In this community post, detroitmatt explores how to make cleanup code reliably run in C# with IAsyncDisposable, highlighting pitfalls with disposable patterns.

Introduction

detroitmatt raises a practical concern about resource cleanup in C#, particularly when using the IAsyncDisposable interface for asynchronous cleanup logic. The question involves ensuring that necessary cleanup code runs reliably, comparing IDisposable and IAsyncDisposable, and observing areas where the C# compiler or runtime might leave gaps.

The Problem with IDisposable

Traditionally, in C#, implementing the IDisposable interface allows objects to release unmanaged or scarce resources. However, this pattern requires consumers of the object to explicitly call Dispose(). The most reliable way is via a using statement (or declaration), which ensures disposal at the end of scope:

using(var resource = new MyResource())
{
    // Use resource
} // Dispose() automatically called here

A challenge arises if the object creator or user forgets to use using. In that case, the disposal method is never called, leading to potential resource leaks. Moreover, the C# compiler does not emit warnings if objects implementing IDisposable are instantiated without using or explicit disposal, so mistakes can go unchecked.

IAsyncDisposable and Asynchronous Cleanup

C# 8.0 introduced the IAsyncDisposable interface for types that need to perform asynchronous cleanup. This is useful when, for example, releasing network resources asynchronously:

await using(var resource = new MyAsyncResource())
{
    // Use resource
} // DisposeAsync() automatically called here

However, similar to IDisposable, the callsite must use await using, otherwise the cleanup is skipped. There is no compiler-enforced guarantee that cleanup will occur.

Why Not Finalizers?

A finalizer (destructor) is sometimes used as a last resort for releasing unmanaged resources. However, finalizers are:

  • Non-deterministic (run when GC happens, not at a predictable time)
  • Synchronous: they cannot handle async operations required by IAsyncDisposable
  • Potentially costly, causing objects to linger in memory longer, affecting performance

Because IAsyncDisposable.DisposeAsync is asynchronous, it’s not compatible with standard finalizer logic.

Is There an “Idiotproof” Way?

detroitmatt asks for a method to ensure that cleanup code at least attempts to run, even if not promptly, or if an exception occurs. In managed languages like C#, the patterns rely on correct usage by the developer. Some guidelines include:

  • Document usage requirements: Make clear in API docs that consumers must use using or await using
  • Code analyzers: Use static analyzers or Roslyn analyzers like CA2000 (Dispose objects before losing scope) to catch missed disposals
  • Fallback patterns: For critical resources, consider implementing both IDisposable and a finalizer, but restrict the finalizer to only vital, synchronous resource cleanup
  • Application-wide cleanup: For certain resources, register for process exit or unhandled exception events (e.g., AppDomain.ProcessExit) to perform last-ditch cleanup, acknowledging that async work here is unreliable

Conclusion

While C# provides patterns for resource management, full guarantees depend on developer discipline or tooling. IAsyncDisposable enables async cleanup but lacks automatic enforcement. To improve reliability:

  • Always use using/await using
  • Supplement with static analysis
  • Document usage
  • Consider fallback cleanup, knowing its limits

References

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