TimeboxExtensions.cs

public static class TimeboxExtensions
{
    public static async Task Timebox(this IEnumerable<Task> tasks, TimeSpan timeoutAfter, string messageWhenTimeboxReached)
    {
        using (var tokenSource = new CancellationTokenSource())
        {
            var delayTask = Task.Delay(Debugger.IsAttached ? TimeSpan.MaxValue : timeoutAfter, tokenSource.Token);
            var allTasks = Task.WhenAll(tasks);

            var returnedTask = await Task.WhenAny(delayTask, allTasks).ConfigureAwait(false);
            tokenSource.Cancel();
            if (returnedTask == delayTask)
            {
                throw new TimeoutException(messageWhenTimeboxReached);
            }

            await allTasks.ConfigureAwait(false);
        }
    }   
}

TimeboxExtensionsTests.cs

[TestFixture]
public class TimeboxExtensionsTests
{
    [Test]
    public void ThrowsTimeoutException()
    {
        var allTasks = new[] { Task.Delay(20), Task.Delay(50)};
        var exception = Assert.ThrowsAsync<TimeoutException>(async () => { await allTasks.Timebox(TimeSpan.FromMilliseconds(20), "Problem due to timeout"); });

        Assert.AreEqual("Problem due to timeout", exception.Message);
    }

    [Test]
    public void DoesNotThrowWhenCompleted()
    {
        var allTasks = new[] {Task.Delay(20), Task.Delay(50)};

        Assert.DoesNotThrowAsync(async () => { await allTasks.Timebox(TimeSpan.FromMilliseconds(100), "Problem"); });
    }

    [Test]
    public void ThrowsExceptionOfTasks()
    {
        var invalidOperationException = new InvalidOperationException();
        var allTasks = new[] { Task.Delay(20), Task.Delay(50), Task.Delay(50).ContinueWith(t => throw invalidOperationException) };

        var exception = Assert.ThrowsAsync<InvalidOperationException>(async () => { await allTasks.Timebox(TimeSpan.FromMilliseconds(100), "Problem"); });
        Assert.AreSame(invalidOperationException, exception);
    }
}