using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        IEnumerable<User> users = new[] { new User() };

        foreach (IUser user in users.OfType<IUser>())

            Console.WriteLine("There is IUser");

    }

}

​

public class User : IUser

{

}

​

public interface IUser

{

}