EF Core 11's New EF1004 Analyzer Catches a Silent Async Mistake

.NET 11 Preview 5, released June 9, 2026, adds a new EF Core analyzer with diagnostic ID EF1004. It catches a mistake that has gotten much easier to make since System.Linq.AsyncEnumerable moved into the BCL in .NET 10: calling ToAsyncEnumerable() on an EF Core query and quietly running it synchronously.

How the wrong call slipped in

Now that System.Linq.AsyncEnumerable ships in the runtime, its extension methods are in scope almost everywhere. One of them is ToAsyncEnumerable(), which adapts any IEnumerable<T> into an IAsyncEnumerable<T>. An EF Core IQueryable<T> is also an IEnumerable<T>, so the call compiles cleanly and looks correct next to an await foreach:

// Looks async. Is not.
await foreach (var blog in db.Blogs.ToAsyncEnumerable())
{
    Console.WriteLine(blog.Name);
}

The problem is that ToAsyncEnumerable() wraps synchronous enumeration. It walks the IQueryable using the blocking enumerator, so the database round trip runs on the calling thread. The await foreach gives you the syntax of streaming without any of the async behavior. Under load this is exactly the shape that starves the thread pool and produces deadlocks that only show up in production.

What EF1004 wants instead

EF Core exposes its own AsAsyncEnumerable() method. That one routes the query through EF Core’s async pipeline, so each row is materialized as it comes off the DbDataReader without blocking a thread:

// Runs through EF Core's async query pipeline.
await foreach (var blog in db.Blogs.AsAsyncEnumerable())
{
    Console.WriteLine(blog.Name);
}

The two method names differ by three characters, both return IAsyncEnumerable<T>, and both compile. Before Preview 5 nothing told you which one you had picked. EF1004 fires at build time the moment you call ToAsyncEnumerable() on an IQueryable<T>, pointing you at AsAsyncEnumerable().

Turning it into an error

The analyzer ships in the EF Core analyzer package and runs during a normal build, so you get the warning with no extra setup on a project referencing Microsoft.EntityFrameworkCore 11.0.0. If you want to guarantee nobody ships the synchronous version, promote it in your project file:

<PropertyGroup>
  <WarningsAsErrors>$(WarningsAsErrors);EF1004</WarningsAsErrors>
</PropertyGroup>

This pairs naturally with the streaming patterns covered in How to use IAsyncEnumerable<T> with EF Core 11: AsAsyncEnumerable() is the call that makes await foreach actually stream. EF1004 is just the guardrail that stops the look-alike from sneaking past code review.

Source: the EF Core .NET 11 Preview 5 release notes and the .NET 11 Preview 5 announcement.