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private AsyncLocal<int> asyncLocal1 = new AsyncLocal<int>();
private AsyncLocal<int> asyncLocal2 = new AsyncLocal<int>();
private AsyncLocal<int> asyncLocal3 = new AsyncLocal<int>();
private AsyncLocal<int> asyncLocal4 = new AsyncLocal<int>();

[Benchmark(OperationsPerInvoke = 4000)]
public void Update()
{
    for (int i = 0; i < 1000; i++)
    {
        asyncLocal1.Value++;
        asyncLocal2.Value++;
        asyncLocal3.Value++;
        asyncLocal4.Value++;
    }
}

Completely out of the blue, we actually looked into AsyncLocal performance, since the AsyncLocal was becoming a bottleneck for us. I wrote this statement a couple days ago, after our investigation:

In real world profiling of ASP.NET Core applications, at least 4 items are generally stored in the async locals. This suggests that the MultiElementAsyncLocalValueMap type is most commonly used.

This is awesome!

Thanks for reviewing, @kouvel.

Activity, HttpContextAccessor, Logging scope, what's the 4th 😄. I need to go look...