# Description
Fix source-generated configuration types whose explicit namespace
references constants emitted by the same generator pass. Roslyn cannot
resolve those constants while `[DataType]` attributes are analyzed, so
the generated XML codec silently fell back to `urn:memorybuffer` /
`urn:testdata` while the shipped configuration uses the model namespace
URIs.
This change:
- adds `MODELGEN021` to fail when an explicitly supplied
`[DataType.Namespace]` expression cannot be resolved;
- uses compile-time-available model namespace URIs for the MemoryBuffer
and TestData configuration types;
- restores production-shaped XML fixtures and exercises the
`UpdateExtension<T>()` / `ParseExtension<T>()` path;
- documents the source-generator namespace requirement and migration
guidance.
Targeted validation passed:
- `TypeGeneratorTests`: 17 tests on `net10.0` and `net48`;
- `ConfigurationRoundTripTests`: 4 tests on `net10.0` and `net48`.
## Related Issues
- Fixes#4014
## Checklist
- [ ] I have signed the
[CLA](https://opcfoundation.org/license/cla/ContributorLicenseAgreementv1.0.pdf)
and read the
[CONTRIBUTING](https://github.com/OPCFoundation/UA-.NETStandard/blob/master/CONTRIBUTING.md)
doc.
- [x] I have added tests that prove my fix is effective or that my
feature works and increased code coverage.
- [x] I have added all necessary documentation.
- [x] I have verified that my changes do not introduce (new) build or
analyzer warnings.
- [ ] I ran **all** tests locally using the **UA.slnx** solution against
at least .net **framework** and .net **10**, and all passed.
- [ ] I fixed **all** failing and flaky tests in the CI pipelines and
**all** CodeQL warnings.
- [x] I have addressed **all** PR feedback received.
Marc Schier