C_SHARP_SKILLS/.ai/skills/11-linq-querying.md

Skill 11 - LINQ Querying

Core Concepts

LINQ lets code describe what sequence result is wanted: filter, project, sort, group, join, aggregate, and transform.

LINQ Parts

• Extension methods such as Where, Select, OrderBy, GroupBy, and Join provide the operations.
• Providers execute expressions against in-memory objects, EF Core, XML, or other sources.
• Lambda expressions express predicates and projections.
• Query comprehension syntax is optional and maps to method calls.

Rules

• Prefer method syntax for simple pipelines.
• Consider query syntax for multi-join/group queries when readability improves.
• Understand deferred execution: queries may not run until enumerated.
• Materialize with ToList, ToArray, or similar only when needed.
• Avoid multiple enumeration of expensive queries.
• Place filters early.
• For EF Core, ensure the query can translate to SQL.
• Do not call custom .NET methods inside EF queries unless they can be translated.

Common Operations

• Where filters.
• Select projects.
• OrderBy / ThenBy sorts.
• GroupBy groups.
• Join, GroupJoin, LeftJoin, and RightJoin combine sequences where available.
• Any checks existence.
• All checks universal conditions.
• Count, Sum, Average, Min, Max aggregate.

Performance Rules

• Avoid materializing before filtering.
• Use Any() instead of Count() > 0 for existence.
• Use TryGetNonEnumeratedCount when count may be available cheaply.
• Be careful with closures in loops.