How do you use CancellationToken in an ASP.NET Core API, and what problem does it solve?

CancellationToken in ASP.NET Core

Use CancellationToken to stop wasted work when clients disconnect, timeouts expire, or background operations are being shut down.

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TL;DR

CancellationToken is .NET's way to request cooperative cancellation. In ASP.NET Core I can accept a token in the action method and pass it to database calls, HTTP calls, and other async operations. The token does not kill the thread; it lets the called code notice cancellation and stop safely. This helps avoid wasted work when a client disconnects or a timeout is reached.

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Cooperative cancellation, request-aborted tokens, dependency propagation, timeout boundaries, and avoiding wasted work.

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Why this question matters
CancellationToken matters because production APIs should not keep expensive work running after the caller has gone away or after the operation is no longer useful.
CancellationToken is .NET's way to request cooperative cancellation. In ASP.NET Core I can accept a token in the action method and pass it to database calls, HTTP calls, and other async operations. The token does not kill the thread; it lets the called code notice cancellation and stop safely. This helps avoid wasted work when a client disconnects or a timeout is reached.
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In practice, I pass the request token through the whole call chain instead of creating random new tokens inside each method. For example, if a user cancels a report download, the API should stop the database query and external calls if those libraries support cancellation. The trade-off is that cancellation must be handled carefully: I do not want to leave partial side effects, half-written files, or inconsistent state just because a token was signaled halfway through a workflow.

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Details

Creating a CancellationToken parameter but never passing it further.
Assuming cancellation forcibly stops work immediately.
Cancelling after irreversible side effects without compensating logic.
Swallowing OperationCanceledException as if it were always an error.
Mixing request cancellation and business timeout rules without clarity.

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