtensions.g.verified.cs	New verified output snapshot for DI registrations for non-generic behaviors (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.ZeroTypeParameterBehavior_assemblies=Msdi#IH.Dummy.GetUsersQuery.g.verified.cs	New verified output snapshot for non-generic behavior pipeline generation (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.ResponseTypeParameterBehavior_assemblies=Normal#IH.Dummy.GetUsersQuery.g.verified.cs	New verified snapshot for single-parameter behavior (response varies) generation (Normal).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.ResponseTypeParameterBehavior_assemblies=Msdi#IH.ServiceCollectionExtensions.g.verified.cs	New verified snapshot for DI registrations for single-parameter behavior (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.ResponseTypeParameterBehavior_assemblies=Msdi#IH.Dummy.GetUsersQuery.g.verified.cs	New verified snapshot for handler pipeline with single-parameter behavior (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.RequestTypeParameterBehavior_assemblies=Normal#IH.Dummy.GetUsersQuery.g.verified.cs	New verified snapshot for single-parameter behavior (request varies) generation (Normal).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.RequestTypeParameterBehavior_assemblies=Msdi#IH.ServiceCollectionExtensions.g.verified.cs	New verified snapshot for DI registrations for single-parameter behavior (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.RequestTypeParameterBehavior_assemblies=Msdi#IH.Dummy.GetUsersQuery.g.verified.cs	New verified snapshot for handler pipeline with single-parameter behavior (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.BehaviorHasMultipleTResponseConstraints_assemblies=Normal#IH.Dummy.GetUsersQuery.g.verified.cs	New verified snapshot ensuring invalid/multi constraints cause behavior to be dropped from pipeline (Normal).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.BehaviorHasMultipleTResponseConstraints_assemblies=Msdi#IH.ServiceCollectionExtensions.g.verified.cs	New verified snapshot for DI registrations in multi-constraint case (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.BehaviorHasMultipleTResponseConstraints_assemblies=Msdi#IH.Dummy.GetUsersQuery.g.verified.cs	New verified snapshot for handler pipeline in multi-constraint case (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.BehaviorHasMultipleTRequestConstraints_assemblies=Normal#IH.Dummy.GetUsersQuery.g.verified.cs	New verified snapshot ensuring invalid/multi constraints cause behavior to be dropped from pipeline (Normal).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.BehaviorHasMultipleTRequestConstraints_assemblies=Msdi#IH.ServiceCollectionExtensions.g.verified.cs	New verified snapshot for DI registrations in multi-constraint case (Msdi).
tests/Immediate.Handlers.Tests/GeneratorTests/BehaviorTests.BehaviorHasMultipleTRequestConstraints_assemblies=Msdi#IH.Dummy.GetUsersQuery.g.verified.cs	New verified snapshot for handler pipeline in multi-constraint case (Msdi).
tests/Immediate.Handlers.Tests/AnalyzerTests/HandlerClassAnalyzerTests/Tests.HandlerUsesBehaviorTypeWithZeroGenericParameters.cs	Adds handler-level analyzer tests for non-generic behaviors.
tests/Immediate.Handlers.Tests/AnalyzerTests/HandlerClassAnalyzerTests/Tests.HandlerUsesBehaviorTypeWithOneGenericParameter.cs	Adds handler-level analyzer tests for single-generic behaviors and IHR0020 mismatch cases.
tests/Immediate.Handlers.Tests/AnalyzerTests/HandlerClassAnalyzerTests/Tests.HandlerUsesBehaviorTypeIsMultiplyInherited.cs	Adds handler-level analyzer tests for behaviors inheriting Behavior<,> indirectly (and mismatch detection).
tests/Immediate.Handlers.Tests/AnalyzerTests/BehaviorClassAnalyzerTests.cs	Adds tests for new behavior constraint validation analyzer (IHR0021).
tests/Immediate.Handlers.Tests/AnalyzerTests/BehaviorAnalyzerTests/Tests.BehaviorTypeWithZeroGenericParameters.cs	Adds BehaviorsAnalyzer tests validating 0-generic behavior types are accepted.
tests/Immediate.Handlers.Tests/AnalyzerTests/BehaviorAnalyzerTests/Tests.BehaviorTypeWithOneGenericParameter.cs	Adds BehaviorsAnalyzer tests validating 1-generic behavior types are accepted.
