Analyzer rule

LC043: Async Enumerable Buffering

LC043 flags an IAsyncEnumerable buffered with ToListAsync or ToArrayAsync only to loop over it once. Use await foreach to stream instead.

Default severity
Info
Code fix
Available
Category
Performance

Change the severity in .editorconfig:

[*.cs]
dotnet_diagnostic.LC043.severity = error

Sample code · Analyzer source

LC043: Async Enumerable Buffering

In Plain Terms

Imagine waiting for every toy to arrive in one giant pile before you start playing, even though you could play with each toy as soon as it shows up.

Goal

Detect immediate buffering of an IAsyncEnumerable<T> into a list or array before a single foreach.

The Problem

Buffering an async stream into memory and then looping exactly once throws away streaming behavior for no benefit.

Example Violation

var users = await stream.ToListAsync();
foreach (var user in users)
{
    Console.WriteLine(user.Name);
}

The Fix

Stream directly with await foreach.

await foreach (var user in stream)
{
    Console.WriteLine(user.Name);
}

Analyzer Logic

ID: LC043

Category: Performance

Severity: Info

Notes

This v1 rule is intentionally narrow. It reports only immediate buffer-then-loop patterns that are safe to rewrite to await foreach. The buffered call must come from a proven IAsyncEnumerable<T> source, so custom non-stream helpers named ToListAsync or ToArrayAsync stay quiet. Calls with buffer-method arguments, such as cancellation tokens, are left alone so the fixer does not drop behavior. Buffers captured by nested lambdas or local functions are also left alone because removing the local would break that captured use.

The code fix works on any receiver, including a chained one such as await db.Users.AsAsyncEnumerable().ToListAsync(), which becomes await foreach (var user in db.Users.AsAsyncEnumerable()). Comments and blank lines above the removed declaration move onto the await foreach.