Spec: LC005 - Multiple OrderBy Calls

Goal

Detect usage of multiple OrderBy or OrderByDescending calls in a single query chain.

The Problem

In LINQ, each OrderBy call completely resets the sort order. If you want to sort by multiple columns, you must use ThenBy or ThenByDescending after the first OrderBy.

Example Violation

// Error: The first sort by Name is discarded and ignored.
var users = db.Users.OrderBy(u => u.Name).OrderBy(u => u.Age).ToList();

// Same reset after a simple local hop.
var sorted = db.Users.OrderBy(u => u.Name);
var users = sorted.OrderBy(u => u.Age).ToList();

The Fix

Use ThenBy or ThenByDescending for the second and later sort keys.

// Correct: Sorts by Name, then by Age for ties.
var users = db.Users.OrderBy(u => u.Name).ThenBy(u => u.Age).ToList();

The code fix rewrites only the later resetting sort call, preserving the selector and any explicit generic type arguments. The same fix is offered when the resetting sort is called on a single-assignment local that still has an ordered type:

var sorted = db.Users.OrderBy(u => u.Name);
var users = sorted.ThenBy(u => u.Age).ToList();

If the local is explicitly widened to IEnumerable<T> or IQueryable<T>, LC005 still reports the reset, but the fix is manual because ThenBy is not available on the widened receiver type. LC005 stays quiet when the local is reassigned before the later OrderBy directly, through deconstruction, or through out/ref, because the analyzer cannot prove which ordering state reaches the reset.

Query-comprehension syntax

Two separate orderby clauses are the query-syntax spelling of the same reset and are flagged:

// Warning: the second orderby resets the first.
var users = from u in db.Users
            orderby u.Name
            orderby u.Age
            select u;

A single orderby clause with multiple keys is correct multi-level sorting (it lowers to OrderBy(...).ThenBy(...)) and is intentionally left quiet:

// Correct: sorts by Name, then by Age for ties.
var users = from u in db.Users
            orderby u.Name, u.Age
            select u;

The diagnostic is report-only for query syntax — there is no method-call node to rewrite to ThenBy, so the automatic fix is withheld there. Collapse the two clauses into one comma-separated orderby to resolve it by hand. The ThenBy fix is still offered for the fluent OrderBy(...).OrderBy(...) form.

Boundaries

LC005 follows direct fluent chains, static Enumerable/Queryable syntax, and single-assignment sorted locals. It does not chase fields, properties, multi-assignment locals, or arbitrary helper methods, because those shapes need broader dataflow proof to avoid noisy warnings.

Analyzer Logic

ID: LC005

Category: Performance

Severity: Warning