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Preview 5 brought the headliners: Runtime Async, unions, vector search. Preview 6 is where they grow up. If my breakdown of .NET 11 Preview 5's features that actually matter was about the ideas, this post is about those ideas taking production shape. Unions now serialize and show up
.NET
Every .NET developer has the same reflex. An API wants a Stream, you have a string or a buffer, so you allocate a MemoryStream, copy everything into it, and hand it over. We've done it for twenty years. It's such muscle memory that nobody even registers
.NET
Your agent started with a tidy 300-token system prompt. Six months later it's 9,000 tokens. Legal added the compliance rules, Support pasted in the escalation playbook, Finance dropped the entire expense policy on top. I've watched this happen on two projects now. Nobody plans it.
.NET
I've deleted more Selenium test suites than I've shipped. Fifteen years of StaleElementReferenceException, Thread.Sleep(3000) landmines all over the codebase, and CI pipelines nobody trusted. A red build meant "re-run it" long before it meant "something's broken". E2E testing
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Traditional event sourcing asks you to solve five problems independently: concurrency control, aggregate hydration, snapshot management, event persistence, and projection delivery. Five infrastructure concerns, five failure modes, and at least three NuGet packages you'll regret choosing. Orleans already solved every one of those problems. It just solved them
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Rate limiting in .NET has always been an afterthought. You either bolted on AspNetCoreRateLimit (which hasn't aged gracefully), hacked together a custom ActionFilter with a ConcurrentDictionary, or shipped without any limiting and hoped nobody would hammer your endpoints. We've all been there. .NET 10's
.NET
Your job scheduler is probably the oldest pattern in your codebase. Hangfire or Quartz.NET, running in production, doing its thing. It works. But every time you touch the scheduling code, you wince a little. Magic strings mapping to methods. Reflection-based serialization that blows up after a rename. XML config
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EF Core 11's vector search announcement was genuinely exciting, if you're on SQL Server. But according to Stack Overflow's 2024 Developer Survey, PostgreSQL is the most-used database among professional developers at 49%. Half of us watched that announcement and thought: "Cool. Doesn'
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The current AI/RAG stack for .NET developers is architecturally absurd. You've got SQL Server holding your domain data, Pinecone or Qdrant sitting in a separate cloud for vectors, a sync pipeline duct-taped between them to keep embeddings consistent, and a completely different SDK with its own query
.NET
.NET 11 Preview 5: The Features That Actually Matter Every November the .NET release train pulls into the station, and every November the "what's new" posts read like someone piped the changelog through a thesaurus. I've sat through fifteen of these cycles now. Most
.NET
Heads-up on versions: The official MCP C# SDK just went stable -- ModelContextProtocol 1.4.0 shipped in early June 2026, so we're past the preview churn. The code below targets the 1.x API. If you're reading this later, sanity-check the current package version against
.NET
If you've shipped a multi-tenant app with EF Core, you already know what I'm about to say. You get one query filter per entity. One. Want tenant isolation AND soft deletes? Mash them into the same lambda and hope nobody ever needs to turn off just