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Short answer

Mark a method async and return Task or Task<T>. await waits without blocking the thread. Start independent work first, then combine with Task.WhenAll. [S1]

Pattern

async Task<Breakfast> MakeBreakfastAsync()
{
    var eggsTask  = FryEggsAsync(2);
    var baconTask = FryBaconAsync(3);
    await Task.WhenAll(eggsTask, baconTask);
    return new Breakfast(await eggsTask, await baconTask);
}

Rules of thumb

• await frees the thread while the work finishes. [S1]
• Awaiting each task as you create it still runs them one after another; start them first to overlap. [S1]
• Factor a series of steps into a method that returns a task, then compose those tasks. [S1]
• Prefer WhenAll over .Result / .Wait() on unfinished tasks. [S1]
• The same async/await model covers I/O-bound and CPU-bound work; CPU work usually goes through Task.Run. [S2]

Claims

• id: C1 claim: await provides a nonblocking way to start a task. source: S1 evidence: "The await keyword provides a nonblocking way to start a task"
• id: C2 claim: Awaiting sequentially delays starting other tasks. source: S1 evidence: "doesn't start other tasks until the current work completes"
• id: C3 claim: WhenAll completes when every listed task completes. source: S1 evidence: "completes when all the tasks in its argument list are complete"
• id: C4 claim: Async code can model both I/O-bound and CPU-bound work. source: S2 evidence: "for both I/O-bound and CPU-bound scenarios"
• id: C5 claim: CPU-bound async work is typically started on a background thread. source: S2 evidence: "starts an operation on a background thread with the Task.Run method"

Sources

• [S1] Asynchronous programming with async and await. Microsoft Learn. https://learn.microsoft.com/en-us/dotnet/csharp/asynchronous-programming/ . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-05.
• [S2] Asynchronous programming scenarios. Microsoft Learn. https://learn.microsoft.com/en-us/dotnet/csharp/asynchronous-programming/async-scenarios . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-05.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

Source notice: .NET documentation © Microsoft. Licensed under Creative Commons Attribution 4.0 International (https://github.com/dotnet/docs/blob/main/LICENSE).

@misc{iil_note_w7wdq1s3,
  author = {{iinventedlight}},
  title = {{C\# async and await: start tasks, don't block, compose with WhenAll}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/csharp-async-await/},
  note = {Permanent ID iil:note:w7wdq1s3. License: CC-BY-4.0.},
}

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