What is advanced LINQ and how do custom operators work?

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**Advanced LINQ** — extension methods and deferred execution. **Deferred vs immediate:** ```csharp // Deferred — executed on enumeration var query = list.Where(x => x > 5); // Immediate — executed now var result = list.Where(x => x > 5).ToList(); ``` **Custom operator:** ```csharp public static IEnumerable WhereIf( this IEnumerable source, bool condition, Func predicate) { return condition ? source.Where(predicate) : source; } ``` **IQueryable vs IEnumerable:** - IEnumerable: in-memory filtering. - IQueryable: expression trees, translated to SQL. **Performance:** - Avoid multiple enumeration. - Use .AsQueryable() for remote data. - Profile generated SQL with EF Core logging.

C#: What is advanced LINQ and how do custom operators work?