Run async work with a hard timeout

using System;
using System.Threading;
using System.Threading.Tasks;

/// <summary>Awaits a task but throws when it exceeds a deadline.</summary>
public static class TimeoutExtensions
{
    public static async Task<T> WithTimeoutAsync<T>(
        this Task<T> task,
        TimeSpan timeout,
        CancellationToken cancellationToken = default)
    {
        using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        cts.CancelAfter(timeout);

        try
        {
            // WaitAsync releases the await when the task or the token completes.
            return await task.WaitAsync(cts.Token);
        }
        catch (OperationCanceledException) when (!cancellationToken.IsCancellationRequested)
        {
            throw new TimeoutException($"operation exceeded {timeout.TotalMilliseconds:0} ms");
        }
    }

    public static async Task Main()
    {
        var slow = Task.Delay(TimeSpan.FromSeconds(5)).ContinueWith(_ => 42);
        try
        {
            Console.WriteLine(await slow.WithTimeoutAsync(TimeSpan.FromMilliseconds(100)));
        }
        catch (TimeoutException e)
        {
            Console.WriteLine(e.Message); // operation exceeded 100 ms
        }
    }
}