[*.cs]
dotnet_diagnostic.LC042.severity = error

LC042: Missing Query Tags

In Plain Terms

Imagine a giant pile of lunch boxes with no names on them. When something goes wrong, nobody knows which lunch belongs to whom.

Goal

Detect complex EF Core queries that run without TagWith(...) or TagWithCallSite().

The Problem

A tagged query carries a SQL comment naming where it came from. When a slow query shows up in logs, a profiler, or the database’s query store, the tag leads straight to the code. Simple lookups are easy to recognize without one. Queries that filter, sort, page, join, or group are not.

Example Violation

var users = db.Users
    .Where(u => u.IsActive)
    .OrderBy(u => u.Name)
    .Take(10)
    .ToList();

The Fix

var users = db.Users
    .Where(u => u.IsActive)
    .OrderBy(u => u.Name)
    .Take(10)
    .TagWith("Users.ActiveByName")
    .ToList();

TagWithCallSite() (EF Core 6+) tags the query with the file and line instead.

Analyzer Logic

ID: LC042

Category: Performance

Severity: Info

1. Start at a method that runs the query: ToList, ToArray, ToDictionary, ToHashSet, First/Single/Last (and their OrDefault forms), Any, All, Count, LongCount, Sum, Min, Max, Average, and their EF Core Async versions.

2. Walk back to the query root and score each operator:

   Operator	Score
   Join, GroupJoin, LeftJoin, RightJoin, GroupBy, SelectMany	2
   Every other Queryable operator (Where, Select, OrderBy, ThenBy, Skip, Take, Distinct, …) and Include / ThenInclude	1
   A predicate or selector passed to the terminal method, as in Count(o => o.IsOpen)	1
   AsNoTracking, AsNoTrackingWithIdentityResolution, AsTracking, AsSplitQuery, AsSingleQuery, IgnoreQueryFilters, IgnoreAutoIncludes, AsQueryable	0

3. Report on the terminal method when the chain has no tag, starts at a DbSet or DbContext.Set<T>(), and scores at least the threshold (3 by default).

Query syntax (from ... join ... where ... select) is scored the same way as the method chain it compiles to.

Configuration

dotnet_code_quality.LC042.query_operator_threshold = 3

When it stays quiet (non-goals)

• A TagWith or TagWithCallSite anywhere in the chain.
• Queries under the threshold, such as Where(...).Select(...). Tracking and split-query options do not add to the score.
• Subqueries inside an outer query’s lambda, such as db.Customers.Where(c => db.Orders.Any(...)). The outer query is scored and tagged as a whole.
• Chains that pass through a method the rule does not know, such as a repository helper. The helper may add its own tag.
• In-memory sources, including AsQueryable() over a list, and LINQ to Objects after AsEnumerable().
• Queries built up in a local variable and run later. Only one fluent chain is scored.

Code Fix

Two fixes insert a tag right before the method that runs the query:

• Tag query with “Type.Member” adds TagWith("Type.Member"), named after the method, property, or constructor that contains the query. Queries in lambdas and local functions use the enclosing member’s name.
• Tag query with its call site adds TagWithCallSite(). It is offered only when the referenced EF Core version has it (6.0 and later).

// Before
var users = db.Users.Where(u => u.IsActive).OrderBy(u => u.Name).Take(10).ToList();

// After
var users = db.Users.Where(u => u.IsActive).OrderBy(u => u.Name).Take(10).TagWith("UserService.GetActive").ToList();

In a multi-line chain the tag goes on its own line. A comment or #pragma above the terminal stays with the terminal and is not copied onto the tag’s line. The fix adds using Microsoft.EntityFrameworkCore; when the tag method is not already in scope. The static form (Enumerable.ToList(query)) is reported without a fix. Rename the generated tag if your team uses a different convention; the rule only checks that a tag exists.

Test Cases

Violations

db.Orders.Where(o => o.Total > 0).OrderBy(o => o.Id).Take(10).ToList();
db.Orders.Join(db.Customers, o => o.CustomerId, c => c.Id, (o, c) => c.Name).Where(n => n != null).ToList();
await db.Orders.Where(o => o.Total > 0).Distinct().CountAsync(o => o.CustomerId == 1);

Valid

db.Orders.AsNoTracking().Where(o => o.Total > 0).Select(o => o.Id).ToList();
db.Orders.Where(o => o.Total > 0).OrderBy(o => o.Id).Take(10).TagWith("orders").ToList();
cachedOrders.AsQueryable().Where(o => o.Total > 0).OrderBy(o => o.Id).Take(10).ToList();