csharp

Use ToLookup when you group once and only read: it is immutable, returns an empty sequence for missing keys, accepts null keys and keeps first-seen order. Use Dictionary<TKey, List<TValue>> when the groups change after construction or cross a JSON boundary.

The ASP.NET Core team mapped out where C# 15 union types flow through .NET 11: minimal API and MVC bodies, SignalR's JsonHubProtocol, Blazor, and OpenAPI anyOf schemas. Route, query, header, and form binding are not supported yet.

Signal, WaitForExitStatus, and ProcessExitStatus land directly on System.Diagnostics.Process in .NET 11 RC 1, so graceful shutdown of a child process no longer needs a kill P/Invoke or a SafeProcessHandle detour.

The BCL has no ProblemDetails type, and adding a FrameworkReference to Microsoft.AspNetCore.App makes your client refuse to start on a runtime-only container. Here is the 20-line model that does the job, the DelegatingHandler that turns problem+json into a typed exception, and the content-type myth that most answers still repeat.

Play Console tells you your ANR rate is over 0.47% and hands you a native stack full of libcoreclr.so frames. Here is how to get from that useless trace to the exact async void event handler that blocked the main thread, using a Looper printer, a SynchronizationContext wrapper that names the state machine, and dotnet-trace over dsrouter.

Agent Framework ships an in-memory chat history provider and nothing else for .NET. Here is a Cosmos DB backed ChatHistoryProvider that survives process restarts, the container design that keeps a 200-turn session cheap, and the second provider you need if you want semantic recall across sessions.

A render mode decides where a Razor component executes and whether it is interactive. Here are the four modes in .NET 11, the propagation rules that decide what your component inherits, and the RendererInfo and AssignedRenderMode properties that tell you at runtime which one actually won.

An EF Core interceptor is a class EF calls before and after operations like executing a command or SaveChanges, and it can modify or suppress them, not just observe. Here are the seven interception points in EF Core 11, the registration and lifetime rules, and the cases where a query filter or plain logging is the better answer.

ModelContextProtocol 2.0.0 flipped HTTP transport to stateless by default, which is the one change that can drop every client still speaking 2025-11-25. HttpServerSessionMode.StatefulForInitializeClients in 2.2.0 serves both revisions on one endpoint. Here is the ordered upgrade, the ten breaking changes that matter, and the discover-probe bug that only a 2.1.0 client fixes.

A step-by-step migration from xunit 2.9.3 to xunit.v3 4.0.0: package swaps, the OutputType Exe change, IAsyncLifetime returning ValueTask, the Xunit.Abstractions removal, and the CI filter syntax that silently stops matching.