Burimi Burimi - 3 months ago 35
C# Question

Why is Thread.Sleep so harmful

I often see it mentioned that

Thread.Sleep();
should not be used, but I can't understand why this is so. If
Thread.Sleep();
can cause trouble, are there any alternative solutions with the same result that would be safe?

eg.

while(true)
{
doSomework();
i++;
Thread.Sleep(5000);
}


another one is:

while (true)
{
string[] images = Directory.GetFiles(@"C:\Dir", "*.png");

foreach (string image in images)
{
this.Invoke(() => this.Enabled = true);
pictureBox1.Image = new Bitmap(image);
Thread.Sleep(1000);
}
}

Answer

The problems with calling Thread.Sleep are explained quite succinctly here:

Thread.Sleep has its use: simulating lengthy operations while testing/debugging on an MTA thread. In .NET there's no other reason to use it.

Thread.Sleep(n) means block the current thread for at least the number of timeslices (or thread quantums) that can occur within n milliseconds. The length of a timeslice is different on different versions/types of Windows and different processors and generally ranges from 15 to 30 milliseconds. This means the thread is almost guaranteed to block for more than n milliseconds. The likelihood that your thread will re-awaken exactly after n milliseconds is about as impossible as impossible can be. So, Thread.Sleep is pointless for timing.

Threads are a limited resource, they take approximately 200,000 cycles to create and about 100,000 cycles to destroy. By default they reserve 1 megabyte of virtual memory for its stack and use 2,000-8,000 cycles for each context switch. This makes any waiting thread a huge waste.

The preferred solution: WaitHandles

The most-made-mistake is using Thread.Sleep with a while-construct (demo and answer, nice blog-entry)

EDIT:
I would like to enhance my answer:

We have 2 different use-cases:

  1. We are waiting because we know a specific timespan when we should continue (use Thread.Sleep, System.Threading.Timer or alikes)

  2. We are waiting because some condition changes some time ... keyword(s) is/are some time! if the condition-check is in our code-domain, we should use WaitHandles - otherwise the external component should provide some kind of hooks ... if it doesn't its design is bad!

My answer mainly covers use-case 2