ThreadPool.QueueUserWorkItem is not working in C# Console

Question

I am trying to use ThreadPool in C# Console application as shown below.

static void Main(string[] args)
{
            for (var i = 1;  i<= 10; i++)
            {
                ThreadPool.QueueUserWorkItem(n =>
                {
                    Console.WriteLine($"Start operation {n}");
                    Thread.Sleep(1000); // Simulate long running operation
                    Console.WriteLine($"Operation {n} completed");
                }, i);
            }
            Console.ReadKey();
}

But it is showing the below error:

 

The type or namespace name "QueueUserWorkItem"  does not exist in the namespace "ThreadPool".

 

What I am doing wrong or which reference I need to add. I have already used the namespace system.Threading.

 

I am working on .Net framework version 4.6.