Async/Await in C#: Patterns, Pitfalls, and Performance

Introduction to Async/Await

Async/await is a C# language feature that simplifies writing non-blocking code. It allows methods to yield control while waiting for long-running operations (I/O, network calls) without blocking threads.

Key Benefits

✔ Improved Responsiveness (UI stays interactive)
✔ Better Scalability (servers handle more concurrent requests)
✔ Simplified Code (avoids callback hell)

1. How Async/Await Works

Execution Flow

Under the Hood

• The compiler transforms async methods into state machines

• await points become potential suspension/resume locations

• The .NET thread pool manages continuations

2. Common Async/Await Patterns

Basic Usage

Parallel Execution

3. Critical Anti-Patterns

❌ Blocking Async Code

✅ Solution:

❌ Async Void

✅ Solution:

❌ Ignoring Tasks

✅ Solution:

4. Performance Considerations

Thread Pool Starvation

✅ Fix:

• Use ConfigureAwait(false) in library code

Avoid excessive parallel tasks

• Use ValueTask for hot paths

Task Allocation Overhead

5. Advanced Patterns

Cancellation

Timeout Pattern

6. Debugging Async Code

Common Issues

SymptomLikely CauseDeadlocksBlocking on async code (.Result, .Wait)Silent failuresUnobserved Task exceptionsHigh memoryUnbounded Task creation

Tools

• Visual Studio Parallel Stacks (Debug > Windows)

• Async Diagnostics (Microsoft.VisualStudio.Threading.Analyzers)

• Dump Analysis (!dumpasync in WinDbg)

7. Best Practices Checklist

✅ Do:

• Use async/await all the way up call stack

• Prefer Task over void for async methods

• Use ConfigureAwait(false) in library code

• Handle cancellation properly

• Log unobserved exceptions

❌ Don't:

• Block async code with .Result/.Wait()

• Ignore returned Task objects

• Use async void except event handlers

• Mix sync/async without careful consideration

Conclusion

Async/await revolutionized .NET performance but requires discipline:
✔ Understand the state machine magic
✔ Avoid blocking calls at all costs
✔ Handle errors and cancellation
✔ Monitor thread pool health

Tip: Use ValueTask for frequently-called methods that often complete synchronously.