How to transform task.Wait(CancellationToken) to an await statement?

await is used for asynchronous methods/delegates, which either accept a CancellationToken and so you should pass one when you call it (i.e. await Task.Delay(1000, cancellationToken)), or they don't and they can't really be canceled (e.g. waiting for an I/O result).

What you can do however, is abandon* these kinds of tasks with this extension method:

public static Task<T> WithCancellation<T>(this Task<T> task, CancellationToken cancellationToken)
{
    return task.IsCompleted // fast-path optimization
        ? task
        : task.ContinueWith(
            completedTask => completedTask.GetAwaiter().GetResult(),
            cancellationToken,
            TaskContinuationOptions.ExecuteSynchronously,
            TaskScheduler.Default);
}

Usage:

await task.WithCancellation(cancellationToken);

---

^{* The abandoned task doesn't get cancelled, but your code behaves as though it has. It either ends with a result/exception or it will stay alive forever.}

To create a new Task that represents an existing task but with an additional cancellation token is quite straightforward. You only need to call ContinueWith on the task, use the new token, and propagate the result/exceptions in the body of the continuation.

public static Task WithCancellation(this Task task,
    CancellationToken token)
{
    return task.ContinueWith(t => t.GetAwaiter().GetResult(), token);
}
public static Task<T> WithCancellation<T>(this Task<T> task,
    CancellationToken token)
{
    return task.ContinueWith(t => t.GetAwaiter().GetResult(), token);
}

This allows you to write task.WithCancellation(cancellationToken) to add a token to a task, which you can then await.