Junior LINQ interviews ask "what does Where do." Senior interviews ask why your query hit the database three times to render one number , why .Where(...) after .ToList() quietly loads a billion rows into memory , and why the same LINQ runs on SQL Server in one place and in C# in another .

LINQ looks like a tidy query language and is really two very different engines — in-memory delegates vs database expression trees — behind one syntax. Knowing which one you're in, and when it runs, is the whole senior game.

This is 30 questions across the execution model, operators & correctness, performance & EF translation, and advanced pitfalls . Each has a short answer, a deep dive with C# code, and the senior signal the interviewer is listening for. Every example is from Mattrx — a real multi-tenant platform on .NET 9 / EF Core 9 over Azure SQL , with Campaigns (~4M rows) and CampaignEvents (~1.2B rows).

Block A — the execution model

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