A simple question about this iasyncresult statement

See more:

C#

threads

I tried a simple demo at an article on here, and got into an issue, which I think it is simple:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;

namespace thread02
{
    public delegate bool MyDelegate(int i, string str);

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("testing threading concept:");
            MyDelegate delInstance = new MyDelegate(MyFunction);
            

            IAsyncResult ref = delInstance.BeginInvoke(100,"I am in Delegate Thread", null, null); //<== can not compile at this statement

            for (int i = 0; i < 100; i++)
                Console.WriteLine("I am in Main Thread {0}", i);
            bool status = delInstance.EndInvoke(ref);

            Console.ReadKey();
        }
        public static bool MyFunction(int count, string str)
        {
            for (int i = 0; i < count; i++)
                Console.WriteLine(str + " {0}", i);

            return true;
        }

    }
}

the error message is:

Error	CS0119	'IAsyncResult' is a type, which is not valid in the given context

What I have tried:

tried microsoft site and google. can not find the reason why.