ConfigureAwait

ConfigureAwait(false) avoids capturing the synchronization context, improving performance in library code.

Why No SynchronizationContext?

ASP.NET Core does not have a SynchronizationContext 1. Library code should still use ConfigureAwait(false) for portability.

GitHub search: repo:dotnet/aspnetcore .ConfigureAwait(false)

Runtime Internals

Task.GetAwaiter 2 returns TaskAwaiter
Task.ConfigureAwait(bool) 3 returns ConfiguredTaskAwaitable
TaskAwaiter struct 4
ConfiguredTaskAwaitable.ConfiguredTaskAwaiter struct 5

State Machine Difference

Without ConfigureAwait(false), the compiler-generated state machine uses TaskAwaiter. With it, the state machine uses ConfiguredTaskAwaitable.ConfiguredTaskAwaiter — the key difference being whether continueOnCapturedContext is set.

AsyncTaskMethodBuilder.AwaitUnsafeOnCompleted 6
AsyncTaskMethodBuilderT.AwaitUnsafeOnCompleted 7
Task.UnsafeSetContinuationForAwait 8
SynchronizationContext source 9

ASP.NET Core Usage

Server.IntegrationTesting RetryHelper 10
HealthCheckPublisherHostedService 11
TaskExtensions.TimeoutAfter 12
System.Private.CoreLib.TaskContinuation 13
TaskAwaiter source 14
AsyncTaskMethodBuilderT.AwaitUnsafeOnCompleted 15

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