Best way to Make Thread.sleep replacement without hang

Thread.sleep.cs

        //Install PM> Install-Package Microsoft.Bcl.Async 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class ExTask : Task
{
    public ExTask(Action action)
        : base(action)
    { }

    public static Task Delay(double milliseconds)
    {
        var tcs = new TaskCompletionSource<bool>();
        System.Timers.Timer timer = new System.Timers.Timer();
        timer.Elapsed += (obj, args) =>
        {
            tcs.TrySetResult(true);
        };
        timer.Interval = milliseconds;
        timer.AutoReset = false;
        timer.Start();
        return tcs.Task;
    }
}