tests/Immediate.Handlers.Tests/AnalyzerTests/BehaviorAnalyzerTests/Tests.BehaviorTypeHasThreeGenericParameters.cs	Renames/adjusts test to match new “3+ generic params invalid” rule.
tests/Immediate.Handlers.FunctionalTests/NoBehaviors/ParameterlessTests.cs	Updates functional test handler signatures to include CancellationToken.
tests/Immediate.Handlers.FunctionalTests/Immediate.Handlers.FunctionalTests.csproj	Adds analyzer reference + diagnostics testing package for functional tests.
tests/Immediate.Handlers.FunctionalTests/Behavior/PartiallySpecifiedBehaviorTests.cs	Adds functional test exercising 0/1/2-parameter behaviors in a pipeline.
tests/Immediate.Handlers.FunctionalTests/Behavior/Constraints/Tests.BehaviorShouldConstrain_A.cs	Disables new IHR0020 in constraint functional tests where mismatch is intentional.
tests/Immediate.Handlers.FunctionalTests/Behavior/Constraints/Tests.BehaviorShouldConstrain_B.cs	Disables new IHR0020 in constraint functional tests where mismatch is intentional.
tests/Immediate.Handlers.FunctionalTests/Behavior/Constraints/Tests.BehaviorShouldConstrain_C.cs	Disables new IHR0020 in constraint functional tests where mismatch is intentional.
tests/Immediate.Handlers.FunctionalTests/Behavior/Constraints/Tests.BehaviorShouldConstrain_D.cs	Disables new IHR0020 in constraint functional tests where mismatch is intentional.
src/Immediate.Handlers.Generators/Utility.cs	Removes generator-local Utility (migrated/shared via src/Common).
src/Immediate.Handlers.Generators/TransformHandler.cs	Uses new return-type helpers; adjusts base-type capture to original definitions.
src/Immediate.Handlers.Generators/TransformBehaviors.cs	Accepts 0–2 type parameters; extracts/serializes constraint info for pipeline filtering and rendering.
src/Immediate.Handlers.Generators/Templates/Handler.sbntxt	Updates handler template to use a precomputed behavior “type_name” instead of forcing <TReq,TRes>.
src/Immediate.Handlers.Generators/Models.cs	Refactors behavior constraint representation for generator filtering/rendering.
src/Immediate.Handlers.Generators/ImmediateHandlersGenerator.cs	Filters behaviors via structured constraints; generates correct behavior type arguments for 0/1/2-parameter behaviors.
src/Immediate.Handlers.CodeFixes/StaticToSealedHandlerCodeFixProvider.cs	Updates attribute checks to use new extension properties.
src/Immediate.Handlers.CodeFixes/Immediate.Handlers.CodeFixes.csproj	Adds Microsoft.Bcl.HashCode package reference.
src/Immediate.Handlers.Analyzers/Properties/launchSettings.json	Updates DebugRoslynComponent targets (adds functional tests target).
src/Immediate.Handlers.Analyzers/InvalidIHandlerAnalyzer.cs	Updates extension usage (IsIHandler property).
src/Immediate.Handlers.Analyzers/Immediate.Handlers.Analyzers.csproj	Adds Microsoft.Bcl.HashCode package reference.
src/Immediate.Handlers.Analyzers/HandlerClassAnalyzer.cs	Adds IHR0020 and handler-level behavior type compatibility analysis.
src/Immediate.Handlers.Analyzers/DiagnosticIds.cs	Adds IDs for IHR0020 and IHR0021.
src/Immediate.Handlers.Analyzers/BehaviorsClassAnalyzer.cs	New analyzer enforcing supported generic constraints on behavior type parameters (IHR0021).
src/Immediate.Handlers.Analyzers/BehaviorsAnalyzer.cs	Allows behavior types with 0–2 generic parameters; keeps unbound generic enforcement for generic types.
src/Immediate.Handlers.Analyzers/AnalyzerReleases.Unshipped.md	Adds unshipped analyzer release tracking file (currently empty).
src/Immediate.Handlers.Analyzers/AnalyzerReleases.Shipped.md	Documents new analyzer rules in release notes (IHR0020/IHR0021).
src/Common/Utility.cs	Adds shared utility extensions (NullIf, WhereNotNull).
src/Common/ITypeSymbolExtensions.cs	Adds behavior constraint extraction + constraint satisfaction helpers; refactors existing symbol helpers into extension properties.
src/Common/BehaviorConstraintInfo.cs	Adds shared constraint model used by analyzers/generator for behavior compatibility.
Directory.Packages.props	Bumps DI package versions and adds diagnostics testing package version.

