[*.cs]
dotnet_diagnostic.LC051.severity = error

LC051: ToAsyncEnumerable() runs an EF Core query synchronously

In Plain Terms

You ask for a delivery service, but the courier walks every parcel over one at a time while you stand at the door. It looks asynchronous from the outside. Underneath, someone is waiting the whole time.

Goal

Detect ToAsyncEnumerable() called directly on an EF Core query.

The Problem

.NET 10 ships System.Linq.AsyncEnumerable in the box (earlier versions get it from the System.Linq.Async or System.Linq.AsyncEnumerable packages). Its ToAsyncEnumerable() accepts any IEnumerable<T>, and an EF Core query is one. So this compiles:

// Violation: looks async, runs the query with synchronous I/O.
await foreach (var blog in db.Blogs.Where(b => b.Rating > 3).ToAsyncEnumerable())
{
    ...
}

ToAsyncEnumerable() only wraps the query’s synchronous enumerator. EF Core opens the connection and reads every row synchronously, blocking a thread-pool thread for the whole query while the code looks asynchronous. Under load this starves the thread pool just like .Result on a task.

The Fix

Use EF Core’s AsAsyncEnumerable(), which returns the same IAsyncEnumerable<T> and streams rows with async I/O:

await foreach (var blog in db.Blogs.Where(b => b.Rating > 3).AsAsyncEnumerable())
{
    ...
}

Analyzer Logic

ID: LC051

Category: Performance

Severity: Warning

1. Find System.Linq.AsyncEnumerable.ToAsyncEnumerable(IEnumerable<T>), fluent or static.
2. Walk its source back through Queryable operators, EF Core query operators (AsNoTracking, Include, TagWith, …), and query syntax.
3. Report when the walk reaches a DbSet<T> or DbContext.Set<T>().

EF Core 11 ships its own check for this (EF1004), so LC051 stays quiet in projects that reference EF Core 11 or later.

When it stays quiet (non-goals)

• In-memory sources: lists, arrays, and AsQueryable() over them. AsAsyncEnumerable() throws on a queryable EF Core does not back, so the rule only reports what it can prove is an EF query.
• Queries held in a variable, parameter, or returned by a helper. The rule cannot prove what backs them.
• Queries that already switched to LINQ to Objects with AsEnumerable(). That is an explicit choice to run the rest in memory.
• ToAsyncEnumerable() overloads for tasks and observables.

Code Fix

The fix renames the call to AsAsyncEnumerable() and adds using Microsoft.EntityFrameworkCore; when the method is not already in scope. The static form (AsyncEnumerable.ToAsyncEnumerable(query)) is reported without a fix.

Test Cases

Violations

db.Orders.ToAsyncEnumerable();
db.Orders.AsNoTracking().Where(o => o.Total > 0).Select(o => o.Id).ToAsyncEnumerable();
(from o in db.Orders where o.Total > 0 select o.Id).ToAsyncEnumerable();

Valid

db.Orders.Where(o => o.Total > 0).AsAsyncEnumerable();
cachedOrders.AsQueryable().Where(o => o.Total > 0).ToAsyncEnumerable();
db.Orders.AsEnumerable().Where(o => o.Total > 0).ToAsyncEnumerable();