Why ThreadLocal<T> fails when used in asynchronous C# code:

When execution resumes after an await, the continuation frequently runs on a different thread from the .NET ThreadPool. ThreadLocal<T> stores data on the physical OS thread, losing your context!

AsyncLocal<T> Context Flow:

public static AsyncLocal<string> CorrelationId = new();

async Task ProcessRequestAsync()
{
    CorrelationId.Value = Guid.NewGuid().ToString(); // Flows automatically across all awaited child tasks!
    await StepOneAsync();
    await StepTwoAsync();
}

AsyncLocal<T> travels with the logical ExecutionContext, ensuring telemetry correlation IDs flow across thread pool thread transitions!

AsyncLocal<T> is the backbone of distributed tracing in OpenTelemetry, ASP.NET Core HttpContextAccessor, and Serilog logging contexts.

Keep objects stored in AsyncLocal small and immutable because context cloning occurs on execution forks.