Avoid compiler gotchas mixing sync and async execution

Avoid compiler gotchas mixing sync and async execution

ℹ️ This documentation describes the previous Polly v7 API. If you are using the new v8 API, please refer to pollydocs.org.

Avoid code such as:

// Synchronous policy
var policy = Policy
    .Handle<Exception>()
    .Retry(3); 

// NB Bad code: synchronous execution of an async delegate!
var something = await policy.Execute(async () => await DoSomethingAsync());

The compiler will permit the above code to compile, because async is not strictly part of a method signature; the code resolves to compiling a delegate which happens to return Task<T>. Polly additionally cannot throw or fail-to-compile for any sync/async mismatch, because both the policy (Retry) and the execute overload (Execute) match as synchronous.

Note however that the Polly policy will not govern the full asynchronous execution lifecycle of DoSomethingAsync() with this code!

Why doesn't it work?

All .NET async methods synchronously return a Task representing the ongoing execution, when execution hits the first await statement. When an async delegate such as async () => await DoSomethingAsync() is run through a synchronous policy.Execute(...), the policy only governs that synchronous execution up to the first await statement returning that Task.

The asynchronous task continues to execute and may throw, placing the error into the returned Task. However, the code is then awaiting that Task (await policy.Execute(...)) outside the policy execution, as synchronous policy execution has already successfully completed synchronously returning the Task. As the above code is then awaiting a faulted Task outside the policy execution, the exception will be rethrown without the policy governing it.