How can I detect async deadlocks in a .NET Core web API?

Understanding the Problem

Async deadlocks in .NET are sneaky. They happen when you mix blocking calls like .Result or .Wait() with asynchronous code, and the synchronization context tries to reclaim a thread that’s already stuck waiting. In a web API, this often surfaces as a request that never returns, leaving connections pooled and threads exhausted.

Common Triggers

• Calling async methods from synchronous event handlers without proper context handling.
• Using ConfigureAwait(false) incorrectly in library code that later needs the context.
• Deadlocks between async methods that lock on shared objects while awaiting.

The first step is instrumenting your application. Tools like PerfView, dotTrace, or the built-in DiagnosticSource events can reveal threads stuck in wait states. Pay attention to threads parked in Task.Wait() or Task.Result calls during request processing.