CancelSearch.cs

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

namespace CancelSearch
{
    class Program
    {
        static void Main()
        {
            _userService = new UserService();

            var query = string.Empty;
            while (true)
            {
                var keyInfo = Console.ReadKey();
                if (char.IsLetter(keyInfo.KeyChar))
                {
                    query += keyInfo.Key;
                    TextChangedHandler(query);
                }
                else
                {
                    Console.WriteLine("\nResetting query string");
                    query = string.Empty;
                }
            }
        }

        private static UserService _userService;
        private static CancellationTokenSource _cts;

        static async void TextChangedHandler(string text)
        {
            try
            {
                _cts?.Cancel();     // cancel previous search
            }
            catch (ObjectDisposedException)     // in case previous search completed
            {
            }

            using (_cts = new CancellationTokenSource())
            {
                try
                {
                    await Task.Delay(TimeSpan.FromSeconds(1), _cts.Token);

                    var users = await _userService.SearchUsersAsync(text, _cts.Token);
                    Console.WriteLine($"\nGot users with IDs: {string.Join(", ", users)}");
                }
                catch (TaskCanceledException)       // if the operation is cancelled, do nothing
                {
                }
            }
        }
    }

    class UserService
    {
        public async Task<IEnumerable<User>> SearchUsersAsync(string query, CancellationToken cancellationToken)
        {
            Console.WriteLine($"\nPerforming search with query {query}");
            await Task.Delay(TimeSpan.FromSeconds(1), cancellationToken);

            return new[] {new User()};
        }
    }

    class User
    {
        public string Id { get; } = Guid.NewGuid().ToString();

        public override string ToString()
        {
            return Id;
        }
    }
}