Custom LINQ Operators

LINQ ships with a long list of operators, but every codebase eventually grows a few queries that show up again and again. "Products that are in stock with rating at least four." "Orders placed in the last seven days." Repeating that predicate in twenty places is how bugs creep in. A custom LINQ operator wraps the predicate (or projection, or aggregation) into an extension method with a domain name, so the query reads like the business rule. This lesson covers how to write those operators, when to compose existing ones, when to write your own iterator with yield return, and the eager-validation pattern that makes argument errors easy to find.