using System;

using System.Threading;

using System.Threading.Tasks;

​

public class Program

{

    public static async Task Main()

    {

        var provider = new InstanceProvider();

        ThreadPool.QueueUserWorkItem(s => Console.WriteLine(provider.GetInstance()));

        ThreadPool.QueueUserWorkItem(s => Console.WriteLine(provider.GetInstance()));

        ThreadPool.QueueUserWorkItem(s => Console.WriteLine(provider.GetInstance()));

        ThreadPool.QueueUserWorkItem(s => Console.WriteLine(provider.GetInstance()));

        await Task.Delay(100);

    }

}

​

public class InstanceProvider

{

    private Object _instance;

    public Object GetInstance()

    {

        if (_instance == null)

        {

            _instance = CreateInstance();

        }

        return _instance;

    }

    private Object CreateInstance()

    {

        return Thread.CurrentThread.ManagedThreadId;

    }

}