using System;

using System.Threading.Tasks;

public interface IInterface

{

    Task TestTask();

}

​

class TestClass : IInterface

{

    public async Task TestTask()

    {

        await Task.Delay(500);

    }

}

​

public class Program

{

    public static async Task Main()

    {

        var cls = new TestClass();

        await cls.TestTask();

        Console.WriteLine("Hello World");

    }

}