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[Copilot]

Pull request overview

Copilot reviewed 52 out of 52 changed files in this pull request and generated 4 comments.

Comments suppressed due to low confidence (1)

src/Immediate.Handlers.Analyzers/AnalyzerReleases.Unshipped.md:2

• AnalyzerReleases.Unshipped.md was added but is currently empty/blank. If release tracking is being used, new rules should typically be recorded in Unshipped until a version is actually released, and empty files tend to cause confusion. Either add the appropriate entries or remove the empty file if it’s not needed.

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[Copilot]

Pull request overview

Copilot reviewed 52 out of 52 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (1)

src/Immediate.Handlers.Analyzers/AnalyzerReleases.Unshipped.md:2

• The analyzer release tracking files look inconsistent: AnalyzerReleases.Unshipped.md is effectively empty, but AnalyzerReleases.Shipped.md already lists IHR0020/IHR0021 under a new "Release 3.2" section. Unless these diagnostics have actually shipped, they should be documented in AnalyzerReleases.Unshipped.md instead (and only moved to Shipped as part of a release).

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[Copilot]

AddMissingConstraint sets ExactType = baseType.TypeArguments[index] when the base Behavior<,> argument is a type parameter. That makes ExactType become something like TRequest/TResponse, which will never equal a concrete handler request/response type, causing IHR0020 to fire incorrectly and behaviors to be filtered out even when they should match (e.g., behaviors that forward a single type parameter through the base Behavior). Consider treating ITypeParameterSymbol arguments as "parameterized" (leave ExactType null and carry constraints), and only set ExactType when the base argument is a non-type-parameter symbol.

Suggested change

	constraints.Add(
	baseConstraints[index].Item2.ExactType != null
	? baseConstraints[index]
	: (name, new() { ExactType = baseType.TypeArguments[index], TypeConstraints = [] })
	);
	var baseConstraint = baseConstraints[index];
	var baseConstraintInfo = baseConstraint.Item2;
	if (baseConstraintInfo.ExactType is not null)
	{
	constraints.Add(baseConstraint);
	return;
	}
	var baseTypeArgument = baseType.TypeArguments[index];
	if (baseTypeArgument is ITypeParameterSymbol typeParameterSymbol)
	{
	// Treat type-parameter arguments as parameterized: do not set ExactType,
	// but carry over the parameter's own type constraints.
	constraints.Add(
	(name, new() { ExactType = null, TypeConstraints = typeParameterSymbol.ConstraintTypes })
	);
	}
	else
	{
	// For non-type-parameter arguments, we can treat the argument as the exact type.
	constraints.Add(
	(name, new() { ExactType = baseTypeArgument, TypeConstraints = [] })
	);
	}

[Copilot]

IHR0020 validation only scans [Behaviors] attributes on the handler's containing type (method.ContainingSymbol.GetAttributes()), but the generator also applies assembly-level [assembly: Behaviors(...)] attributes. This means mismatched fixed/partially-specified behaviors declared at assembly scope won't be diagnosed. Consider also scanning context.Compilation.Assembly.GetAttributes() (and/or modules) for BehaviorsAttribute and reporting IHR0020 at the corresponding assembly attribute argument location.