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BackgroundService runs all of ExecuteAsync as a Task

BackgroundService now runs the entirety of ExecuteAsync on a background thread. Previously, the synchronous portion of ExecuteAsync (before the first await) ran on the main thread during service startup, blocking other services from starting. Only code after the first await ran on a background thread.

Version introduced

.NET 10

Previous behavior

Previously, the synchronous portion of ExecuteAsync ran on the main thread and blocked other services from starting.

New behavior

Starting in .NET 10, all of ExecuteAsync runs on a background thread, and no part of it blocks other services from starting.

Type of breaking change

This change is a behavioral change.

Reason for change

The previous behavior was a common pitfall that didn't meet user expectations. Most implementers of BackgroundService didn't understand that the synchronous portion before the first await blocked other services from starting during application startup.

If you require any part of your BackgroundService.ExecuteAsync to run earlier during startup (synchronously and blocking other services), you can do any one of the following:

  • Place the code that needs to run synchronously in the constructor, and it executes as part of the service construction.
  • Override StartAsync and do some work before calling base.StartAsync. StartAsync retains the behavior that its synchronous portion runs synchronously during startup and blocks other services from starting.
  • If you want to run code at a more specific time during service startup, implement Microsoft.Extensions.Hosting.IHostedLifecycleService on your BackgroundService.
  • Forgo BackgroundService entirely and implement your own Microsoft.Extensions.Hosting.IHostedService.

Affected APIs