Call TransformOutbound for optional route parameters

[GOVINSAGA]

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Call TransformOutbound for optional route parameters

Description

DefaultParameterPolicyFactory.InitializeRouteConstraint wraps an optional parameter's constraint in OptionalRouteConstraint, which implements only IRouteConstraint. TemplateBinder.Initialize discovers transformers with a separate is IOutboundParameterTransformer test, and the wrapper fails that test — so when the inner constraint also implements IOutboundParameterTransformer, the transformer is silently dropped and TransformOutbound is never called during URL generation.

The visible effect is that a transforming constraint applies to a required segment but not to an optional one:

Route template	Outbound value today	Expected
{id:myconstraint}	transformed	transformed
{id:myconstraint?}	not transformed	transformed

Approach

Optional constraints that also implement IOutboundParameterTransformer are now wrapped in a new internal sealed subclass of OptionalRouteConstraint that additionally implements IOutboundParameterTransformer and forwards to the inner constraint. Matching behaviour is inherited unchanged.

Constraints that are not transformers keep using OptionalRouteConstraint exactly as before, so the wrapper type callers already observe is unchanged for every existing case.

The public OptionalRouteConstraint type is untouched, so there is no public API change and no PublicAPI.Unshipped.txt entry.

Note on the Components build

src/Http/Routing/src/Constraints/**/*.cs is compiled into Microsoft.AspNetCore.Components via Microsoft.AspNetCore.Components.Routing.targets, and IOutboundParameterTransformer is not part of that source set. The new file is excluded with a Compile Remove entry, alongside the three constraints already excluded there. Verified that Microsoft.AspNetCore.Components still builds clean.

Behaviour change

URLs generated for an optional segment with a transforming constraint now contain the transformed value where they previously contained the raw value. That is the fix, but it is an observable output change for anyone relying on the current behaviour — happy to gate it if you would prefer that.

Tests

• DefaultParameterPolicyFactoryTest.Create_CreatesParameterPolicy_FromRoutePattern_Constraint_Optional_PreservesOutboundParameterTransformer — the policy returned for an optional transforming constraint still surfaces IOutboundParameterTransformer and forwards TransformOutbound.
• TemplateBinderTests.BindValues_ParameterTransformer_OptionalParameter — end-to-end URL generation applies the transform, using a policy built through DefaultParameterPolicyFactory so the parameter is wrapped exactly as it is at runtime.

Both fail without the change. Full Microsoft.AspNetCore.Routing.Tests suite passes (3480 tests).

Fixes #23063

[dotnet-policy-service]

Thanks for your PR, @GOVINSAGA. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[halter73]

@javiercn Do you have any thoughts on this change?

I see that IParameterLiteralNodeMatchingPolicy also implements IParameterPolicy. ICachableParameterPolicy is also commonly implemented by IParmaterPolicy, so would we eventually need to have a special wrapper for those as well? I assume not, since the number of combinations would become unwieldy.

I wonder if option B from the issue (#23063) would be the better way to address this:

Option B — leave the factory alone and unwrap in TemplateBinder.Initialize, i.e. also accept OptionalRouteConstraint { InnerConstraint: IOutboundParameterTransformer }. Narrower, but puts routing-specific unwrapping knowledge into the binder.