[release/10.0] Fix ModelMetadata null reference exception in emitted XMLComment transformers

[github-actions]

Backport of #63872 to release/10.0

/cc @captainsafia @desjoerd

OpenAPI: Fix ModelMetadata null reference exception in emitted XMLComment transformers

Description

OpenAPI XML comment support walks ParameterDescriptions and previously assumed that parameterDescription.ModelMetadata was always non-null. For controller route parameters (including those coming from [Route] on the controller or inherited controllers), ModelMetadata can be null, which caused a NullReferenceException in the generated XML comment operation transformer while trying to inspect metadata.MetadataKind.

This change:

• Updates the emitted transformer code to guard against null before checking MetadataKind and other members:

var metadata = parameterDescription.ModelMetadata;
if (metadata is not null
    && metadata.MetadataKind == ModelMetadataKind.Property
    && metadata.ContainerType is { } containerType
    && metadata.PropertyName is { } propertyName)
{
    // existing logic to look up XML comments
}

• Regenerates the corresponding source-generator snapshots so the emitted code matches the new null-check.
• Adds a new controller-based test (SupportsRouteParametersFromControllers) that verifies:
  • A controller route parameter defined via [Route("[controller]")] and [HttpGet("{userId}")] does not throw during OpenAPI document generation.
  • The XML <summary> on the route parameter is correctly applied to the OpenAPI parameter description in the generated document.

Fixes #64185

Customer Impact

Without this fix, apps that:

• Use controllers with route parameters supplied via [Route] on the controller (common in versioned APIs), and
• Enable XML documentation-based OpenAPI support via AddOpenApi,

will see OpenAPI document generation fail with a NullReferenceException when hitting their OpenAPI endpoint or UI. This prevents customers from using the built-in OpenAPI + XML comments experience in affected controller scenarios and can break development and diagnostics workflows that depend on the generated spec.

With this fix:

• OpenAPI document generation no longer throws when ModelMetadata is null for a parameter.
• XML comments are still applied where metadata is available.
• There are no behavior changes for parameters that already had non-null ModelMetadata.

Regression?

• Yes
• No

This is a bug in the new XML documentation–backed OpenAPI support introduced in .NET 10; there wasn’t a previously working version of this feature for this specific scenario.

Risk

• High
• Medium
• Low

The change is limited to:

• Adding a null check before accessing ModelMetadata in the generated XML comment transformers.
• Updating source-generator snapshots and adding a focused test.

If ModelMetadata is non-null, behavior is unchanged. When it is null (previously a hard crash), the transformer now simply skips property-based XML comment lookup for that parameter, which is a safe and expected fallback. The change is localized to OpenAPI XML comment handling and does not affect other MVC or OpenAPI paths.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[captainsafia]

Approved via email.