.NET 11 Preview 5 adds JSON Lines serialization and full outer joins

Microsoft released .NET 11 Preview 5 on June 9, adding data-processing capabilities to the standard libraries. The release announcement lists JSON Lines serialization in System.Text.Json and full outer joins in LINQ among the preview’s additions.

Data operations move closer to the platform

The preview also adds generic numeric APIs to Random and X25519 key agreement. Runtime work covers asynchronous suspension, JIT optimization, and garbage-collector trimming and compaction. These are preview capabilities, so experiments should identify the specific build under evaluation.

For teams maintaining their own record-stream serializer, the JSON Lines addition offers a reason to compare responsibilities. Determine whether the existing helper merely formats records or also owns buffering, cancellation, error reporting, and restart behavior. Replacing the formatting portion does not automatically replace those surrounding contracts.

Exercise the awkward join cases

A useful full-outer-join evaluation starts with unmatched entries on both sides, repeated keys, and an empty input. Write down the intended output for each case before comparing the new API with a hand-built implementation. That exposes whether the old code’s behavior was deliberate or simply an artifact of its implementation.

For serialized records, inspect the emitted bytes with the downstream reader that will consume them. Include a failure midway through production so the application has a clear answer about which records are complete and which need retrying.

The preview’s library additions can reduce custom plumbing, but the engineering benefit depends on preserving the application’s semantics. A small equivalence experiment around one existing helper is a stronger starting point than replacing every similar utility at once.