gistfile1.cs

private static List<byte> TestList = new List<byte>();

static async Task Do()
{
    var source = new CancellationTokenSource();
    var t = Task.Run(() =>
    {
        source.Token.Register(() => TestList = null);

        for (var i = 0; i < 100000000; i++)
        {
            TestList.Add(0);
        }
    }, source.Token);
    await Task.Delay(50);
    source.Cancel();
    await t;
}